computer network module 2

COMPUTER NETWORKS(BCSE 3306)

Ajit K [email protected]

Department of Computer Science Engineering & Application

Lecture NotesModule II

Computer Networking / Module II / AKN / 2

Out Line of Module IIData-Link Layer

Error detection and correctionData link control and protocolsPoint-to-Point access (PPP)Multiple AccessLocal Area Networks: EthernetWireless LANSVirtual Circuit Switching: Frame Relay and ATM

Text: “Data Communications and Networking” Third Edition,Behrouz A Forcuzan, Tata Mc Graw-Hill.Chapter 10 - Chapter 15 and Chapter 18


Lecture ILecture I•• Error Detection and CorrectionError Detection and Correction

•• Types of ErrorsTypes of Errors•• DetectionDetection•• Error CorrectionError Correction

•• Data Link Control and ProtocolsData Link Control and Protocols•• Stop and Wait ARQStop and Wait ARQ•• GoGo--BackBack--N ARQN ARQ•• Selective Repeat ARQSelective Repeat ARQ•• HDLCHDLC•• PPPPPP


The Link layer

Datalink layer is subdivided into two sub-layers

Logical Link Control (LLC): non architecture specific, same for all LANS (IEEE)

Media Access Control (MAC): contains a number of distinct module each carries proprietary information specific to the LAN product being used

Project 802 of IEEE sets standards to enable intercommunication between equipment from a variety of manufacturers


Errors in TransmissionData can be corrupted during transmission, for reliable communication, errors must be detected and corrected Two types of transmission errors

Single bit error: only one bit of the data unit is changed

Burst error: two or more bits in the data unit have changed


Detection mechanismsTo detect errors in transmission the concept of redundancy is used i.e. adding extra bits along with data and transmitted to other end.

Detection Methods

Parity Check

Cyclic Redundancy Check (CRC)

Checksum


Parity checkIt can be simple or two dimensionalSimple Parity Check

A redundant bit called parity bit is added to every data unit so that the total no of 1s (including parity bit) in the unit becomes even or odd Performance:

it can detect all singlebit errors,

also it can detectburst errors of odd size i.e. 1, 3, 5 etc.

However it can not detect burst errors of even size, i.e. 2, 4, 6 etc


Example: Simple Parity CheckSuppose the sender wants to send the word ‘world’. In ASCII the five characters are coded as 1110111 1101111 1110010 1101100 1100100The following shows the actual bits sent after adding parity bit

11101110 11011110 11100100 11011000 11001001Now suppose the word ‘world’ is received by the receiver without being corrupted in transmission.

11101110 11011110 11100100 11011000 11001001The receiver counts the 1s in each character and comes up with even numbers (6, 6, 4, 4, 4). No errors, The data are accepted.Now suppose the word ‘world’ is corrupted during transmission as follows. 11111110 11011110 11101100 11011000 11001001The receiver counts the 1s in each character and comes up with even and odd numbers (7, 6, 5, 4, 4). Contains error, the data are discards them, and asks for retransmission.


Two-Dimensional Parity CheckA block of bits is organised in a table, now parity bit is calculated for each row then parity bit of each column thus generating one more row (redundant data unit).The extra data unit is also transmittedPerformance

A redundancy of n bits can easily detect a burst error of n bits

If some even number of bits of two data units gets corrupted in same position then it can not be detected


Example: Two dimensional Parity CheckSuppose the following block is sent:10101001 00111001 11011101 11100111 10101010However, it is hit by a burst noise of length 8, and some

bits are corrupted. 10100011 10001001 11011101 11100111 10101010When the receiver checks the parity bits, some of the bits do not follow the even-parity rule10100011 10001001 11011101 11100111 10101010and the whole block is discarded.


Cyclic Redundancy Check (CRC)It is the Most powerful method based on binary divisionThe redundancy bits used by CRC are derived by dividing the data unit by a predetermined divisor; the remainder is the CRC

MethodA string of n 0s appended to data unit, n is one less then the number of

bits present in the predetermined divisor

Data unit appended with 0s is divided by the divisor using modulo-2 division and the remainder is collected

Remainder replaces n no of 0s appended to data unit and transmitted

At the receiving end the data along with CRC is divided by the divisor and if remainder is Zero than no error


Binary Division in CRC: An Example

Sending End Receiving End


CRC contd.Modulo-2 division

If left most bit of dividend is 1 then quotient will be 1 else 00-0 = 0, 1-1=0, 1-0=0, 0-1=1Note: we are dealing with bit-patterens not with quantitative values

DivisorIt is calculated from an algebraic poly

nomial

PropertiesIt should not be divisible by x, guarantees that

all burst errors of a length equal to the degree of polynomial are detected

It should be divisible by x+1, guarantees that all burst errors affecting an odd number of bits are detected


Standard Polynomials

An ExampleIt is obvious that we cannot choose x (binary 10) or x2 + x (binary 110) as both are divisible by x.However, we can choose x + 1 (binary 11) because it is not divisible by x, but is divisible by x + 1.We can also choose x2 + 1 (binary 101) because it is divisible by x + 1 (binary division).

Name Polynomial Application

CRCCRC--88 x8 + x2 + x + 1x10 + x9 + x5 + x4 + x 2 + 1

x16 + x12 + x5 + 1

x32 + x26 + x23 + x22 + x16 + x12 + x11 +x10 + x8 + x7 + x5 + x4 + x2 + x + 1

ATM headerCRCCRC--1010 ATM AAL

ITUITU--1616 HDLC

ITUITU--3232 LANs


ChecksumA simple but effective method based on redundancy

MethodSubdivide the data unit into equal segment of n bits

These segments are added using ones complement arithmetic so that result is also n bits

The sum is complemented and appended at the end of original data unit as redundant bits

At the receiving end all the groups are added again and if the result is zero then there is no error

PerformanceDetects all errors involving an odd

number of bits as well as most errors involving an even number of bits


Example Example Suppose the following block of 16 bits is to be sent using a checksum of 8 bits. 10101001 00111001 The numbers are added using one’s complement

10101001 00111001------------

Sum 11100010 , Checksum 00011101The pattern sent is 10101001 00111001 00011101Now suppose the receiver receives the pattern sent without any error. Receiver adds all sections using same method

10101001

00111001

00011101

Sum 11111111 , Complement 00000000 means that the pattern is OK.


ExampleExampleNow suppose there is a burst error of length 5 that affects 4 bits.

10101111 11111001 00011101

When the receiver adds the three sections, it gets

10101111

11111001

00011101

Partial Sum 1 11000101

Carry 1

Sum 11000110

Complement 00111001 the pattern is corrupted.


Error CorrectionTwo ways of error correction

Error correction by retransmissionWhen an error is detected, receiver asks sender to retransmit the whole data unitThis method is normally used in TCP/IP. i.e. CRC is used in Link layer and Checksum is used in Network and Transport Layer

Forward Error CorrectionA receiver uses an error correcting code, but it requires more number of redundant bits to know the position of error in comparison to error detection methodse.g. for a single bit error, one bit parity is sufficient to know if there is error or no error, i.e. 0 or 1


Forward Error correctionRedundancy bits needed is

Let number of data bits be m, and redundant bits be rTherefore total no of bits sent is m+rThen r must be able to indicate m+r+1 different states. i.e. to indicate if there is an error in one of the m+r positions and one state is needed to say that there is no errorOne bit can represent 2 states, then r bit can represent 2r states therefore the inequation

2r ≥ m+r+1 must be satisfiedNow given the value of m, r can be calculatede.g. if m=7 then r has to be 4, i.e. 24 ≥ 7+4+1 is satisfied


Hamming Code for Single bit error CorrectionProvides a practical solution to FECMethod:

Let number of data bits (m) be 7 => number of redundant bits (r) are 4 Position of redundant bits are defined as 1st, 2nd, 4th, and 8th, i.e. 2x, x=0,1,2,…

Value of each of r bits is calculated as parity bit for one combination of data bits given as follows

r1: data bits 1, 3, 5, 7, 9, 11 (binary value containing 1 at 1st position) r2: data bits 2, 3, 6, 7, 10, 11 (binary value containing 1 at 2st position)r3: data bits 4, 5, 6, 7 (binary value containing 1 at 3rd position)r4: data bits 8, 9, 10, 11 (binary value containing 1 at 4th position)


Redundancy bits calculation


Error detection using Hamming code


Burst error CorrectionHamming code cannot correct a burst error directly. But by rearranging the sending data the code may be appliedMethod:Instead of sending all bits in a data unit together, we organise N units in a column and then send first unit of each unit followed by second bit of each and so on If a burst error of M bits occur (M<N), then only one bit from each of the M units is corrupted We can then correct the corrupted bit applying the Hamming code N times


Data link control and ProtocolsFlow Control and Error control are the important responsibilities of link layer, collectively called as data link controlFlow Control

refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgement

Error Control It is based on the Automatic Repeat reQuest (ARQ), i.e. any time an error is detected, specified frames are retransmitted also called Positive Acknowledgement with Retransmission(PAR)


Stop and Wait ARQThe sending device sends a frame and waits for the acknowledgement before sending anotherThe sending device keeps a copy of the last frame transmitted until it receives an ACKBoth data and ACK frames are numbered and a data frame 0 acknowledged by ACK 1 frame indicating that the receiver has received data 0 and expecting Data 1.If a receiver receives a damaged frame then it is discarded without any response to senderSender maintains a variable ‘S’, that holds the number of recently sent frame. Receiver maintains a variable ‘R’ that holds the number of next expected frameThe sender starts a timer when a fame is sent, if ACK is not received within a pre allotted time period (Time out) the same frame is retransmittedThe receiver sends ACK if a correct frame is received


OperationNormal

Lost or damaged frame

Lost ACK Delayed ACK


PiggybackingTo achieve full duplex (bi-directional transmission) we use a concept called piggybacking It is a method to combine data frame with an ACK instead of sending two separate frames, thus ACK gets a free ride on the next out going data frame. It saves bandwidth.Characteristics of Stop & Wait

It is simple but inefficient because

a) The network will be completely idle during times that the receiver responses

b) Wasting of substantial amount of BW

c) Duplication due to premature retransmission or lost ACK

Round Trip Time(RTT): The time required for a frame to arrive at the receiver plus the transmission time for ACK to come back


ExampleGiven Channel BW=50Kbps, RTT=500ms, 1 frame contains 1000bits, then Find channel utilizationAnswer (Method 1)

At t=0 first frame transmission startsThe time required to push 1000 bits to medium (1 frame) is t=1000/50,000=20ms1st bit arrives at t=250ms (RTT/2), the last bit arrives at t=270msIgnoring the processing time, and length of ACK, At t=520ms ACK arrives at the sender, i.e. At t=520ms the next frame transmission startsi.e. 20ms utilization time and 500ms waiting time for a total of 520 ms communication timeUtilization = 4%, waiting=96% loss of BW

Answer (Method 2)The system can send 50000×.5=25,000 (BW×delay)bits during the time it takes for the data to go from the sender to the receiver and then back again.However, the system sends only 1000 bits. i.e. the link utilization is only 1000/25,000, or 4%.


Sliding WindowThe main disadvantage of Stop and wait method is loss of bandwidth and efficiency is lowTo improve efficiency, multiple frames should be in transition while waiting for ACKIn this method an window amount of frames can be sent without receiving ACKAn imaginary window with fixed size slides over the frames at sending end. It contains the frames to be transmitted nextIt is implemented in two ways. 1. Go Back n ARQ 2. Selective Repeat ARQ

Window size = 7 frames

Sequence number 0, 1, 2, . . ., 7 then wrap around

Direction of transmission


Go Back n ARQThe sender maintains three variables

S: holds the sequence number of the recently sent frameSF: holds the sequence number of the first frame in the windowSL: holds sequence number of last frame in the windowThe size of the window W = SL – SF + 1

The receiver maintains only one variable R: holds the sequence number of the next frame expected

Direction of transmission


Go Back n ARQ

ACK 2

Normal Lost or damaged frame


Go Back n ARQ contd.A damaged or lost ACK results in two situations

If next ACK arrives before the timer expires, no retransmission is requiredIf next ACK does not arrive before timer expires then all the frames starting from last ACK received is retransmitted again, thus the name Go Back n

A delayed ACK results in two situationsIf next ACK arrives before the timer expires, no retransmission is requiredIf no ACK is received before timer expires then all the frames starting from last ACK is resent again and these frames are discarded at the other end if already received before

PiggybackingEach direction needs a window


Go Back n ARQ: Sender Window sizeSize of Sender window < 2m

m is the number of bits available in the header to store the sequence number of the framesLet m = 2 then

sequence nos are 0, 1, 2, 3 (i.e. 4 frames)

Sender window size = 3 frames? Other wise there will be problem


Selective Repeat ARQGo Back n method is inefficient in case of noisy links where there is higher probability of damage, as this method has to resent multiple frames making the condition more worseTo avoid this we have another method called Selective repeat ARQ, which resends only the damaged/lost frame instead of all frames.Sender and Receiver windows

Both have a window having size at most on-half of the value 2m


Selective Repeat ARQ, lost frame

• Draw the figures for

• lost Ack

• delayed ACK

• Piggybacking


Selective Repeat ARQ, sender window size

Bandwidth-Delay Product

It is a measure of efficiency of an ARQ, it is a measure of no of bits that can be sent while waiting from the receiver

i.e. BW (bits/sec) × round trip time (sec) = no of bits


Example Example In a Stop-and-Wait ARQ system, the bandwidth of the line is 1 Mbps, and 1 bit takes 20 ms to make a round trip. What is the bandwidth-delay product? If the system data frames are 1000 bits in length, what is the utilization percentage of the link? What is the utilization percentage of the link, if the link uses Go-Back-N ARQ with a 15-frame sequence?

SolutionSolutionStop & Wait ARQ

The bandwidth-delay product is 1 × 106 × 20 × 10-3 = 20,000 bits , can be sent during one RTT, But system sends only 1000 bits. i.e. link utilization is 1000/20,000 = 5%.

Go Back n ARQThe bandwidth-delay product is still 20,000. The system can send up to 15 frames or 15,000 bits during a round trip. This means the utilization is 15,000/20,000 = 75%. Of course, if there are damaged frames, the utilization percentage is much less

because frames have to be resent.


HDLC ProtocolHigh-level Data Link Control is an actual protocol designed(by ISO in 1979) to support half-duplex and full-duplex communication over point-to-point and multipoint linksTwo Transfer Modes defined

Normal Response Mode (NRM)Asynchronous Balance Mode (ABM)

NRM: in this unbalanced mode one station is designated as primary station and others are secondary

A primary station can send commands and secondary stations can only respond to the commands

Uses both point-to-point and multipoint links


HDLC Protocol contdAsynchronous Balance Mode (ABM): in this balanced mode each station can command and response. It uses point-to-point link

Frames: HDLC defines three types of framesInformation Frames (I-Frame):used to carry user data and control information relating to user dataSupervisory frames (S-Frame): used only to carry control informationUnnumbered Frames (U-Frames): reserved for system management or link management


HDLC Frame Formats

Each frame contains up to 6 fields as shown aboveFlag: An 8-bit sequence with a bit-pattern that identifies both beginning and end of a frame and also used for synchronization.Address: It contains the secondary station’s address i.e. a ‘to address’ if frame created by primary or contains a ‘from address’ if created by secondary.

It can be on byte or multiple bytes. If the last bit of one byteis a 1 then there is no other byte. If the last bit is zero thennext byte is also contains addressThe system that does not use primary/secondary configuration like ethernet needs two address field for sender and receiver respectively


HDLC Frame FormatsControl: 1 or 2 byte segment used for flow and error controlInformation: contains user information or network management information. Length is fixed within each networkFCS: Frame Check Sequence field contains either a 2-byte or 4 byte CRC


HDLC I-Frame Format

Control fieldfirst bit is zero for I-FrameNext thee bits called N(S) represents sequence number of data framesP/F bit used for dual purpose i.e. if value is one then it is either POLL bit or a FINAL bit. If 0 then it is unused

Poll: when a frame is sent by primary to secondaryFinal: when frame sent from secondary to primary

Next thee bits called N(R) represents sequence number of ACK frames when piggybacked

Information fieldContains user users data, i.e. data received from upper layer


HDLC S-Frame Format

These frames are used for ACK, flow and error control, i.e. used for ACK whenever piggybacking is inappropriate or not possibleControl field

First two bits are 10 for S-FrameNext two code bits defines 4 types of S-FrameReceive Ready (RR): value 00, it can be used in 4 different ways

ACK: P/F=0, used by receiver to send an +ve ACK for a received I-Frame, N(R) contains next frame expectedPoll: P=1, used by primary for pollingFinal: F=1, used by secondary to tell that nothing t be sent+ve response to select, F=1, used by secondary, ready to receive


HDLC S-Frame Control field contd.Receive Not Ready (RNR): value 10, can be used in 3 different ways

ACK, RNR: used by receiver, acknowledging receipt of all frames up to & excluding N(R), and announces busy condition, i.e. unable to receive more framesSelect: P=1, used by primary to send information to secondaries-ve response to Selcet: F=1, used by secondary, unable to receive data

Reject (REJ): value 01, used by receiver to represent –ve ACK (NAK)Selective reject (SREJ): code 11, used by receiver, NAK for Selective repeat ARQP/F bit is as discussed for I-FramesNext thee bits called N(R) represents sequence number of ACK or NAK valueIt has no N(S) field as it is never used for data transmission


HDLC U-Frame

Theses frames are used to exchange session management and control informationIt contains 5 code bits having following options

Command Meaning Response

00 001 SNRM Set Normal Response Mode

11 011 SNRMESet Normal Response Mode (Extended)

11 100 SABMSet Asynchronous Balance Mode, Disconnect Mode DM

11 110 SABMESet Asynchronous Balance Mode (Extended)

00 000 UI Unnumberd Information UI

00 110 Unnumberd Acknowledgement UA

00 010 DISCDisconnect, Request Disconnect RD

10 000 SIMSet Initialization Mode, Request Information Mode RIM

00 1000 UP Unnumbered poll11 001 RSET Reset11 101 XID Exchange ID XID10 001 Frame Reject FRMR

Code


Data TransparencyIf a data field of an HDLC frame contains a pattern that is same as flag sequence then receiver interprets it the sequence as ending flag which causes problem called lack of data transparencyTo ensure that the flag sequence does not appear in the any where else in the frame HDLC uses technique called Bit StuffingAccording to this technique each time a sender wants to transmit a bit sequence having more than five consecutive 1s, it stuffs (inserts) one redundant 0 after the fifth 1.At the receiving end receiver deletes (unstuffs) one 0 after fifth 1 in the received bit stream.


Point-to-Point Access: PPPIt is deigned to connect home computers to the server of ISP. They are connected to the Internet either through a telephone line or cable TV connection using a modemPPP is a datalink layer protocolPPP provides following services

It defines the data format of the frameEstablishment of link and exchange of dataAuthentication


Point-to-Point Access: PPP

It employs a version of HDLC frameFlag: 011111110, identifies boundaries of a PPP frameAddress: 11111111, uses broadcast address, as it is a point to pointControl: 11000000, format of U-frame and shows that does not contain any sequence number, thus no flow or error controlProtocol: it defines the type of data field, i.e. either user data or any other informationData: carries either user data or other informationFCS: 2-byte or 4-byte CRC.


Point-to-Point Access: PPPIdle state: link is not being used, no active carrier, line is quitEstablishing State: one of the endpoints starts communication, options are negotiated. If negotiation is successful, system goes to authentication stateAuthenticating State: is optional, if successful, connection goes networking state, otherwise goes to termination stageTerminating stage: closing the connection


PPP StackPPP uses a stack of protocols to provide the service. Each time, a PPP packet can carry packets related to one of these protocols in its data field. The type of protocol is defined by the protocol fieldLink Control Protocol (LCP)

It is responsible for establishing, maintaining, configuring, and terminating linksAlso provides negotiation mechanism to set options


PPP StackCode: Type of LCP PacketID: A value used to match request with reply.

Code Packet Type

0116 Configure-request

0216 Configure-ack

0316 Configure-nak

0416 Configure-reject

0716 Code-reject

0A16 Echo-reply

0B16 Discard-request

0816 Protocol-reject

0916 Echo-request

0516 Terminate-request

0616 Terminate-ack

Length: length of LCP PacketInformation: extra information needed for LCP Packet

Options: There are many options that can be negotiated between two end pointsOptions are inserted in the information fieldSome of the options are as follows

Option Default

Maximum receive unit 1500NoneOff

Off

Authentication protocolProtocol field compressionAddress and control field compression


LCP Packet OptionsConfiguration Packets: are used to negotiate options

Configuration Request: initiating communicationConfiguration ACK: if options of configuration Request are accepted.Configure NAK: if configurations are revised, then sender sends a totally new configuration requestConfigure Reject: if some options are not recognized then these are marked with reject packet, sender send a new configure request

Link Termination Packets: are used to disconnect the linkTermination Request: either of the two parties Termination Reply: other answers

Link Monitoring and Debugging PacketsCode Reject: receives a packet with unknown codeProtocol Reject: receives a packet with unknown protocolEcho Request & Echo Reply: to monitor the linkDiscard Request: loop-back test packet, used by sender to check internal condition, request is simply discarded


Authentication ProtocolPPP is designed to be used over dial-up links where verification of user identity is necessaryTwo protocols are used for authentication(PAP & CHAP).Password Authentication Protocol(PAP)

User needs to access sends an authentication identification(user name) and a passwordThe system checks the validity of the identification and password and either accepts or denies connection


PAP contd.PAP packet is embedded in information frame of PPP

Authentication request: request accessAuthentication-ACK: accept accessAuthentication-NAK: to deny access


Challenge Handshake Authentication ProtocolIs a three way handshaking protocol that provides more security than PAP

System sends the user a challenge packet containing a challenge value, usually a few bytesUser applies a predefined function to received value and own password and produces a result. Sends the result back to systemSystem also performs same operation with same data and compares with the received result.


CHAP Contd.4 packet types as shown


Network Control Protocol (NCP)After successful authentication connection goes to network stateNCP used to encapsulate data coming from network layer to PPP frameInternetwork Protocol Control Protocol(IPCP)

Before the packet can be sent one more connection at the network layer has to establishedThe set of packets that establish and terminate a network layer connection for IP Packets is called IPCP


Network Control Protocol Contd.It uses 7 types of packetsConfiguration request packet is used to negotiate options with other partyAfter configuration link is ready to carry IP data in the payload field of PPP, and the value of protocol field is changed to 002116

After data transmission again IPCP takes control to terminate the network connection

Code IPCP Packet

01 Configure-request

Configure-ack

Configure-nakConfigure-rejectTerminate-requestTerminate-ackCode-reject

02

0304050607

Types of IPCP packetsA Complete Example


Lecture IILecture II•• Multiple AccessMultiple Access

•• Random AccessRandom Access•• Controlled AccessControlled Access•• ChannelizationChannelization

•• Wired LAN: Ethernet (802.3)Wired LAN: Ethernet (802.3)•• Traditional EthernetTraditional Ethernet•• Fast EthernetFast Ethernet•• Gigabit EthernetGigabit Ethernet

•• Wireless LANWireless LAN•• IEEE 802.11IEEE 802.11•• BluetoothBluetooth (802.15)(802.15)


Access ControlThe way to coordinate access to a link when used by more than one deviceThe methods may be classified as follows


Random AccessAccording to this method , each station has the right to the medium without being controlled by any other stationSome procedures are defined to resolve the access conflict called collision.

Multiple Access (ALOHA)Any station sends when a frame is readyWaits for an ACK, if not received in allotted time then the frame is resent


Carrier Sense Multiple Access (CSMA)To minimize the chance of collision, The CSMA method is developedEach station listens before transmitting. i.e. transmission is possible only if line is idlePossibility of collision still exists because of propagation delayThe garbled signal reaches at times t4and t5 respectivelyTwo strategies to sense a busy medium

Non persistentpersistent


CSMA, Persistence StrategiesIf the line is idle, station sends immediately, otherwise waits a random period of time before resendingReduces the chance of collision as it is unlikely that two stations will wait same amount of time. But decreases efficiency as waits unnecessarily random amount of time

If the line is idle, the station sends with a probability1-persistent: station sends frame with a probability 1 (i.e. 100%). Increases chance of collisionP-persistent: station sends a frame with a probability p and refrains with a probability 1-p


CSMA/CD (Collision Detection)After sending the system monitors for a collision

If collision detected, it informs others by sending a jam signaland waits for some time(?) before resending

To reduce the probability of collision, station waits for a little time for first time. Changes waiting time for subsequent collision


CSMA/CD Contd.Back-off methods

Exponential Back-offSystem waits between 0 – 2n×max-propagation time‘n’ is the attempted no of transmissions

Random Back-offsystem waits for a random amount of time before resending. The random number is generated based on current value of parameter n. ‘n’ is initially 0 and incremented by one each time a collision occurs

When jam signal is received by others, they will discard the part of the frame received.This method is used in traditional Ethernet


CSMA/CA (Collision Avoidance)This procedure avoids collision, uses one of the persistence strategiesAfter it finds a line is idle

System waits an Inter Frame Gap (IFG) amount of time then another random amount of time before sending and sets a timerIf it does not receive ACK before timer expires then it increments back-off and waits this amount of time before sensing the line again.

Used in W-LANs.


Controlled AccessA Station cannot send unless authorized by other stationsReservation Method

A station needs to reserve before sendingTime is divided into intervals, In each interval, a reservation frame precedes the data frames sent in that intervalIf N stations are there in the system, there are exactly N reservation mini slots. Each belonging to a station


PollingOne device designated as primary and others are as secondaryPrimary has control over the link, the secondaries follows instructionsPolling: Primary asks secondary if they have anything to sendSelect: primary needs to send to a secondary


Token PassingA station authorized to send data when it receives a special frame called tokenA token keeps circulating around the ringIf a station needs to send it captures the token and sends data frames.Finally it releases the

token when finishes. Now the token may be used by others


Channelization

In this method the available bandwidth of a link is shared in space, time, frequency or through codeSDMA: space is dividedFDMA: The bandwidth is divided into channelsTDMA: bandwidth is one channel that is time sharedCDMA: one channel carries all transmissions simultaneously with different code


FDMASeparation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole timeAdvantages:no dynamic coordination necessaryworks also for analog signals

Disadvantages:waste of bandwidth if the traffic is distributed unevenlyinflexibleguard spaces

f

t

k2 k3 k4 k5 k6k1

c


f

t

c

k2 k3 k4 k5 k6k1

TDMAA channel gets the whole spectrum for a certain amount of time

Advantages:only one carrier in themedium at any timethroughput high even for many users

Disadvantages:precise synchronization necessary


FTMACombination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: GSM Advantages:

better protection against tappingprotection against frequency selective interferencehigher data rates compared tocode multiplex

but precise coordinationrequired

f

t

c

k2 k3 k4 k5 k6k1


CDMAEach channel has a unique code

All channels use the same spectrum at the same timeAdvantages:

bandwidth efficientno coordination and synchronization necessarygood protection against interference and tapping

Disadvantages:lower user data ratesmore complex signal regeneration

Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

f

t

c


Access method - Mathematical RepresentationEach station is assigned a code, which is a

sequence of numbers called chips0 represented as –1 and 1 as +1


Disadvantages:higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)all signals should have the same strength at a receiver

Advantages:

all terminals can use the same frequency, no planning needed

huge code space (e.g. 232) compared to frequency space

interferences is not coded

forward error correction and encryption can be easily integrated


Codes in CDMAWhat is a good code for CDMA?

A code for one user should have a good autocorrelation and should be orthogonal to other codesIf inner product of two vectors is zero then two vectors are said to be Orthogonal

(2,5,0) × (0,0,17) = 0 + 0 + 0 = 0 It has a similar meaning in code space alsoIf the inner product of a code with itself is very large and drops when shifted by one chip. It stays at that low value until the code matches itself again perfectlyExample Barker code

+ - + + - + + + - - -+ + - + + - + + + - -+ - - + - - + + - + +

Original

Shifted once

sopThus Barker code has a good autocorrelation and this property helps

receiver to reconstruct the original data precisely


Sequence GenerationWalsh table is used to generate the sequence.It is a two-dimensional having equal number of rows and columnsEach row is a sequence of chips

W2=4W1, with last w1 complemented

W4=4W2, with last W2 complemented


Properties of Orthogonal SequencesIf –1 is multiplied with the sequence, every element is complemented

C.-1 = [+1, +1, -1, -1].-1 = [-1, -1, +1, +1]

Inner product of same sequence = N (N is the no of chips in the sequence)

C.C = [+1, +1, -1, -1]. [+1, +1, -1, -1] = 1+1+1+1 = 4

Inner product of diff sequences = 0B.C = [+1, -1, +1, -1].[+1, +1, -1, -1] = 1-1-1+1=0

Inner product of a sequence with its complement = -NC.-C = [+1, +1, -1, -1]. [-1, -1, +1, +1]=-1-1-1-1=-4


Local Area Networks: EthernetDesigned for a limited geographical area such as a building or campusMost dominant technology used today for LAN is EthernetThe original ethernet was designed at Xerox’s Palo Alto Research Center (PARC)The LLC sub-layer is not used often todayLANs differ in their MAC sub-layer and physical layer,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,


Three Generations of Ethernet


Traditional EthernetDesigned to operate at 10MbpsAccess method used is 1-persistent CSMA/CDFrame defined by 802.3, contains seven fields

Preamble: 7bytes of alternating 0s and 1s that alert the receiving system to the coming frame and enable it to synchronizeStart Frame Delimiter: 1 byte (10101011) signals beginning of frame. Last chance for synchronization. Destination and Source address: 6 bytes, contains the physical address of source and destinations


Traditional EthernetLength/Type: if val ≤ 1518 then it represents length of data

if val ≥ 1536 -> type of PDU that is encapsulatedData: carries data encapsulated from the upper layer protocols. Min of 46 bytes and a max of 1500 bytesCRC: it uses CRC –32 for error detectionFrame length

is restricted to 64 bytes min, 1518 bytes max for correct operation of CSMA/CDA collision must be heard before the total frame is sent otherwise MAC will discard the frame and start sending the next frameStandard defines the smallest length for 10 Mbps Ethernet as 512 bits or 64 bytes without preamble and SFDThese 64 bytes contains 18 bits of header and 46 bytes of data. Max frame length is 1518, thus data can be upto 1518-18=1500 bytes


Physical AddressThe 6-byte (48-bit) physical address is embedded into NIC.The address is normally represented in two-digit hexadecimal notationThey are written hyphenated by octets

e.g. 12:34:56:78:9A:BC

This gives a theoretical 281,474,976,710,656 addresses.This is more than 56,000 MAC addresses for each person on the planet! In practice, the address assignment policy will inevitably lead to some wastage - but even so, there are likely to be enough addresses for ever.computer, and a large number of household/office devices (heating systems, ovens, washing machines, fridges, dispensers, security alarms, video recorders, ...) which could in the future be internet enabled!!!


Physical layer• PLS Encodes using Manchester scheme

• For a datarate of 10 Mbps, a BW of 20 Mbaud is needed?

• AUI defines the interface between PLS and MAU

• MAU is medium dependent, It creates appropriate signal for a particular medium

• It transmits, receives and detects collision


Categories of traditional Ethernet


10Base5 10Base2

• called Thicknet, One segment < = 500 mts and uses RG 8 cable

• Access method is CSMA/CD

• AUI is 15 wire cable and DB-15 connector

• AUI length < 50 mts

• Uses BUS topology

• Called Thinnet, uses CSMA/CD

• RG-58 cable, span length <= 185 mts

• Bus topology LAN, uses BNC-T connectors


10Base-T, Twisted Pair Ethernet

• Uses star topology LAN, with a hub to connect multiple devices

• max length 100 mts

• UTP cable and RJ-45 connectors

• Star topology LAN with a hub

• Uses two pairs of fiber optic cables

10Base-FL (Fiber Link)


Sharing bandwidth, Changes in standard• If more than one station starts sending then the capacity is shared

• If two stations have a lot of frames to send via a 10Mbps link, they alternate the usage of link

• On average, each station sends 5Mbps, i.e. the capacity for each station is BW/n


Bridged Ethernet

• A bridge divides a LAN into two or more segments

• Now the full capacity may be used independently by each segment, i.e. theoretically the capacity of each segment is 10/6 Mbps instead of 10/12Mbps

• Thus Bridged LAN raises the bandwidth of a LAN


Collision domains• Another advantage of using a bridge is the separation of collision domain, but broadcast domain remains the same

• smaller collision domains results in better performance of the LAN


Switched Ethernet• Like a bridge, Switch divides the LAN into N number of segments where each segment contains only one station. i.e. equivalent to a bridged LAN with one computer per segment

• i.e. Collision domain is divided into N domains. Communication among any two station will not disturb others

• The bandwidth is shared by the node and switch

Port 1Port 2

Port 3SWITCH


Full-duplex switched Ethernet• In full-duplex switched Ethernet, each station is connected to the switch via two separate channels

•The full duplex mode increases the capacity of each domain

• No need for CSMA/CD as each link is a point to point dedicated path between switch and node

• Traditional Ethernet was designed as a connection less protocolwithout flow and error control

• A new sub-layer is added called MAC controlbetween LLC and MAC for full-duplex switched Ethernet to provide flow and error control


Fast Ethernet (100 Mbps)IEEE 802.3uAccess method, frame format, min and max frame length and addressing are same as Traditional Ethernet (10 Mbps)

There is no need for CSMA/CD, but it is there to be compatible with traditional Ethernet

Auto negotiation: A new feature added to Fast Ethernet that allows a station or a hub to

Allow incompatible devices to connect one another. i.e. back word compatible with traditional Ethernet to work in 10MbpsAllow one device to have multiple capabilities.Allow a station to check hub’s capabilities


Fast Ethernet physical layerFour sub-layers:

1. Reconciliation(RS)It replaces the PLS sub-layer in 10Mbps Ethernet

It is specifically used to pass data in a 4-bit format (nibble)

2. Medium Independent Interface (MII)

AUI is replaced with MII

This is improved to be used with both 10 and 100 Mbps data rate i.e. backward compatible


Fast Ethernet Contd.It provides a parallel data path (nibble) between the PHY and RS

Management functions are added

3. Physical layer entity(PHY)This is the transceiver in fast EthernetIt is responsible for encoding and decodingThe transceiver can be external or internalAn external transceiver is installed close to medium and is connected via an MII cable

4. Medium dependent interface (MDI)Used to connect the transceiver to the medium and is implementation specificIs a hardware and implementation specific


Fast Ethernet Implementations


100Base-TX implementation

• Uses 2 pairs of cat-5/STP twisted pair with Star topology

• Transceiver is responsible for transmitting, receiving, detecting collisions, and encoding and decoding of data

Encoding and decoding•To maintain synchronization, the encoder first performs block encoding.

• The 4 parallel bits received from NIC are encoded into 5 serial bits using 4B/5B (requires a bw of 125MHz i.e 125Mbps)

• The data is then encoded in to MLT-3


100Base-FX Implementation

• uses two pairs of fiber-optic cables in a physical star topology

• The transceiver is responsible for transmitting, sending, detecting the collision, and encoding/decoding.

Encoding and Decoding•To maintain synchronization, the encoder performs block encoding

• The four parallel bits received from NIC is encoded into 5 serial bits using 4B/5B

• Then the data is encoded using NRZ-I


100Base-T4 Implementation

Uses Cat-3, 4-pair UTP

uses 8B/6T for encoding and decoding, which reduces the bW from 100 to 75Mbaud.

100Base-T4 is designed to operate on 25-Mbaud BW

Two pairs are designed for unidirectional transmission, other two for bi-directional transmission. Unidirectional pairs are kept free to carry collision signals.


Gigabit EthernetIEEE 802.3zUses full-duplex mode without CSMA/CDLack of collision implies that the max length of the cable is determined by the signal attenuation in the cable, not by the collision detection processGMII specifies a parallel data path (8 bits at a time) between RS sub-and transceiver Implementation

Gigabit Ethernet can be categorized as either a two wire or a four wire implementation.The two wire implementations use either fiber-optic cable (1000Base-SX, shortwave or 1000Base-LX, long wave) or STP (1000Base-CX)


Gigabit EthernetThe four wire version uses category-5 twisted pair cable (1000Base-T)

Two wire implementations use NRZ encoding after 8B/10B block encodingOne wire is used for sending another for receiving (fiber or STP)The four wire implementation uses 4D-PAM5 to reduce the bandwidthAll four wires used for input and output and carries 250Mbps each


Wireless LANSDefined by IEEE 802.11 & Bluetooth

The standard 802.11 defines two kinds of services

Basic Service Set(BSS): the basic building block of WLan

Extended Service Set(ESS): communicating among BSSs

Basic Service Set

Is made of stationary or mobile wireless stations and an optional central base station known as Access Point (AP)


Service SetsThe BSS without AP is a stand alone network and can’t send data to other BSSs

It is called an Ad hoc Network or Infrastructure Less Network

An BSS with an AP is called Infrastructure based network

Extended Service Sets

Is made up of two or moreBSSs with Aps

Communication between two stations in two different BSSsusually occurs via two Aps

APs are connected via distribution System


Station typesThree types of stations are defined based on their mobility

No-transitionis either stationary or moving only inside a BSS

BSS-transitioncan move from one BSS to another, but the movement is confined to one ESS

ESS-transitioncan move from one ESS to another. But standard does not guarantee the continuity in communication


Protocol ArchitectureIt covers MAC and physical layerThe basic tasks of MAC is medium access, fragmentation, and encryption.The MAC management supports the association and re-association of a station to an access point and roaming between APs, it also controls authentication mechanisms, synchronization with AP etc.Physical layer provides carrier sense signal, modulation encoding/decoding of signals


Physical Layer

• FHSS: Frequency hopping spread spectrum for signal generation in a 2.4 GHz ISM band in 79 sub-bands, each with a BW of 1MHz

• 802.11 supports three different physical layers

• one layer based on infra-red and two layers based on radio transmission (in ISM band at 2.4 GHz)

• A pseudorandom number generator selects the hopping sequence and the sender and receiver agree on the sequence of the allocated bands.

• The amount of time spent at each sub-band is called dwell time(400ms)

• Modulation is either a 2-level GFSK at 1Mbaud/s or 4-level GFSK at 2bit/baud resulting in the data rate of 1 or 2 Mbps

Figure from chap 6, 4ed


Direct Sequenced Spread Spectrum(DSSS)• Each bit sent by the sender is replaced by a sequence of bits called a chip code (CDMA). It uses 11 bit Barker sequence(10110111000). Uses entire band of 2.4 GHz

• The data rate for sending chip codes is 11 times the data rate of the original bit stream

• Modulation is Differential Binary PSK at 1Mbaud/s or DQPSK for 2Mbps

Infrared• uses Infrared light in the range of 850 to 950 nm

• max range is 10m if no sunlight or heat sources interfere.

• Today no products are available that offer this type of communication


IEEE 802.11a OFDMOrthogonal frequency division multiplexing in 5GHz ISM band52 sub-bands, 48 used for data and 4 for control informationUses PSK and QAMDatarate is 18Mbps, and 54Mbps

IEEE 802.11b HR-DSSSHigh-rate DSSS in 5GHz ISM bandUses Complementary Code Keying, which encodes 4 or 8 bits into one CCK symbolUses 2.4 MHz ISM bandDefines 1, 2, 5.5 and 11 MbpsIEEE 802.11g uses OFDM with 2.4 GHz ISM band and achieves 54Mbps data rate


MAC sub-layers in IEEE 802.11Defines two MACs and is called

as Distributed Foundation Wireless Medium Access Control (DFWMAC)

Distributed coordination function(DCF)

The CSMA/CD access method of ethernet fails in wireless medium

so it uses the basic method CSMA/CA and an optional method avoiding hidden terminal problem

Point coordination function (PCF)

uses contention free polling method by AP for time bounded service


Motivation for CSMA/CAWLANs cannot implement CSMA/CD because

It requires that the station must be able to send data and receive collision at the same time. (i.e. two paths and increased BW!)Collision may not be detected due to hidden terminal problemSignal fading could prevent a station at one end from hearing a collision from other end called near/far terminal problem

Thus DCF implements CSMA/CA


Hidden terminalsA sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails)collision at B, A cannot receive the collision (CD fails)A is “hidden” for C

Hidden and Exposed terminals

BA CExposed terminalsB sends to A, C wants to send to another terminal (not A or B)C has to wait, CS signals a medium in usebut A is outside the radio range of C, therefore waiting is not

necessaryC is “exposed” to B


Terminals A and B send, C receivessignal strength decreases proportional to the square of the distancethe signal of terminal B therefore drowns out A’s signalC cannot receive A

Near and Far terminals

A B C

Also severe problem for CDMA networks as all signals arrive with more or less same strength - precise power control needed!


Inter-frame spacing times802.11 MAC uses three different parameters for waiting time that define the priorities of medium access

Short inter-frame spacing(SIFS): shortest waiting time and hence highest priority is defined for control messages like ACK or polling responses, for DSSS SIFS is 10µs and for FHSS it is 28µsPCF inter-frame spacing(PIFS): A medium priority waiting time used for a time-bounded service like AP polling. Defined as SIFS plus one slot timeDCF inter-frame spacing(DIFS): longest waiting time and has the lowest priority for medium access. Used for data service within a contention period Defined as SIFS plus two slot times Slot time is 50µs for FHSS and 20µs for DSSS


Basic DFWMAC-DCF without RTS/CTS5 stations competes for sending at arrow marksAt first sta3 accesses the medium at t1Sta1, sta2, sta5 waits till the carrier is free (t2)then waits for DIFS then start there back-off timer within the contention window

Sta2 gets access as it has smallest back-off time at t4

sta1, sta5 stops their back-off timer and stores their residual back-off timesta4 wants to send between t4 and t5, all three starts their timer at t6.

Accidentally timers of sta4 and sta5 finishes same time(t7). Thus there is a collision and sta5 stops its timer.

finally sta5 gets the medium at t10

sta1

sta1sta2

sta3

sta4

sta5

t0 t1 t2 t3 t6t4 t5 t7 t8 t9 t10


Basic DFWMAC-DCF Unicastingstation has to wait for DIFS before sending data

receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC)

no other station can transmit because DIFS is greater than SIFS, hence no chance of collision

If no ACK is received sender automatically retransmits the frame

but the sender has to wait again and compete for the access right to retransmit

contention


Process CSMA with CTS and RTS

Station now waits for a period of time called the Distributed Inter-Frame Space; then sends a control frame called RTS (request to send)

Before sending a frame source senses the mediumChannel uses a persistent strategy with back-off until the

channel is idle

Source sends data after waiting an amount of time equal to SIFS

Destination waits for SIFS before sending ACK

After receiving the RTS and waiting a short period of time called short inter-frame space, the destination station sends a control frame called CTS(clear to Send)


CSM

A/C

A fl

owch

art


Process Collision Avoidance

If two or more stations try to send RTS at the same time and there is a collision, then sender tries again if it does not receive CTS

As the wireless medium is noisy, It is more efficient to replace a large frame with some smaller frames.

When station sends RTS frame, it includes the duration of the time that it needs to occupy the channelThe stations that are affected by this transmission create a timer called NAV.Each other station first checks NAV before sensing the channel


Hidden node provision for contention free access

Sender issues a RTS after DIFS (if medium was not busy)RTS contains duration of transmission, every node listening to it sets NAVReceiver answers with CTS which includes another duration and all listening nodes readjust their NAVThis method reserves the medium for one sender thus called a virtual reservation schemeSender sends data and receives ACK after SIFS each.When NAV expires the standard cycle starts againUsing RTS/CTS is an overhead. Usually it is essential for large frames and not needed in case of short frames


Fragmentation, DFWMAC-DCFWireless LAN have much more bit error rate than any other guided media.

Wireless LAN have much more bit error rate than any other guided medium for same frame length.Standard specifies a fragmentation mode to reduce bit error rateHere frag1 also contains a duration which is used to set NAV in case of mobile nodes or for a different set of nodes, this mechanism further reduces the collision.If frag2 is not the last frame then it should contain another duration. It is not required for last fragment.


DFWMAC – PCF with pollingDCF could not cant guarantee a maximum access delay or minimum transmission BWTo provide a time bound service, the standard specifies a point coordination function (PCF) on top of standard DCF mechanismsUsing PCF requires a access point that controls medium access and polls one node at a time.Ad-hoc networks cant use this functionThe point coordinator in access point splits the access time into super frame periods.A super frame comprises a contention-free period and a contention period.It is because due to the priority of PCF over DCF, stations thatonly use DCF may not gain access to the medium.To prevent this the super frame other wise called as repetition interval has been designed to cover both traffic


Contention free access using PollingTheoretical contention period starts at t0, but as another station still transmitting thus start of super frame postponed till t1

The point coordinator starts polling sta1 by sending D1 after PIFS and it responds with U1 after another SIFSIf stai has nothing to send then it will not respond so that after another PIFS the coordinator polls nextstai+1

Finally, the coordinator issues an end marker called CFend so that the contention period starts. After which the cycle starts again with next super frame

Using PCF automatically sets NAV


MAC sub-layer Frame format

Field Frame Control sub-fields Explanation

VersionVersion The current version is 0.The current version is 0.TypeType Type of information: management (00), control (01), or data (10)Type of information: management (00), control (01), or data (10)..SubtypeSubtype Defines the subtype of each typeDefines the subtype of each typeTo DSTo DS Defined later.Defined later.From DSFrom DS Defined later.Defined later.

More dataMore data When set to 1, means station has more data to send.When set to 1, means station has more data to send.WEPWEP Wired equivalent privacy. When set to 1, means encryption implemWired equivalent privacy. When set to 1, means encryption implemented. ented. RsvdRsvd Reserved.Reserved.

More flagMore flag When set to 1, means more fragments.When set to 1, means more fragments.RetryRetry When set to 1, means retransmitted frame.When set to 1, means retransmitted frame.PwrPwr mgtmgt When set to 1, means station is in power management mode.When set to 1, means station is in power management mode.


Other fieldsD: This field defines the duration of the transmission used to set NAVAddresses: Four address fields, each 6byte longSequence Control: Defines the sequence# of the frame used for flow controlFrame body: Contains information based on the type and sub-type field in FCFCS: 4byte CRC-32 error detection sequenceFrame types: standard defines three types of frames

Management frames: Are used for the initial communication between station and access pointsControl frames: Used for accessing the channel and ACK framesData frames: used for carrying data and control information


Control Frame format Sub-type Meaning

1011 Request to send (RTS)Request to send (RTS)

1100 Clear to send (CTS)Clear to send (CTS)

1101 Acknowledgment (ACK)Acknowledgment (ACK)

For control frame type field is 01

Addressing Mechanism (FC)ToToDSDS

FromFromDSDS

AddressAddress11

AddressAddress22

AddressAddress33

AddressAddress44

DestinationDestinationstationstation

SourceSourcestationstationSendingSendingAPAPSourceSourcestationstationSendingSendingAPAP


N/AN/ABSS IDBSS ID

SourceSourcestationstation

ReceivingReceivingAPAP


N/AN/A

N/AN/A


SourceSourcestationstation

ReceivingReceivingAPAP

00

11

00

11

00

00

11

11

Add 1: address of next device

Add 2 : the address of previous device

Add 3 : the address of Final destination if not defined by Add 1

Add 4 : the address of original source if not same as Add 2.


Addressing mechanism: case 1 Case 2

To DS=0, from DS=0

• the frame is not going and is not coming from a distribution system.

• i.e the frame is going from one station in a BSS to another system without passing through distribution system

To DS=0, from DS=1

• the frame is coming from a distribution system.

• i.e the frame is coming from an AP and going to a station

• The ACK is sent to the AP


Addressing mechanism: case 3 case 4

To DS=1, from DS=0

• the frame is going from a station to an AP

• The ACK is sent to the original station

• Address 3 contains the final destination in another BSS

To DS=1, from DS=1

• The frame is going from one AP to another AP via wireless medium

• As in wire medium ethernet frame has to be used


BluetoothIs a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so onA Bluetooth LAN can be connected to Internet if one of the gadgets has this capabilityIt can not be largeThis technology can be used to connect peripheral devices to computer, monitoring devices can be connected to sensors etc.Originally developed by Ericsson and named after Harald Blaatand (king of Denmark, 940-981). Later in 1998 five companies (Ericsson, Intel, IBM, Nokia, Toshiba) founded Bluetooth consortium to develop a low cost, single chip, radio based wireless networkToday the Bluetooth technology is the implementation of a protocol defined by 802.15 and is called WPAN that is operable in a small area with small number of devices


BluetoothA Bluetooth network is called a piconet and needs no infrastructure, this type of WPANS may be used in various scenarios

Connection of peripheral devices to a desktopSupport of ad-hoc networking

e.g. teacher distributing data to students PDAs inside the class

Bridging of networksUsing piconets, a mobile phone can be connected to a PDA or laptop. The mobile phone can now act as a bridge between local piconet and GSM

Bluetooth is designed to provide local wireless access at very low cost with a limited BW and without any extra infrastructure


Can have up-to eight stations, one of which is called the Master, rest are slaves

Master determines hopping pattern

Communication may be on-to-one, or one-to-many

Bluetooth ArchitectureOperates on 79 channels in 2.4GHz band with 1MHz carrier spacingEach device performs frequency hopping with 1600 hops/sec in a pseudo random fashionA piconet is a collection of Bluetooth devices which are synchronized to the same hopping sequence with following properties

An additional eighth slave may be in parked state, i.e. cant participate actively in piconet

The limit is eight because the address field contains 3 bits and any parked device can be active if one of the seven slaves goes to parked state


A master of one piconet can act as a slave in other piconet but one cant be master in two piconets

Bluetooth ArchitectureA group of piconets is combined to form a scatternet and can accommodate more devices.Different piconets in a scatternet use different hopping sequencesOne device may participate in more than one piconet one at a time

communication between different piconets takes place by devices jumping back and forth between these nets

however scatternets are not yet supported by all devices


Bluetooth layers (Protocol Stack)Radio Layer

Specifications of air interface and equivalent to physical layer of internet modelBand: 2.4GHz ISM band divided into 79 channels of 1MHz eachFHSS: 1600 hops/sec, i.e. one frequency is used for 625 µsModulation: uses Gaussian FSK and are available in three classes

Power class 1: max power of 100mW and min power of 1mW, 100m range without obstaclesPower class 2: max power of 2.5mW and min power of 0.25mW, 10m range without obstaclesPower class 3: max power of 1mW


Base Band LayerUses Time Division Duplex-TDMAIf the piconet has only one slave, then the master uses even numbered slots and slave uses odd numbered slotsTDD-TDMA allows the communication in half-duplex mode. i.e. in slot 0 master sends and slave receives and in slot 1 slave sends master receivesIf it has more slaves then master uses even numbered slots where as odd numbered slots are used by slaves.

if master sends to slave 1 at slot 0 then slave 1 answers at slot 1 then master sends to slave 2 in slot 2 slave 2 answers atslot 3, and the cycle continues


Base Band Layer Frame format A frame can be of three types

One slot frame: 259µs is used for hopping and control mechanism thus the frame can last upto 366µs or of 366 bitsThree slot frame: length of frame = 3*625-259=1616 bitsFive slot frame: length of frame = 2866 bits

Access code: contains synchronization bits and the identifier of masterHeader: is a repeated 18 bit pattern, each pattern contains

Address: represents active slave address. can define seven secondaries, 0 is used for broadcasting from master to slaves


Base Band Layer Frame format contd.

Type: type of data coming from upper layerF: used for flow controlA: used for ACK, uses stop and wait ARQ without retransmissionS: sequence number of frameHEC: header error correction uses checksumThe header has three identical 18-bit sections. Receiver compares these three sections bit by bit. If all three corresponding bits are same than the bit is accepted otherwise majority is taken.This is a form of Forward error correctionPayload: it contains the data or control information from upper layers.


L2CAP data packet formatThe logical link control and adaptation protocol is used for data exchange on an asynchronous connection link

Length defines the size of the data, in bytesCID defines a unique identifier for the virtual channel createdL2CAP duties

Multiplexing: supports many application layer protocolsSegmentation and reassembly: divides large packets of upper layer (65535) to smaller sizeQoS: uses best effort serviceGroup management: allows devices to create logical addressing among themselves to form a multicast group


Lecture IIILecture III

•• Wide Area NetworksWide Area Networks•• Virtual CircuitVirtual Circuit•• Frame RelayFrame Relay•• ATMATM


Wide Area NetworksWANs provide long distance transmission over large geographical areasIt utilizes public, leased or private communication equipments to span over countries or continentsIt uses one of the packet switching methods called virtual circuit switching. Frame relay is a link layer protocol used in WAN technology, which provides relatively high speed transmissionATM (Asynchronous Transfer Mode) is another protocol for WAN technology that works as a super highway of communication.


Virtual circuit switching in WANGlobal Addressing

A source or destination need to have a global address, which should be unique in the scope of WAN

Virtual Circuit IdentifierIs a small number that only has switch scope and is actually used for data transferi.e it is used by a frame between two switchWhen a frame arrives a switch, it has one VCI; when it leaves, it has another


VCI PhasesTo communicate, the source and destination need to go through three phases. i.e. setup, data transfer, & tear down

Data Transfer phaseAll switches have table called a switching table with at least 4 columnsWhen a frame arrives it checks its VCI and incoming port no.


Data transfer PhaseIt finds corresponding out going port and changes the VCI as in the table before sending outThe data transfer phase is active until the source sends all its frames


Setup PhaseSetup phase

Source and destination use their global addresses to help switches make table entries for connection Two approaches: PVC, SVC

Permanent Virtual CircuitFor each destination one circuit is dedicated (always present) whether in use or notThe corresponding table entry is recorded for all switches by the administrator An out going VCI is given to the source, and an incoming VCI is given to the destinationIf there is a need for duplex communication then two virtual circuits are established


Switched Virtual CircuitSVC creates a temporary connection, that exists only when data are transmitted between source and destinationSetup request

Source sends the set up frame to the switch, switch finds out going port from its routing table and creates entry in the switching table for the incoming virtual circuit

AcknowledgementThe last field of switching table entry is made from ACK

setup ACK


Frame RelayIs a virtual circuit switching WAN used between 1980 – 1990It operates at a higher speed (1.544Mbps, now 44.376Mbps)Operates in physical and datalink layerAllows bursty data with a frame size of 9000 bytesIt is less expensive than other traditional WANsIt has error detection mechanism at datalink layer. No flow or error control schemes available.It is so designed to provide fast transmission capability for more reliable media and for those protocols that have flow and error control at the higher layers


Frame Relay ArchitectureFrame relay network can be a part of InternetFrame relay uses both PVC and SVC. The virtual circuit is identified by a number called Data Link Control Identifier(DLCI is same as VCI)

Frame Format


Frame format contd.

DLCI: total 10 bit used as connection identifierCommand/response (C/R): used by upper layers to identify the frame as either a command or responseExtended Address(EA): if 0 then another address byte to follow; if 1 then current byte is the final byte


Frame format contd.FECN: can be set by any switch to indicate that the traffic is congested in the direction of flowBECN: is set to indicate a congestion in the opposite directionDiscard eligibility(DE): this bit indicates the priority level of the frame according to which the frame may be discarded in case of congestionFrame Relay Assembler/Dissembler (FRAD)

This device is used to to handle frames coming from other protocols


Other featuresVoice Over Frame Relay (VOFR)

Used to send voice by digitizing the voice using PCM and then sent in data frames

Local Management Information (LMI)Is a protocol added to frame relay to provide more management features, It providesA keep alive mechanism to check if data are flowingA multicast mechanism to allow a local end system to send frames to more than one remote end systemA mechanism to allow an end system to check the status of a switch (if switch is congested)

Congestion Control and Quality of Services are also provided by frame relay


ATM NetworksIs a cell relay protocol designed by ATM forum and adopted by ITU-T

A cell is a basic unit of data exchange. It is a small, fixed sized block of informationFrames of different sizes and formats reach the cell network, these frames are split into some cells of fixed lengthNow cells are multiplexed with other cells and routed

frame of line 1 and lne 2 are splited into 3 cells each then interleaved so that none suffers long delay

suitable for voice or real-time transmissions


ATM Networks ArchitectureATM uses asynchronous time division multiplexing

The user access devices, called the end points are connected to the switches through a user-to-network Interface(UNI)Switches are connected with each other through Network to Network Interface (NNI)


ATM Architecture, Virtual ConnectionConnection between two end points are accomplished through transmission paths (TPs). i.e. TPs are physical connection between endpoints via a set of switches and linksA TP is divided into several Virtual Paths (VPs). A VP provides a connection or set of connections between two switchesAll VP contains more than one Virtual circuits (VCs)


Virtual Connection

In a UNI, the VPI is represented using 8bits and in NNI it represented using 12 bits.VCI is represented using 16 bits in both cases

A virtual connections are identified by a pair of parameters called The Virtual path identifier (VPI) and virtual circuit identifier (VCI)VPI defines the VP and VCI defines a particular VC inside VP


Miscellaneous Cell structure

A cell is of 53 bytes with 5 byte header and 48 byte payload

SwitchingUses both PVC and SVCA switch routes the cell using both VPIs and VCIs


ATM LayersPhysical layer

Original design was based on SONET as the physical layer carrierBut it also can use other carriers

ATM LayerProvides routing, traffic management, switching, and multiplexing servicesIt accepts 48 byte segments from AAL and adds 5byte header to form a cell of 53 bytes


ATM LayerIt uses two different header formats for UNI and NNI

GFC: 4bit field provides flow control at UNI levelVPI: virtual path identifier (8bit for UNI, 12 bit for NNIVCI: 16 bit for both UNI and NNIPT: 3 bit field defines user data or management infoCLP: 1 bit field for congestion controlHEC: 8 bit CRC for header part only


Application Adaptation LayerIt is designed to accept any type of payload like internet data or multimedia loadAAL defines a sub-layer called Segementation and reassembly sub-layer to segment incoming data in to 48byte segments and at the destination these segments are reassembledThe converge sub-layer is defined to provide integrity of dataATM defines 4 versions of AAL: AAL1, AAL2, AAL3/4, AAL5AAL1 is used for streaming audio and video communication and AAL5 for data communication


AAL1Supports applications that transfer information at constant bit rates like video or voiceCS sub-layer divides bit stream into 47 byte segments and passes them to SAR sub-layerSAR adds 1 byte header to each segment. The header contains

Sequence number(SN): out of 4 bits, 3bits are used to define sequence no to order the bits

Sequence number protection(SNP): second 4 bit protects the first field. First 3 bit protects sequence # and last bit is for parity for all 7 bits


AAL5It assumes that all cells belonging to a message travel sequentiallyCS sub-layer adds a trailer with following fields

User-to-user(UU): used by end usersCommon part identifier (CPI): defines how the the subsequent fields are to be interpreted Length(L): 2-byte

L field indicates the length of original data

CRC: The last 4 bytes is for error control in the entire data unit


END of Module II

Date post:	12-Nov-2014
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