Presentation4-Making Connections Www

8/3/2019 Presentation4-Making Connections Www

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Making ConnectionsMaking Connections

Prof. Aiman Hanna Prof. Aiman Hanna

Department of Computer ScienceDepartment of Computer Science

Concordia UniversityConcordia University

Montreal, CanadaMontreal, Canada


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CCommunication Carriers &ommunication Carriers &

DevicesDevicesThe Telephone NetworkThe Telephone Network Connects 100s of millions of usersConnects 100s of millions of users Calls are routed first to the local office (local exchange or central office)Calls are routed first to the local office (local exchange or central office)

Calls within the same area code can be made through direct connectionsCalls within the same area code can be made through direct connections

Other calls are routed depending on the destinationOther calls are routed depending on the destination

Private Branch Exchange (PBX) computer is used to route telephone callsPrivate Branch Exchange (PBX) computer is used to route telephone calls

within a company or organizationwithin a company or organization

Figure 4.2 Telephone Network


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DevicesDevicesC

ellular PhonesC

ellular Phones A geographic region is divided intoA geographic region is divided into cellscells each with aeach with a

base station. A cellular phone is a twobase station. A cellular phone is a two--way radioway radiocapable of communicating with the base stationcapable of communicating with the base station

The cell phone may be within more than one boundary,The cell phone may be within more than one boundary,however it communicates with the base station fromhowever it communicates with the base station fromwhere the signal is strongerwhere the signal is stronger

Base stations communicate with a MTSO (MobileBase stations communicate with a MTSO (MobileTelephone Switching Office), which connects to theTelephone Switching Office), which connects to theregular telephone networkregular telephone network

Receiving a cell call is more complexReceiving a cell call is more complex


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DevicesDevicesCellular PhonesCellular Phones

Figure 4.3 Cellular Grid


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DevicesDevicesCellular PhonesCellular Phones

Figure 4.4 Cellular Phone Communication


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DevicesDevicesFacsimile (Fax) MachinesFacsimile (Fax) Machines A paper sheet is divided into a dot matrix; each dotA paper sheet is divided into a dot matrix; each dot (Pixel)(Pixel) is sois so

tiny (200 dots per inch, 40,000 dots per square inch)tiny (200 dots per inch, 40,000 dots per square inch)

Each dot is a bit: 1 if dot is white, 0 if blackEach dot is a bit: 1 if dot is white, 0 if black

8.5x11 inches paper would produce 3,740,000 dots, and it takes8.5x11 inches paper would produce 3,740,000 dots, and it takes

2 minutes (approx.) at the rate of 33.6 bps2 minutes (approx.) at the rate of 33.6 bps

Fax machines useFax machines useData CompressionData Compression schemes; instead ofschemes; instead of

sending dot by dot, the fax groups the dots and defines binarysending dot by dot, the fax groups the dots and defines binary

representation of them using fewer bitsrepresentation of them using fewer bits


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TTransmissionransmission ModesModes Defines the way in which a bit group travels fromDefines the way in which a bit group travels from

device to anotherdevice to another

Also defines whether bits travel in both directionsAlso defines whether bits travel in both directions

simultaneously or must take turnssimultaneously or must take turns

Different transmission modes exist:Different transmission modes exist:

Serial & ParallelSerial & Parallel Asynchronous, Synchronous & IsochronousAsynchronous, Synchronous & Isochronous

Simplex, HalfSimplex, Half--Duplex & FullDuplex & Full--DuplexDuplex


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TTransmissionransmission ModesModes (continue...)(continue...)Serial & Parallel TransmissionSerial & Parallel Transmission

Parallel transmission sends bits of a byteParallel transmission sends bits of a bytesimultaneously on separate wires; used between PCsimultaneously on separate wires; used between PCand printerand printer

Only recommended for short distances due to syncOnly recommended for short distances due to syncproblemsproblems

Serial transmission uses one wire, and can be used forSerial transmission uses one wire, and can be used forlong distance communication; cheaper, more reliablelong distance communication; cheaper, more reliable

but slowerbut slower

Figure 4.7 Parallel & Serial transmission


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TTransmissionransmission ModesModes (continue...)(continue...)Asynchronous, Synchronous & Isochronous TransmissionAsynchronous, Synchronous & Isochronous Transmission

These are ways to provide serial communicationThese are ways to provide serial communication

Asynchronous transmission:Asynchronous transmission:

Bits are divided into small groups, usually bytes, and sent independentlyBits are divided into small groups, usually bytes, and sent independently

The receiver never knows when the bits will arriveThe receiver never knows when the bits will arrive

For example, typing keyboard charactersFor example, typing keyboard characters

TypicalTypical bytebyte--oriented inputoriented input--outputoutput; that is data is transmitted one byte at; that is data is transmitted one byte at

a timea time

AAstart bitstart bitis needed to alert the receive that some data is coming;is needed to alert the receive that some data is coming;

otherwise the first few bit may get lost by the time the receiver detect andotherwise the first few bit may get lost by the time the receiver detect andreacts to data receptionreacts to data reception

Similarly, aSimilarly, astop bitstop bitis neededis needed


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Figure 4.8 Asynchronous CommunicationOverhead is 2/8 = 25%

Figure 4.9 Asynchronous Transmission of

ASCII Digits 321 using NRZ Coding


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TTransmissionransmission ModesModes (continue...)(continue...)Asynchronous, Synchronous & Isochronous TransmissionAsynchronous, Synchronous & Isochronous Transmission Synchronous transmission:Synchronous transmission:

Allows transmission of larger bit groupsAllows transmission of larger bit groups

Characters are grouped into aCharacters are grouped into a Data FrameData Frame (simply(simply FrameFrame) them be transmitted) them be transmittedas a wholeas a whole

A generic data frame has the following pieces:A generic data frame has the following pieces:

SYNSYN: unique bit pattern that alert the receiver of frame arrival: unique bit pattern that alert the receiver of frame arrival

Also used to ensure the receivers sampling rate and the consistency of the arrival rateAlso used to ensure the receivers sampling rate and the consistency of the arrival rate

The receiver can then synchronize itself to the rate at which bits arriveThe receiver can then synchronize itself to the rate at which bits arrive

ControlControl: these bits may include the following elements: these bits may include the following elements

Source addressSource address

Destination addressDestination address: Needed if frame needs to go through different nodes before reaching: Needed if frame needs to go through different nodes before reachingthe destinationthe destination

DataData: Actual number of data bytes: Actual number of data bytes

SequenceNumberSequenceNumber: Used to assemble frames at the destination in case they arrive out: Used to assemble frames at the destination in case they arrive outof orderof order

Frame TypeFrame Type: Distinguished by some protocols: Distinguished by some protocols

ErrorError: Error checking bits: Error checking bits

EndEnd: End: End--ofof--frame bitsframe bits


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TTransmissionransmission ModesModes (continue...)(continue...)Asynchronous, Synchronous & Isochronous TransmissionAsynchronous, Synchronous & Isochronous Transmission Synchronous transmission:Synchronous transmission:

Much faster and has small overhead, howeverMuch faster and has small overhead, however

Larger frames require higher buffering; they may also occupy the link forLarger frames require higher buffering; they may also occupy the link forlonger timelonger time

Figure 4.10 Synchronous Transmission Frame


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Isochronous transmission:Isochronous transmission:

With asynchronous & synchronous data do not necessarilyWith asynchronous & synchronous data do not necessarily

arrive at a fixed ratearrive at a fixed rate

Time between different synchronous frames may varyTime between different synchronous frames may vary

(asynchronous nature!)(asynchronous nature!)

Errors may force the frame to be reset, which affects theErrors may force the frame to be reset, which affects the

transfer rate furthertransfer rate further

For some applications, such as file transfer, that is fine sinceFor some applications, such as file transfer, that is fine sincecorrect information is more important than delayscorrect information is more important than delays

Isochronous transmission is used to ensure a fixedIsochronous transmission is used to ensure a fixed

transmission rate without gaps in betweentransmission rate without gaps in between


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TTransmissionransmission ModesModes (continue...)(continue...)Simplex, HalfSimplex, Half--Duplex & FullDuplex & Full--Duplex CommunicationDuplex Communication

Figure 4.11 Simplex, HalfSimplex, Half--Duplex & FullDuplex & Full--DuplexDuplex communication


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IInterface Standardnterface Standard Communication may not occur even if both parties are using the same mechanisms!Communication may not occur even if both parties are using the same mechanisms!

For example, if both send at the same time, no information may reach any of themFor example, if both send at the same time, no information may reach any of them ififone is not ready to listen then information is also lostone is not ready to listen then information is also lost

Hence, communication must be guided by protocolsHence, communication must be guided by protocols

Data Terminal EquipmentData Terminal Equipment ((DTEDTE), such as PCs, do not communicate directly; rather), such as PCs, do not communicate directly; ratherthey communicate tothey communicate to Data Communication EquipmentData Communication Equipment ((DCEDCE), such as a modem,), such as a modem,which connect to the networkwhich connect to the network

The connection between

DTE &

DCE is called

The connection between

DTE &

DCE is called

DTE

DTE--

DCE Interface

DCE Interface

Figure 4.12 DTE-DCE Interface


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IInterface Standardnterface Standard (continue...)(continue...)EIA

EIA

--232 Interface (RS

232 Interface (RS

--25S

erial25S

erialPort)Port)

RS232: 25RS232: 25--line cable withline cable with

2525--pin connector (DB25).pin connector (DB25).

Every line has a function;Every line has a function;

for example:for example:

Pin 1: protective groundPin 1: protective ground

Pin 2: Transmit date DTEPin 2: Transmit date DTE

DCEDCE

Pin 22: Ring Indicator;Pin 22: Ring Indicator;

indicates DCE is receivingindicates DCE is receiving

a ringing signal (whena ringing signal (when

modem receives a call)modem receives a call)

RS-232 Connector


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IInterface Standardnterface Standard (continue...)(continue...)EIAEIA--232 Interface (RS232 Interface (RS--232 Serial Port)232 Serial Port)

Figure 4.14 Sending & Receiving over RS-232


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IInterface Standardnterface Standard (continue...)(continue...)EIAEIA--232 Subset232 Subset Driven by economicsDriven by economics

and actual user needs,and actual user needs,

some vendors onlysome vendors only

implemented a part ofimplemented a part ofthe interface usingthe interface using

only 9 circuits insteadonly 9 circuits instead

of 25 (9of 25 (9--binbin

connectors)connectors)

RS-232 Subset 9-bin Connector


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IInterface Standardnterface Standard (continue...)(continue...)Null ModemNull Modem

Sometimes, it is needed to connect two computers directlySometimes, it is needed to connect two computers directly

A first attempt to establish connection is plug in the wire to both endsA first attempt to establish connection is plug in the wire to both ends

This however wont work; Why?This however wont work; Why?

Same circuit in each end is expected to perform the same functionality; forSame circuit in each end is expected to perform the same functionality; forexample send/send or receive/receiveexample send/send or receive/receive

One solution in such case is to use a null modemOne solution in such case is to use a null modem

The null modem can be simply a cableThe null modem can be simply a cable

Figure 4.15 Null Modem


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IInterface Standardnterface Standard (continue...)(continue...)X.21 InterfaceX.21 Interface

Figure 4.16 Sending & Receiving over an X.21 Connection


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IInterface Standardnterface Standard (continue...)(continue...)Universal Serial BusUniversal Serial Bus ((USB)USB) Not long ago, we had to deal with Serial ports, Parallel ports,Not long ago, we had to deal with Serial ports, Parallel ports,

Special connections for Game controllers, KeySpecial connections for Game controllers, Key--boards, Mice,boards, Mice,

etc.etc.

USB was the proper replacements to those many connectorsUSB was the proper replacements to those many connectors

Very flexible in connecting many different devicesVery flexible in connecting many different devices

Has 7Has 7--bit addressing schemes to reference the devices, whichbit addressing schemes to reference the devices, which

enables connections to 127 (excluding the DTE host itself)enables connections to 127 (excluding the DTE host itself)


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IInterface Standardnterface Standard (continue...)(continue...)U

niversalS

erial BusU

niversalS

erial Bus((USB)USB)

Figure 4.17 Connecting USB Devices


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IInterface Standardnterface Standard (continue...)(continue...)

Universal Serial BusUniversal Serial Bus ((USB)USB)

USB cable contains 4 wires:USB cable contains 4 wires:2 wires for data carrying2 wires for data carryingsignal in modified NRZ (0signal in modified NRZ (0

changing, 1 same)changing, 1 same) The other two wires provideThe other two wires provide

lowlow--amplitude power sourceamplitude power sourceto USB devicesto USB devices

USB 1.1 at 12 Mbps, USBUSB 1.1 at 12 Mbps, USB2.0 at 480 Mbps.2.0 at 480 Mbps.

Limited to 4.5 meters; ifLimited to 4.5 meters; iflonger, there is no guaranteelonger, there is no guaranteeof electrical signal integrityof electrical signal integrity

Figure 4.19 USB Cable & Plugs

Figure 4.18 USB Wires


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IInterface Standardnterface Standard (continue...)(continue...)U

niversalS

erial BusU

niversalS

erial Bus ((USB

)USB

) Operates on Master/Slave mode, where the host is the masterOperates on Master/Slave mode, where the host is the master

USBFrameUSBFrame: 1: 1--milisecond slice of time.milisecond slice of time.

During this 1During this 1--ms time frame,ms time frame,packetspackets are sent (packet is a groupare sent (packet is a groupof bits)of bits)

All devices are clock synchronized in respect to a frameAll devices are clock synchronized in respect to a frame

The synchronization isThe synchronization is notnotdone by a common clock; rather bydone by a common clock; rather bythe host sending a special packet at the beginning of each framethe host sending a special packet at the beginning of each frame

This special packet indicates that a new frame is beginningThis special packet indicates that a new frame is beginning


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IInterface Standardnterface Standard (continue...)(continue...)Universal Serial BusUniversal Serial Bus ((USB)USB) USB defines 4 different transmission types:USB defines 4 different transmission types:

Control TransferControl Transfer

Bulk TransferBulkTransfer

Interrupt TransferInterrupt Transfer

Isochronous TransferIsochronous Transfer

Control TransferControl Transfer::

USB devices areUSB devices are hot pluggablehot pluggable

Once plugged, the host queries the device to determine its type & bit rateOnce plugged, the host queries the device to determine its type & bit rate

The devices respondsThe devices responds the host assigns an address to that devicethe host assigns an address to that device

Once this is done, the device is connected and can receive commandsOnce this is done, the device is connected and can receive commands

from the host such as requesting their status or initiating data exchangefrom the host such as requesting their status or initiating data exchange


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IInterface Standardnterface Standard (continue...)(continue...)Universal Serial BusUniversal Serial Bus ((USB)USB) Interrupt TransferInterrupt Transfer::

The worldThe world interruptinterrupthere is not that proper!here is not that proper!

USB devices hold the information until the host asks for them, which isUSB devices hold the information until the host asks for them, which is

literallyliterally PollingPolling

The major advantage here is avoiding the complexity involved with theThe major advantage here is avoiding the complexity involved with the

interrupt system/protocolinterrupt system/protocol

For example, if the host sets its polling time to the keyboard at 50 framesFor example, if the host sets its polling time to the keyboard at 50 frames

(each 50 ms), then it can get up to 20 characters each second(each 50 ms), then it can get up to 20 characters each second


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IInterface Standardnterface Standard (continue...)(continue...)Universal Serial BusUniversal Serial Bus ((USB)USB) USB PacketsUSB Packets

Several exchange of packets could take place during a singleSeveral exchange of packets could take place during a single

frameframe

Packet types:Packet types: TokenToken,,DataData,,HandshakeHandshake All packets haveAll packets have SYNSYNandand PIDPID (packet ID)(packet ID)

SYNSYN is a bit pattern that forces the receiving device tois a bit pattern that forces the receiving device to

synchronize its clock with the sender and adjust theirsynchronize its clock with the sender and adjust their

receiving bit ratereceiving bit rate

PIDPID identifies the packet typeidentifies the packet type SOFSOF indicates the Start o Frameindicates the Start o Frame


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IInterface Standardnterface Standard (continue...)(continue...)Universal Serial BusUniversal Serial Bus ((USB)USB) USB PacketsUSB Packets

ININ&& OUTOUTpackets represent a request from the host to initiate datapackets represent a request from the host to initiate data

transfertransfer

AddressAddress is a 7is a 7--bit address that identifies the device to be usedbit address that identifies the device to be used

CRCCRC(Cyclic Redundancy Check) is used for error detection(Cyclic Redundancy Check) is used for error detection If errors occur, aIf errors occur, a NAKNAKis sent to the hostis sent to the host

Some devices may have more than one address; for example a gameSome devices may have more than one address; for example a game

controller with multiple buttons would have multiple addresses associatedcontroller with multiple buttons would have multiple addresses associated

with them. Thewith them. The endpointendpointis needed to identify the exact source oris needed to identify the exact source or

destination of the data within the device.destination of the data within the device.

For example, a game controller may have many buttons sending orFor example, a game controller may have many buttons sending orreceiving different information. Each of these buttons will be indicated byreceiving different information. Each of these buttons will be indicated by

an endpointan endpoint


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IInterface Standardnterface Standard (continue...)(continue...)FireWireFireWire FireWire (Apple), i.Link (Sony)FireWire (Apple), i.Link (Sony)

Share common characteristics with USBShare common characteristics with USB

Provide a speed of 400Mbps (USB provides 12Mbps, USB 2.0Provide a speed of 400Mbps (USB provides 12Mbps, USB 2.0

provides 480 Mbpsprovides 480 Mbps Can be used with many devices, but the main focus is onCan be used with many devices, but the main focus is on

multimedia devices, especially with digital video applicationmultimedia devices, especially with digital video application

Connects multiple devices usingConnects multiple devices usingDaisy ChainDaisy Chain, which means, which means

many devices can be connected in sequence and there is no needmany devices can be connected in sequence and there is no need

for a hubfor a hub

Devices have one or more FireWire port, so they can also act asDevices have one or more FireWire port, so they can also act as

regenerators/repeatersregenerators/repeaters


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IInterface Standardnterface Standard (continue...)(continue...)FireWireFireWire

Figure 4.21 Connecting FireWire Devices


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IInterface Standardnterface Standard (continue...)(continue...)F

ireWireF

ireWire Uses 6 wires (2 twisted pairsUses 6 wires (2 twisted pairs TPATPA && TPBTPB + 2 wires for power+ 2 wires for power

source)source)

UsesUses Data Strobe EncodingData Strobe Encoding

TPA uses some form of NRZ, where 1 is high, 0 is lowTPA uses some form of NRZ, where 1 is high, 0 is low

This is however errorThis is however error--prone due to misprone due to mis--synchronization with the sendersynchronization with the senderclockclock

The sender sends a strobe signal overTPB, which stays constantThe sender sends a strobe signal overTPB, which stays constantwhenever the data change from 1 to 0 and vise versawhenever the data change from 1 to 0 and vise versa

The receiver gets both TPA& TPB signals and by XORing them, it canThe receiver gets both TPA& TPB signals and by XORing them, it cancreate the exact sender clockcreate the exact sender clock

This is a bit like Manchester Encoding, with one great difference; theThis is a bit like Manchester Encoding, with one great difference; thebaud rate is the same as the bit rate, so there is no double BW utilizationbaud rate is the same as the bit rate, so there is no double BW utilization

The only cost here is one additional twisted pairThe only cost here is one additional twisted pair


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IInterface Standardnterface Standard (continue...)(continue...)FireWireFireWire

Figure 4.22 Data Strobe Encoding


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IInterface Standardnterface Standard (continue...)(continue...)Multiple

FireWire BusesMultipleF

ireWire Buses USB uses Master/Slave protocol whereas FireWire usesUSB uses Master/Slave protocol whereas FireWire uses peerpeer--toto--

peerpeerprotocolprotocol

Devices may be daisy chained together to form aDevices may be daisy chained together to form a bus groupbus group

Figure 4.23 Multiple FireWire Buses


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IInterface Standardnterface Standard (continue...)(continue...)Multiple FireWire BusesMultiple FireWire Buses FireWire supports two communication modes:FireWire supports two communication modes:

Asynchronous, IsochronousAsynchronous, Isochronous

Asynchronous Communication:Asynchronous Communication:

Involves exchange & acknowledgmentInvolves exchange & acknowledgment

Send a packetSend a packet Wait for a ACK or NACKWait for a ACK or NACK

IsochronousTransfer:IsochronousTransfer: With this mode, FireWire guarantee that data is sent at aWith this mode, FireWire guarantee that data is sent at a

steady rate; there is no waiting forACKs or resending ofsteady rate; there is no waiting forACKs or resending of

packetspackets


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IInterface Standardnterface Standard (continue...)(continue...)F

ireWireA

rbitrationF

ireWireA

rbitration Since there is noSince there is no mastermasterhost, what happens if two deviceshost, what happens if two devices

attempt to send at the same timeattempt to send at the same time

Devices are configured in a tree hierarchy, with one device at theDevices are configured in a tree hierarchy, with one device at the

root; each device selects an ID based on its location in the treeroot; each device selects an ID based on its location in the tree

The root device acts an arbiter; when devices under it wish toThe root device acts an arbiter; when devices under it wish to

transfer, the root decides which one gets the bus based on sometransfer, the root decides which one gets the bus based on some

form of priorityform of priority

This process is only part of the arbitration, and it works withThis process is only part of the arbitration, and it works with

some arbitration methods:some arbitration methods:

Fairness Arbitration, andFairness Arbitration, and

Urgent ArbitrationUrgent Arbitration


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IInterface Standardnterface Standard (continue...)(continue...)FireWireArbitrationFireWireArbitration

Figure 4.23 FireWire Arbitration


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MMultiplexingultiplexing It is possible to connect each device of a networkIt is possible to connect each device of a networkdirectly to that network, however each of thesedirectly to that network, however each of these

connection carries its costconnection carries its cost

Alternatively, multiplexing can be usedAlternatively, multiplexing can be used

A multiplexer, orA multiplexer, ormuxmux, routes transmission from, routes transmission frommultiple sources to a single destinationmultiple sources to a single destination

Multiplexer


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MMultiplexingultiplexing (continue...)(continue...)F

requencyF

requency--D

ivision Multiplexing (FD

M)D

ivision Multiplexing (FD

M) Used with analog signals; a common uses are TV & radioUsed with analog signals; a common uses are TV & radio

The available BW is divided into separate ranges or channelsThe available BW is divided into separate ranges or channels

Each device shares a part of the available BW, a channel, andEach device shares a part of the available BW, a channel, and

keeps that portion at all timeskeeps that portion at all times

Figure 4.27 Amplitude Modulation


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MMultiplexingultiplexing (continue...)(continue...)F

requencyF

requency--D

ivision MultiplexingD

ivision Multiplexing The modulated signals from all inputs are combinedThe modulated signals from all inputs are combined

into as a single, more complex analog signalinto as a single, more complex analog signal

The channels themselves are separated by aThe channels themselves are separated by a guardguard

bandband

Figure 4.29 FDM


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MMultiplexingultiplexing (continue...)(continue...)Statistical TimeStatistical Time--Division MultiplexingDivision Multiplexing

In practice, it may not be possible to keep input & output rates the sameIn practice, it may not be possible to keep input & output rates the same

Keeping the frame size fixed would simply the protocol but underutilize theKeeping the frame size fixed would simply the protocol but underutilize thechannelschannels

An alternative is to use Statistical Multiplexer, sometimes calledAn alternative is to use Statistical Multiplexer, sometimes calledConcentratorConcentrator

Since the order in one frame is not the guaranteed, a more complex logic isSince the order in one frame is not the guaranteed, a more complex logic isthere to resolve the frame correctlythere to resolve the frame correctly

Figure 4.31 Statistical TDM


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MMultiplexingultiplexing (continue...)(continue...)

WaveWave--Division MultiplexingDivision Multiplexing

Similar to FDM, but based onSimilar to FDM, but based onopticsoptics Potential bit rate is 1000Potential bit rate is 1000Gbps (Tera bps)Gbps (Tera bps)

Light consists of severalLight consists of several

wavelengths (refer to spectrum ofwavelengths (refer to spectrum offrequencies)frequencies)

Prism spreads the light intoPrism spreads the light intodifferent colors (to differentdifferent colors (to differentwavelengths)wavelengths)

Each source can operate at aEach source can operate at aspecific wavelengthspecific wavelength

All signals are combinedAll signals are combined

before transmission, andbefore transmission, and

separated at the receiverseparated at the receiver

Figure 4.32 Light Reflecting through a Prism

Figure 4.33 Wave-Division Multiplexing


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DDigital Carriersigital CarriersT1T1 A standard used for longA standard used for long--distance communicationdistance communication

Uses TDM to combine many voice channels into oneUses TDM to combine many voice channels into oneDS1DS1 frameframe

T1T1 refers to the circuit,refers to the circuit,DS1DS1 refers to the signalrefers to the signal

DS1 frame has 24 channels of 8 bits each, and one framing bitDS1 frame has 24 channels of 8 bits each, and one framing bitfor synchronizationfor synchronization

Figure 4.34 DS1 Frame


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DDigital Carriersigital Carriers (continue...)(continue...)T1T1 88--bit voice samples are taken from each of the 24 channels at abit voice samples are taken from each of the 24 channels at a

rate of 8000 samples per secondrate of 8000 samples per second

Each sample occupies one slot in the DS1 frameEach sample occupies one slot in the DS1 frame

The receiving mux extract the bits from each slot and route themThe receiving mux extract the bits from each slot and route them

to the appropriate destination (the voice is heard at the otherto the appropriate destination (the voice is heard at the otherside)side)

Figure 4.34 T1 Carrier System


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DDigital Carriersigital Carriers (continue...)(continue...)T1T1

T1 rate:T1 rate: 88--bit sample * 8000 samples/secondbit sample * 8000 samples/second 64 Kbps64 Kbps

To support this rate, T1 must transmit a DS1 frame each 1/8000 secondsTo support this rate, T1 must transmit a DS1 frame each 1/8000 seconds must transmit 8000 * 193 bits each secondmust transmit 8000 * 193 bits each second

Date rate of 1.544 MbpsDate rate of 1.544 Mbps

This rate is considered slow compared to optical fiber capabilitiesThis rate is considered slow compared to optical fiber capabilities

That is the reason there are other carriers with more channels and faster bitThat is the reason there are other carriers with more channels and faster bitraterate

T1 is not only used for voice communication; other companies lease phoneT1 is not only used for voice communication; other companies lease phonelines to transfer digital information between computerslines to transfer digital information between computers

North American Communication Carriers

Carrier Digital Signal No. No. of Channels Bit Rate, MbpsT1 DS1 24 1.544

T2 DS2 96 6.312

T3 DS3 672 44.736

T4 DS4 4,032 274.176


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CContention Protocolsontention Protocols Access to the medium from many entry points is called contentionAccess to the medium from many entry points is called contention

Unless controlled, contention may lead to fatal problemsUnless controlled, contention may lead to fatal problems Contention protocols are used to avoid such problemsContention protocols are used to avoid such problems

Figure 4.39 No Contention Protocol

Figure 4.40 Stop-and-Go Access Protocol


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CContention Protocolsontention Protocols (continue...)(continue...)Aloha ProtocolsAloha Protocols

Earliest contention protocol in 1970s by Univ. of Hawaii, calledEarliest contention protocol in 1970s by Univ. of Hawaii, calledPure ALOHAPure ALOHA

Several stations to central station (Menehune) by radioSeveral stations to central station (Menehune) by radiocommunicationcommunication

ff11 for broadcast,for broadcast,ff22 (different frequency than(different frequency than ff11) forACK) forACK Any station can transmit; if collision then wait random timeAny station can transmit; if collision then wait random time

Figure 4.41 Aloha System


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CContention Protocolsontention Protocols (continue...)(continue...)Slotted Aloha ProtocolsSlotted Aloha Protocols

Any overlap in signals, even a small one, would forceAny overlap in signals, even a small one, would forceretransmissionretransmission

Hence, a minimal safe period to transmit two signals is 2T (T isHence, a minimal safe period to transmit two signals is 2T (T istime period)time period)

So, to allow a device to transmit, you should reserve 2T for thatSo, to allow a device to transmit, you should reserve 2T for that

Not to waste such time, Slotted Aloha is usedNot to waste such time, Slotted Aloha is used

Devices can only send at the beginning of each slotDevices can only send at the beginning of each slot

Figure 4.42 Transmission Using Pure Aloha & Slotted Aloha


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CContention Protocolsontention Protocols (continue...)(continue...)

Slotted Aloha ProtocolsSlotted Aloha Protocols SlottedAloha has a higher success rate than Pure AlohaSlottedAloha has a higher success rate than Pure Aloha

However, with increased traffic, the different may notHowever, with increased traffic, the different may not

be that significantbe that significant

Figure 4.43 Success Rate for Pure Aloha & Slotted Aloha


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CContention Protocolsontention Protocols (continue...)(continue...)C

ollisionD

etection (CD

)C

ollisionD

etection (CD

) Instead of sending entire frame then discover that collision hasInstead of sending entire frame then discover that collision has

occurred when no ack is received, sense the medium for collisionoccurred when no ack is received, sense the medium for collision

and stop transmitting if occursand stop transmitting if occurs

This will avoid the medium from being unusable during collisionThis will avoid the medium from being unusable during collision

Commonly used with CSMACommonly used with CSMA called CSMA/CDcalled CSMA/CD

Figure 4.45

Collision with and without Detection


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CollisionDetection (CD)CollisionDetection (CD) Two issues worth considering:Two issues worth considering:

Frame sizeFrame size

DistanceDistance

Frame Size:Frame Size:

The frame has to be of a minimum size so the device canThe frame has to be of a minimum size so the device can

detect collision before it finishesdetect collision before it finishes

If too large, a device can monopolize the mediumIf too large, a device can monopolize the medium

So, how small should a frame be?So, how small should a frame be?

Depends on the maximum time it takes to detect collisionDepends on the maximum time it takes to detect collision


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CContention Protocolsontention Protocols (continue...)(continue...)Collision Detection (CD)Collision Detection (CD)

Example:Example:Assume:Assume:

10 Mbps bit rate,10 Mbps bit rate,

Largest distance between two devicesis 2 KMLargest distance between two devicesis 2 KM

Signalpropagate at a rate of200 meter/Signalpropagate at a rate of200 meter/secsec

To propagate 2 KM it takes 10To propagate 2 KM it takes 10secsec

To propagate 4 KM (worst case, go & come back), we need 20To propagate 4 KM (worst case, go & come back), we need 20secsec

Rate of 10 Mbps is the same as 10 bits eachRate of 10 Mbps is the same as 10 bits eachsecsec

In 20In 20secsecwe have 200 bits or 200/8 = 25 byteswe have 200 bits or 200/8 = 25 bytes

This is the minimum size a frame can be so CD can be madeThis is the minimum size a frame can be so CD can be made


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CContention Protocolsontention Protocols (continue...)(continue...)Collision Detection (CD)Collision Detection (CD)

The other issue with CD isThe other issue with CD is distancedistance

For exampleFor example CDCD does not work well with satellite since the timedoes not work well with satellite since the timeneeded to travel back and forth between ground and satellite isneeded to travel back and forth between ground and satellite istoo big due to the large distancetoo big due to the large distance

Binary exponential backBinary exponential back--off algorithmoff algorithm

Varies the waiting time before sending again if collision occurredVaries the waiting time before sending again if collision occurred

If first collision then wait 0, or 1 slotsIf first collision then wait 0, or 1 slots

Second collision then wait randomly for 0, 1, 2, or 3 slotsSecond collision then wait randomly for 0, 1, 2, or 3 slots

....................................................

....

IfIfnn successive collisions then wait for random # of slots between 0 andsuccessive collisions then wait for random # of slots between 0 and22nn--1, when n > 16 give1, when n > 16 give--up and signal to higher layer!up and signal to higher layer!


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CContention Protocolsontention Protocols (continue...)(continue...)Token PassingToken Passing

Instead of sending whenever it wishes, a device will take turns inInstead of sending whenever it wishes, a device will take turns insending with the other onessending with the other ones

Capture token to send data frameCapture token to send data frame

If data then remove token and transmit data frame; else passIf data then remove token and transmit data frame; else passtoken to neighbortoken to neighbor

Only sender can put the token back on ring after receiving itOnly sender can put the token back on ring after receiving itbackback

One frame per tokenOne frame per token

Advantage: contention is much controlled than the previousAdvantage: contention is much controlled than the previousprotocolsprotocols

Disadvantages:Disadvantages: All devices must be knownAll devices must be known

Complexity (what happen if the token is lost or if the device that hasComplexity (what happen if the token is lost or if the device that hascontrol over it fails)control over it fails)


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Token PassingToken Passing

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Date post:	06-Apr-2018
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