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EC1009 HIGH SPEED NETWORKS
ECE VII SEMESTER
UNIT I AND II Question bank
UNIT IHIGH SPEED NETWORKS
2 marks
1. Define frame relay.
A form of packet switching based on the use of variable-length link-layer frames.
There is no network layer, and many of the basic functions have been streamlined or
eliminated to provide for greater throughput.
2. Define ATM.
ATM is a connection-oriented packet switching technique that generalizes the notion of a
virtual connection to one that Provides quality-of-service guarantees. (Or) A form of packet
transmission using fixed size packets, called cells. ATM is the data transfer interfaces for B-
ISDN. Unlike X.25, ATM does not provide error control and flow control mechanisms.
3. Write down the advantages of packet switching network over circuit switching.
a). Line efficiency is greater, because a single node-to-node link can be
dynamically shared by many packets over time.
b). A packet-switching network can carry out data-rate conversion.
c). Priorities can be used.
4. What are the main features of ATM?
1. The service is connection-oriented, with data transfer over a virtual circuit.
2. The data is transferred in 53 byte packets called cells.
3. Cells from different VCs that occupy the same channel or link are statistically
multiplexed.
4. ATM switches may treat the cell streams in different VC connections
unequally over the same channel in order to provide different qualities of
services (QOS).
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5. What are the traffic parameters of connection-oriented services?
1. Peak Cell Rate (PCR)
2. Sustained Cell Rate (SCR)
3. Initial Cell Rate (ICR)
4. Cell Delay Variation Tolerance (CDVT)
5. Burst Tolerance (BT)
6. Minimum Cell Rate (MCR)
6. What are the quality service (QoS) parameters of connection-oriented services?
1. Cell Loss Ratio (CLR)
2. Cell Delay Variation (CDV)
3. Peak-to-Peak Cell Delay Variation (Peak-to-Peak CDV)
4. Maximum Cell Transfer Delay (Max CTD)
5. Mean Cell Transfer Delay (Mean CTD)
7. Types of ATM network interface.
Two most important interfaces are:
1. User-network interface (UNI)
2. Network-network interface or network-node interface (NNI).
8. What are the two sub layers of AAL?
1. Convergence Sub layer (CS)
2. Segmentation and Reassembly Sub layer (SAR).
9. What is the function of CS?
The Convergence Sublayer (CS) converts the information stream into four
types of packets streams, called AAL Type1, Type2, Type3/4, and Type5.The packet
formats match the requirements of the information stream.
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10. What are the subdivisions of CS?
1. Upper, service-specific or SSCS sub layer
2. Lower, common part or CPCS sub layer.
11. What do you mean by Type1 traffic?
Type1 traffic is a traffic generated at constant bit rate, and it is required to be
delivered at the same rate (with a fixed delay).
12. What are the functions of management and control?
1. Fault management
2. Traffic and congestion control
3. Network status monitoring and configuration
4. User/network signaling.
13. What are the functions of user plane?
It compromise the functions required for the transmission of user information
for instance, for an internet protocol over ATM, these layers could be
HTTP/TCP/IP/AAL5.
14. What are the two basic tasks required for internetworking over ATM?
The first is encapsulation of the protocol data units, and the second is Routing or
Bridging of these PDUs.
15 Define fast Ethernet
Fast Ethernet refers to a set of specifications developed by the IEEE 802.3 committee
to provide a low-cost, Ethernet-compatible LAN operating at 100 Mbps.
16. Define gigabit Ethernet?
Gigabit Ethernet, which has a data rate of 1000 Mbps (Or) 1 Gbps. In which collision
domain is reduced. Gigabit Ethernet is mainly designed to use optical fiber, although the
protocol does not eliminate the use of twisted pair cables.
There are four implementations have been designed for gigabit Ethernet:
a) 1000Base-LX
b) 1000Base-SX
c)
1000Base-CX
d) 1000Base-T
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17.List requirements for WLAN?
a). Throughput
b). Number of nodes
c). Connection to backbone LAN
d). Service area
e). Battery power consumption
f). Transmission robustness and security
g). Collocated network operation
h). License-free operation
i). Handoff/roaming
j). Dynamic configuration
18. List out the important services of IEEE 802.11?
a) Association
b) Reassociation
c) Disassociation
d) Authentication
e) Privacy
19. Mention the requirements for fibre channels?
a) Full duplex links with two fibers per link.
b) Performance from 100 Mbps TO 800 Mbps on a single line.
c) Small connectors
d) Support for distances up to 10 km.
e) High capacity utilization with distance insensitivity.
f) Broad availability.
g) Small systems
h) Interface and network protocols.
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20. List out the fibre channel elements?
a) Node: The key elements of a fibre channel network are the end systems.
b) Fabric: The collection of switching elements
21.What is the datalink control functions provided by LAPF?
LAPF core provides a minimal set of datalink control functions consisting of the
following
(i)Frame delimiting, alignment & transparency.
(ii)Frame multiplexing/demultiplexing using the address field.
(iii)Inspection of the frame to ensure that it consist of an integer no. of octets prior to
zero bit insertion or following zero bit extraction.
(iv)Inspection of the frame to ensure that it is neither too long nor too short.
(v)Detection of transmission errors.
(vi)Congestion control functions.
22. list the levels of fiber channel & the function of each level?
FC-0 PHSICAL MODE
Includes optical fiber for long distance application, co-axial for high speeds over shortdistances & shielded twisted pair for lower speeds over short distance.
FC-1 TRANSMISSION POROTOCOL
Defines the signal encoding scheme.
FC-2 FRAMING PROTOCOL
Deals with defining topologies, frame format, flow & error control & grouping of
frames into logical entities called sequences & exchanges.
FC-3 COMMON SERVICES
Include multicasting.
FC-4 MAPPING
Defines the mapping of various channel & network protocol to fiber channel,
including IEEE 802, ATM, IP & the Small Computer System Interface (SCSI).
23. What is meant by SAR & CS?
The AAL layer is organized in two logical sub layers: SAR & CS
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SAR: Segmentation And Reassembly sub layer is responsible for packing information at
the other end.
CS: The Convergence Sublayer provides the function needed to support specific application
using AAL.
24. Difference b/w AAL & AAL 3/5
AAL AAL 3/5
(i)In this MID field is used to
multiplex diff streams of data on the
same virtual ATM connection.
(ii)A 10 bit CRC is provided for
each SAR PDU.
(iii)In this 8 octets per AAL
SDU, 4 octets per ATM cell.
(i)In this MID field is assumed to
that the higher layer software takes care of
such multiplexing.
(ii)A 32 bit CRC protects the entire
cpus PDU, provides strong protection
against bit errors.
(iii)8 octets per AAL SDU, 0 octets
per ATM cell.
25. Give the data rates for frame relay & X.25?
The lower bit rate for X.25 is 64 kbps. The fixed data for frame relay is 1.544mbps.
The higher data rate for frame relay is 44.376mbps.
26. Define Ethernet.
As packet switching has dominated wide area data networking, Ethernet dominates
local area networking. The original experimental Ethernet operated at 3mbps over coaxial
cable. This remarkable over twisted pair & optical fiber as well as coaxial cable. It was
released commercially at 10 mbps & then was scaled up first to 100bps & none 1 & 10 gbps
16 Marks.
1.Explain Frame relay Networks in detail.
Frame relay Networks
Frame Relay often is described as a streamlined version of X.25, offering fewer of the robust
capabilities, such as windowing and retransmission of last data that are offered in X.25.
Frame Relay Devices
Devices attached to a Frame Relay WAN fall into the following two general categories:
Data terminal equipment (DTE) Data circuit-terminating equipment (DCE)
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DTEs generally are considered to be terminating equipment for a specific network and
typically are located on the premises of a customer. In fact, they may be owned by the
customer. Examples of DTE devices are terminals, personal computers, routers, and bridges.
DCEs are carrier-owned internetworking devices. The purpose of DCE equipment is to
provide clocking and switching services in a network, which are the devices that actuallytransmit data through the WAN. In most cases, these are packet switches. Figure 10-1 shows
the relationship between the two categories of devices.
Standard Frame Relay Frame
Standard Frame Relay frames consist of the fields illustrated in Figure 10-4.
Figure Five Fields Comprise the Frame Relay Frame
Each frame relayPDUconsists of the following fields:
1.
Flag Field. The flag is used to perform high level data link synchronization which indicatesthe beginning and end of the frame with the unique pattern 01111110. To ensure that the
01111110 pattern does not appear somewhere inside the frame, bit stuffing and destuffing
procedures are used.
2. Address Field. Each address field may occupy either octet 2 to 3, octet 2 to 4, or octet 2 to 5,
depending on the range of the address in use. A two-octet address field comprising the
EA=ADDRESS FIELD EXTENSION BITS and the C/R=COMMAND/RESPONSE BIT.
3. DLCI-Data Link Connection Identifier Bits. The DLCI serves to identify the virtual connection
so that the receiving end knows which information connection a frame belongs to. Note that
this DLCI has only local significance. A single physical channel canmultiplexseveral different
virtual connections.
4.
FECN, BECN, DE bits. These bits report congestion:o
FECN=Forward Explicit Congestion Notification bit
o
BECN=Backward Explicit Congestion Notification bit
o DE=Discard Eligibility bit
5. Information Field. A system parameter defines the maximum number of data bytes that a
host can pack into a frame. Hosts may negotiate the actual maximum frame length at call
set-up time. The standard specifies the maximum information field size (supportable by any
network) as at least 262 octets. Since end-to-end protocols typically operate on the basis of
larger information units, frame relay recommends that the network support the maximum
value of at least 1600 octets in order to avoid the need for segmentation and reassembling
by end-users.
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Frame Check Sequence (FCS) Field. Since one cannot completely ignore the bit error-rate of
the medium, each switching node needs to implement error detection to avoid wasting
bandwidth due to the transmission of erred frames. The error detection mechanism used inframe relay uses thecyclic redundancy check(CRC) as its basis.
Congestion-Control Mechanisms
Frame Relay reduces network overhead by implementing simple congestion-notification
mechanisms rather than explicit, per-virtual-circuit flow control. Frame Relay typically is
implemented on reliable network media, so data integrity is not sacrificed because flow
control can be left to higher-layer protocols. Frame Relay implements two congestion-
notification mechanisms:
Forward-explicit congestion notification (FECN)
Backward-explicit congestion notification (BECN) FECN and BECN each is controlled
by a single bit contained in the Frame Relay frame header. The Frame Relay frame headeralso contains a Discard Eligibility (DE) bit, which is used to identify less important traffic
that can be dropped during periods of congestion.
Frame Relay versus X.25
The design of X.25 aimed to provide error-free delivery over links with high error-rates.
Frame relay takes advantage of the new links with lower error-rates, enabling it to eliminate
many of the services provided by X.25. The elimination of functions and fields, combined
with digital links, enables frame relay to operate at speeds 20 times greater than X.25.
X.25 specifies processing at layers 1, 2 and 3 of the OSI model, while frame relay operates at
layers 1 and 2 only. This means that frame relay has significantly less processing to do at
each node, which improves throughput by an order of magnitude.
X.25 prepares and sends packets, while frame relay prepares and sends frames. X.25 packets
contain several fields used for error and flow control, none of which frame relay needs. The
frames in frame relay contain an expanded address field that enables frame relay nodes to
direct frames to their destinations with minimal processing .
X.25 has a fixed bandwidth available. It uses or wastes portions of its bandwidth as the load
dictates. Frame relay can dynamically allocate bandwidth during call setup negotiation atboth the physical and logical channel level.
2.Explain ATM architecture , Logical connections and its services.
Asynchronous Transfer Mode (ATM )
Asynchronous Transfer Mode (ATM) is an International Telecommunication Union-
Telecommunications Standards Section (ITU-T) standard for cell relay wherein information
for multiple service types, such as voice, video, or data, is conveyed in small, fixed-size cells.
ATM networks are connection-oriented.
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ATM is a cell-switching and multiplexing technology that combines the benefits of circuit
switching (guaranteed capacity and constant transmission delay) with those of packet
switching (flexibility and efficiency for intermittent traffic). It provides scalable bandwidth
from a few megabits per second (Mbps) to many gigabits per second (Gbps). Because of its
asynchronous nature, ATM is more efficient than synchronous technologies, such as time-
division multiplexing (TDM).
With TDM, each user is assigned to a time slot, and no other station can send in that time
slot. If a station has much data to send, it can send only when its time slot comes up, even if
all other time slots are empty. However, if a station has nothing to transmit when its time slot
comes up, the time slot is sent empty and is wasted. Because ATM is asynchronous, timeslots are available on demand with information identifying the source of the transmission
contained in the header of each ATM cell.
ATM transfers information in fixed-size units called cells. Each cell consists of 53octets, or bytes. The first 5 bytes contain cell-header information, and the remaining 48
contain the payload (user information). Small, fixed-length cells are well suited totransferring voice and video traffic because such traffic is intolerant of delays that result from
having to wait for a large data packet to download, among other things. Figure illustrates the
basic format of an ATM cell. Figure :An ATM Cell Consists of a Header and Payload Data
ATM Protocol archi tecture:
ATM is almost similar to cell relay and packets witching using X.25and framerelay.likepacket switching and frame relay,ATM involves the transfer of data in discrete
pieces.also,like packet switching and frame relay ,ATM allows multiple logical connections
to multiplexed over a single physical interface. in the case of ATM,the information flow on
each logical connection is organised into fixed-size packets, called cells. ATM is a
streamlined protocol with minimal error and flow control capabilities :this reduces theoverhead of processing ATM cells and reduces the number of overhead bits required with
each cell, thus enabling ATM to operate at high data rates.the use of fixed-size cells
simplifies the processing required at each ATM node,again supporting the use of ATM at
high data rates. The ATM architecture uses a logical model to describe the functionality that
it supports. ATM functionality corresponds to the physical layer and part of the data link
layer of the OSI reference model. . the protocol referencce model shown makes reference to
three separate planes:
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user planeprovides for user information transfer ,along with associated controls (e.g.,flow
control ,error control).
control planeperforms call control and connection control functions.
management plane includes plane management ,which performs management functionrelated to a system as a whole and provides coordination between all the planes ,and layer
management which performs management functions relating to resource and parameters
residing in its protocol entities .
The ATM reference model is composed of the following ATM layers:
Physical layerAnalogous to the physical layer of the OSI reference model, the ATM
physical layer manages the medium-dependent transmission.
ATM layerCombined with the ATM adaptation layer, the ATM layer is roughlyanalogous to the data link layer of the OSI reference model. The ATM layer is responsible for
the simultaneous sharing of virtual circuits over a physical link (cell multiplexing) and
passing cells through the ATM network (cell relay). To do this, it uses the VPI and VCI
information in the header of each ATM cell.
ATM adaptation layer (AAL)Combined with the ATM layer, the AAL is roughly
analogous to the data link layer of the OSI model. The AAL is responsible for isolating
higher-layer protocols from the details of the ATM processes. The adaptation layer preparesuser data for conversion into cells and segments the data into 48-byte cell payloads.
Finally, the higher layers residing above the AAL accept user data, arrange it into packets,and hand it to the AAL. Figure :illustrates the ATM reference model.
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Structure of an ATM cell
An ATM cell consists of a 5 byte header and a 48 byte payload. The payload size of 48 byteswas a compromise between the needs of voice telephony and packet networks, obtained by a
simple averaging of the US proposal of 64 bytes and European proposal of 32, said by some
to be motivated by a European desire not to need echo-cancellers on national trunks.
ATM defines two different cell formats: NNI (Network-network interface) and UNI (User-
network interface). Most ATM links use UNI cell format.
Diagram of the UNI ATM Cell
7 4 3 0
GFC VPI
VPI VCI
VCI
VCI PT CLP
HEC
Payload (48 bytes)
Diagram of the NNI ATM Cell
7 4 3 0
VPI
VPI VCI
VCI
VCI PT CLP
HEC
Payload (48 bytes)
GFC =Generic Flow Control(4 bits) (default: 4-zero bits)
VPI =Virtual Path Identifier(8 bits UNI) or (12 bits NNI)
VCI =Virtual channel identifier(16 bits)
PT =Payload Type(3 bits)
CLP =Cell Loss Priority(1-bit)
HEC =Header Error Correction(8-bit CRC, polynomial = X8+ X2+ X + 1)
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The PT field is used to designate various special kinds of cells for Operation and
Management(OAM) purposes, and to delineate packet boundaries in some AALs.
Several of ATM's link protocols use the HEC field to drive aCRC-Based Framingalgorithm,
which allows the position of the ATM cells to be found with no overhead required beyond
what is otherwise needed for header protection. The 8-bit CRC is used to correct single-bitheader errors and detect multi-bit header errors. When multi-bit header errors are detected,
the current and subsequent cells are dropped until a cell with no header errors is found.
In a UNI cell the GFC field is reserved for a local flow control/submultiplexing system
between users. This was intended to allow several terminals to share a single networkconnection, in the same way that two ISDN phones can share a single basic rate ISDN
connection. All four GFC bits must be zero by default.The NNI cell format is almost identicalto the UNI format, except that the 4-bit GFC field is re-allocated to the VPI field, extending
the VPI to 12 bits. Thus, a single NNI ATM interconnection is capable of addressing almost212 VPs of up to almost 216 VCs each (in practice some of the VP and VC numbers are
reserved).
A Virtual Channel (VC)denotes the transport of ATM cells which have the same unique
identifier, called the Virtual Channel Identifier (VCI). This identifier is encoded in the cell
header. A virtual channel represents the basic means of communication between two end-
points, and is analogous to an X.25 virtual circuit.
A Virtual Path (VP)denotes the transport of ATM cells belonging to virtual channels which
share a common identifier, called the Virtual Path Identifier (VPI), which is also encoded in
the cell header. A virtual path, in other words, is a grouping of virtual channels which
connect the same end-points. This two layer approach results in improved network
performance. Once a virtual path is set up, the addition/removal of virtual channels is
straightforward
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ATM Classes of Services
ATM is connection oriented and allows the user to specify the resources required on a per-
connection basis (per SVC) dynamically. There are the five classes of service defined for
ATM (as per ATM Forum UNI 4.0 specification). The QoS parameters for these service
classes are summarized in Table 1.
Service Class Quality of Service Parameter
constant bit rate
(CBR)
This class is used for emulating circuit switching. The cell rate is constant
with time. CBR applications are quite sensitive to cell-delay variation.
Examples of applications that can use CBR are telephone traffic (i.e.,
nx64 kbps), videoconferencing, and television.
variable bit rate
non-real time
(VBRNRT)
This class allows users to send traffic at a rate that varies with time
depending on the availability of user information. Statistical multiplexing
is provided to make optimum use of network resources. Multimedia e-
mail is an example of VBRNRT.
variable bit rate
real time (VBR
RT)
This class is similar to VBRNRT but is designed for applications that are
sensitive to cell-delay variation. Examples for real-time VBR are voice
with speech activity detection (SAD) and interactive compressed video.
available bit rate
(ABR)
This class of ATM services provides rate-based flow control and is aimed
at data traffic such as file transfer and e-mail. Although the standard does
not require the cell transfer delay and cell-loss ratio to be guaranteed or
minimized, it is desirable for switches to minimize delay and loss as much
as possible. Depending upon the state of congestion in the network, the
source is required to control its rate. The users are allowed to declare a
minimum cell rate, which is guaranteed to the connection by the network.
unspecified bit
rate (UBR)This class is the catch-all, other class and is widely used today for TCP/IP.
Technical
ParameterDefinition
cell loss ratio
(CLR)
CLR is the percentage of cells not delivered at their destination
because they were lost in the network due to congestion and buffer
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overflow.
cell transfer
delay (CTD)
The delay experienced by a cell between network entry and exit
points is called the CTD. It includes propagation delays, queuing
delays at various intermediate switches, and service times atqueuing points.
cell delay
variation
(CDV)
CDV is a measure of the variance of the cell transfer delay. High
variation implies larger buffering for delay-sensitive traffic such
as voice and video.
peak cell rate
(PCR)
The maximum cell rate at which the user will transmit. PCR is the
inverse of the minimum cell inter-arrival time.
sustained cell
rate (SCR)
This is the average rate, as measured over a long interval, in the
order of the connection lifetime.
burst tolerance
(BT)
This parameter determines the maximum burst that can be sent at
the peak rate. This is the bucket-size parameter for the
enforcement algorithm that is used to control the traffic entering
the network.
Benefits of ATM
The benefits of ATM are the following:
high performance via hardware switching
dynamic bandwidth for bursty traffic
class-of-service support for multimedia
scalability in speed and network size
common LAN/WAN architecture
opportunities for simplification via VC architecture
international standards compliance
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3. Explain ATM adaptation layer in detail
ATM Adaptation Layers (AAL)
The use ofAsynchronous Transfer Mode (ATM) technology and services creates the need for
an adaptation layer in order to support information transfer protocols, which are not based onATM. This adaptation layer defines how to segment and reassemble higher-layer packets into
ATM cells, and how to handle various transmission aspects in the ATM layer.
Examples of services that need adaptations are Gigabit Ethernet, IP, Frame Relay,SONET/SDH,UMTS/Wireless, etc.
The main services provided by AAL (ATM Adaptation Layer) are:
Segmentation and reassembly
Handling of transmission errorsHandling of lost and misinserted cell conditionsTiming and flow control
The following ATM Adaptation Layer protocols (AALs) have been defined by the ITU-T.It
is meant that these AALs will meet a variety of needs. The classification is based on whether
a timing relationship must be maintained between source and destination, whether the
application requires a constant bit rate, and whether the transfer is connection oriented or
connectionless.
AAL Type 1 supports constant bit rate (CBR), synchronous, connection oriented
traffic. Examples include T1 (DS1), E1, and x64 kbit/s emulation.AAL Type 2 supports time-dependent Variable Bit Rate (VBR-RT) of connection-
oriented, synchronous traffic. Examples include Voice over ATM. AAL2 is also
widely used in wireless applications due to the capability of multiplexing voice
packets from different users on a single ATM connection.
AAL Type 3/4supports VBR, data traffic, connection-oriented, asynchronous traffic
(e.g. X.25 data) or connectionless packet data (e.g. SMDS traffic) with an additional
4-byte header in the information payload of the cell. Examples include Frame Relayand X.25.
AAL Type 5is similar to AAL 3/4 with a simplified information header scheme. ThisAAL assumes that the data is sequential from the end user and uses the Payload Type
Indicator (PTI) bit to indicate the last cell in a transmission. Examples of services thatuse AAL 5 are classic IP over ATM, Ethernet Over ATM, SMDS, and LAN
Emulation (LANE). AAL 5 is a widely used ATM adaptation layer protocol. This
protocol was intended to provide a streamlined transport facility for higher-layer
protocols that are connection oriented.
AAL 5 was introduced to:
reduce protocol processing overhead.
reduce transmission overhead.
ensure adaptability to existing transport protocols.
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T AAL1 PDU
The structure of the AAL1 PDU is given in the following illustration:
SN SNP
CSI SC CRC EPC SAR PDU
Payload
1 bit 3
bits
3 bits 1 bit 47 bytes
AAL1 PDU
SN
Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16). The
sequence number is comprised of the CSI and the SN.
CSI
Convergence sublayer indicator. Used for residual time stamp for clocking.
SC
Sequence count. The sequence number for the entire CS PDU, which is generated by the
Convergence Sublayer.
SNP
Sequence number protection. Comprised of the CRC and the EPC.
CRC
Cyclic redundancy check calculated over the SAR header.
EPC
Even parity check calculated over the CRC.
SAR PDU payload
47-byte user information field.
AAL2
AAL2 provides bandwidth-efficient transmission of low-rate, short and variable packets in
delay sensitive applications. It supports VBR and CBR. AAL2 also provides for variable
payload within cells and across cells. AAL type 2 is subdivided into the Common Part
Sublayer (CPS ) and the Service Specific Convergence Sublayer (SSCS ).
AAL2 CPS Packet
The CPS packet consists of a 3 octet header followed by a payload. The structure of the
AAL2 CPS packet is shown in the following illustration.
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CID LI UUI HEC Information payload
8 bits 6 bits 5 bits 5 bits 1-45/64 bytes
AAL2 CPS packet
CID Channelidentification.
LI
Length indicator. This is the length of the packet payload associated with each individual
user. Value is one less than the packet payload and has a default value of 45 bytes (may be set
to 64 bytes).
UUI
User-to-user indication. Provides a link between the CPS and an appropriate SSCS that
satisfies the higher layer application
HEC
Header error control.
AAL2
The structure of the AAL2 SAR PDU is given in the following illustration.
Start field CPS-PDU payload
OSF SN P AAL2 PDU payload PAD
6 bits 1 bit 1 bit 0-47
bytes
AAL2 CPS PDU
OSF
Offset field. Identifies the location of the start of the next CPS packet within the CPS-PDU.
SN
Sequence number. Protects data integrity.
P
Parity. Protects the start field from errors.
SAR PDU payload
Information field of the SAR PDU.
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PAD
Padding.
AAL2 SSCS Packet
The SSCS conveys narrowband calls consisting of voice, voiceband data or circuit mode
data. SSCS packets are transported as CPS packets over AAL2 connections. The CPS packet
contains a SSCS payload. There are 3 SSCS packet types.
Type 1 Unprotected; this is used by default.
Type 2 Partially protected.
Type 3 Fully protected: the entire payload is protected by a 10-bit CRC which is computed as
for OAM cells. The remaining 2 bits of the 2-octet trailer consist of the message type field.
AAL2 SSCS Type 3 Packets:
The type 3 packets are used for the following:
Dialled digits
Channel associated signalling bits
Facsimile demodulated control data
AlarmsUser state control operations.
The following illustration gives the general sturcture of AAL2 SSCS Type 3 PDUs. The
format varies and each message has its own format according to the actual message type.
Redundancy Time
stamp
Message
dependant
information
Message
type
CRC-
10
2 14 16 6 10 bits
AAL2 SSCS Type 3 PDU
Redundancy
Packets are sent 3 times to ensure error correction. The value in this field signifies the
transmission number.
Time stamp
Counters packet delay variation and allows a receiver to accurately reproduce the relative
timing of successive events separated by a short interval.
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Message dependant information
Packet content that varies, depending on the message type.
Message type
The message type code.
CRC-10
The 10-bit CRC.
AAL3/4
AAL3/4 consists of message and streaming modes. It provides for point-to-point and point-
to-multipoint (ATM layer) connections. The Convergence Sublayer (CS) of the ATM
Adaptation Layer (AAL) is divided into two parts: service specific (SSCS ) and common part
(CPCS ). This is illustrated in the following diagram:
AAL3/4 packets are used to carry computer data, mainly SMDS traffic.
AAL3/4 CPCS PDU
The functions of the AAL3/4 CPCS include connectionless network layer (Class D), meaning
no need for an SSCS; and frame relaying telecommunication service in Class C. The CPCS
PDU is composed of the following fields:
Header Info
Trailer
CPI Btag Basize CPCS
SDU
Pad 0 Etag Length
1 1 2 0-65535 0-3 1 1 2 bytes
AAL3/4 CPCS PDU
CPI
Message type. Set to zero when the BAsize and Length fields are encoded in bytes.
Btag
Beginning tag. This is an identifier for the packet. It is repeated as the Etag.
BAsize
Buffer allocation size. Size (in bytes) that the receiver has to allocate to capture all the data.
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CPCS SDU
Variable information field up to 65535 bytes.
PAD
Padding field which is used to achieve 32-bit alignment of the length of the packet.
0
All-zero.
Etag
End tag. Must be the same as Btag.
Length
Must be the same as BASize.
AAL3/4 SAR PDU
The structure of the AAL3/4 SAR PDU is illustrated below:
ST SN MID Information LI CRC
2 4 10 352 6 10 bits
2-byte header 44 bytes 2-byte trailer
48 bytes
AAL3/4 SAR PDU
ST
Segment type. Values may be as follows:
SN
Sequence number. Numbers the stream of SAR PDUs of a CPCS PDU (modulo 16).
MID
Multiplexing identification. This is used for multiplexing several AAL3/4 connections over
one ATM link.
Information
This field has a fixed length of 44 bytes and contains parts of CPCS PDU.
LILength indication. Contains the length of the SAR SDU in bytes, as follows:
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CRC
Cyclic redundancy check.
Functions of AAL3/4 SAR include identification of SAR SDUs; error indication and
handling; SAR SDU sequence continuity; multiplexing and demultiplexing.
AAL5 The type 5 adaptation layer is a simplified version of AAL3/4. It also consists of
message and streaming modes, with the CS divided into the service specific and common
part. AAL5 provides point-to-point and point-to-multipoint (ATM layer) connections.
AAL5 is used to carry computer data such as TCP/IP. It is the most popular AAL and is
sometimes referred to as SEAL (simple and easy adaptation layer).
AAL5 CPCS PDU
The AAL5 CPCS PDU is composed of the following fields:
Info
Trailer
CPCS payload Pad UU CPI Length CRC
0-65535 0-47 1 1 2 4 bytes
AAL5 CPCS PDU
CPCS
The actual information that is sent by the user. Note that the information comes before any
length indication (as opposed to AAL3/4 where the amount of memory required is known in
advance).
Pad
Padding bytes to make the entire packet (including control and CRC) fit into a 48-byte
boundary.
UU
CPCS user-to-user indication to transfer one byte of user information.
CPI
Common part indicator is a filling byte (of value 0). This field is to be used in the future for
layer management message indication.
Length
Length of the user information without the Pad.
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CRC
CRC-32. Used to allow identification of corrupted transmission.
AAL5 SAR PDU The structure of the AAL5 CS PDU is as follows:
Information PAD UU CPI Length CRC-32
1-48 0-47 1 1 2 4 bytes
8-byte trailer
AAL5 SAR PDU
4.Explain Fast Ethernet and Gigabit Ethernet in detail.
Emergence of High-Speed LANs
2 Significant trends
Computing power of PCs continues to grow rapidlyNetwork computingExamples of requirements
Centralized server farmsPower workgroups
High-speed local backboneClassical Ethernet
Bus topology LAN
10 Mbps
CSMA/CD medium access control protocol
2 problems:
A transmission from any station can be received by all stationsHow to regulate transmission
Solution to First Problem
Data transmitted in blocks called frames:
User dataFrame header containing unique address of destination station
CSMA/CD
Carrier Sense Multiple Access/ Carrier Detection
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If the medium is idle, transmit.
If the medium is busy, continue to listen until the channel is idle, then transmit immediately.
If a collision is detected during transmission, immediately cease transmitting.
After a collision, wait a random amount of time, then attempt to transmit again (repeat from
step 1).
Medium Options at 10Mbps
10Base5
10 Mbps
50-ohm coaxial cable bus
Maximum segment length 500 meters
10Base-TTwisted pair, maximum length 100 meters
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Star topology (hub or multipoint repeater at central point)
Hubs and Switches
Hub
Transmission from a station received by central hub and retransmitted on all outgoing linesOnly one transmission at a time
Layer 2 Switch
Incoming frame switched to one outgoing line
Many transmissions at same time
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Bridge
Frame handling done in software
Analyze and forward one frame at a time
Store-and-forward
Layer 2 Switch
Frame handling done in hardware
Multiple data paths and can handle multiple frames at a time
Can do cut-throughLayer 2 Switches
Flat address space
Broadcast storm
Only one path between any 2 devices
Solution 1: subnetworks connected by routers
Solution 2: layer 3 switching, packet-forwarding logic in hardware
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Benefits of 10 Gbps Ethernet over ATM
No expensive, bandwidth consuming conversion between Ethernet packets and ATM cells
Network is Ethernet, end to end
IP plus Ethernet offers QoS and traffic policing capabilities approach that of ATM
Wide variety of standard optical interfaces for 10 Gbps Ethernet
5.Explain Fiber channel in detail.
Fibre Channel
2 methods of communication with processor:
I/O channel
Network communicationsFibre channel combines both
Simplicity and speed of channel communications
Flexibility and interconnectivity of network communications
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I/O channel
Hardware based, high-speed, short distance
Direct point-to-point or multipoint communications link
Data type qualifiers for routing payload
Link-level constructs for individual I/O operationsProtocol specific specifications to support e.g. SCSI
Fibre Channel Network-Oriented Facilities
Full multiplexing between multiple destinations
Peer-to-peer connectivity between any pair of ports
Internetworking with other connection technologies
Fibre Channel Requirements
Full duplex links with 2 fibres/link
100 Mbps800 Mbps
Distances up to 10 km
Small connectors
high-capacity
Greater connectivity than existing multidrop channels
Broad availability
Support for multiple cost/performance levels
Support for multiple existing interface command sets
Fibre Channel Protocol Architecture
FC-0 Physical Media
FC-1 Transmission Protocol
FC-2 Framing Protocol
FC-3 Common Services
FC-4 Mapping
6.Explain IEEE 802.11- WLAN in detail.
Wireless LAN Requirements
Throughput
Number of nodes
Connection to backbone
Service area
Battery power consumption
Transmission robustness and securityCollocated network operation
License-free operation
Handoff/roaming
Dynamic configurationIEEE 802.11 Services
Association
Reassociation
Disassociation
Authentication
Privacy
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UNIT II CONGESTION AND TRAFFIC MANAGEMENT
2 marks
1. When queue will be formed in a network
Queue will be formed, if the current demand for a particular service exceeds the
Capacity of service provider.
2. What are the characteristics of queuing process/
Characteristics of queuing process depend on:
a)
Arrival patternb) Service pattern
c) Number of server
d) Queue discipline
e) System capacity
f)
Number of channels
3. What is meant by traffic intensity in queuing analysis? And write littles formula forsingle server queue?
Traffic intensity (or) utilization factor = / = arrival rate / rate service
Littles formula = TS
r = Tr
w = Tw
4. Compare Single Server and Multi Server Queue.
S.NoSingle server model Multiserver model
1 Congestion statistics for this model
are:M/M/1, M/D/1, M/G/1
Congestion statistics for this model
is M/M/N.
2 Arrival rate = Arrival rate for each server = /N
5. What is meant by implicit congestion signaling?
When network congestion occurs, packets get discard and acknowledgement will be
delayed. As a result, sources understand that there is congestion implicitly. Here, users arenotified about congestion indirectly.
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6. What is meant by explicit congestion signaling?
In this method, congestion is indicated directly by a notification. The notificat ion may
be in backward or forward direction.
7. Define committed burst size (BC)
It is defined as the maximum number of bits in a predefined period of time that the
network is committed to transfer with out discarding any frames.
8. Define committed information rate (CIR)
CIR is a rate in bps that a network agrees to support for a particular frame mode
connection. Any data transmitted in excess of CIR is vulnerable to discard in event of
congestion.
CIR < Access rate
10. Define access rate.
For every connection in frame relay network, an access rate (bps) is defined. The
access rate actually depends on bandwidth of channel connecting user to network.
12. Write Littles formula.
Littles formula is defined as the product of item arrive at a rate of , and Served time
of items Tr(or) product of item arrive at a rate of and waiting time of an items Tw.
It is given as, r = Tr (or) w = Tw
14. List out the model characteristics of queuing models.
a) Item population.
b) Queue size
c) Dispatching discipline
15. List out the fundamental task of a queuing analysis.
Queuing analysis as the following as a input information.
a) Arrival rate
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b) Service rate
c) Number of servers
Provide as output information concerning:
a) Items waiting
b) Waiting time
c) Items queued
d) Residence time
16. State Kendalls notation.
Kendalls notation is X/Y/N, where X refers to the distribution of the interarrival
times, Y refers to the distribution of service times, and N refers to the number of servers.
The most common distributions are denoted as follows:
G = General distribution of interarrival times or service times
GI = General distribution of interarrival times with the restriction that
Interarrival times are independent.
M = Negative exponential distribution
D = Deterministic arrivals or fixed-length service.
Thus, M/M/1 refers to a single-server queuing model with poisson arrivals
(Exponential interarrival times) and exponential service times.
17. List out the assumptions for single server queues.
a) Poisson arrival rate.
b) Dispatching discipline does not give preference to items based on service times
c) Formulas for standard deviation assume first-in, first-out dispatching.
d) No items are discarded from the queue.
18. List out the assumptions for Multiserver queues.
a) Poisson arrival rate.
b) Exponential service times
c) All servers equally loaded.
d) All servers have same mean service time.
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e) First-in, first-out dispatching.
f) No items are discarded from the queue.
19. State Jacksons theorem.
Jacksons theorem can be used to analyse a network of queues. The theorem is based
on three assumptions:
1. The queuing network consists of m nodes, each of which provides an independent
exponential service.
2. Items arriving from outside the system to any one of the nodes arrive with a
poisson rate.
3. Once served at a node, an item goes (immediately) to one of the other nodes with a
fixed probability, or out of the system.
20. Define Arrival rate and service rate.
Arrival Rate: The rate at which data enters into a queuing system i.e., inter arrival
rate. It is indicated as .
Service Rate: The rate at which data leaves the queuing system i.e., service rate.
It is indicated as .
21. What is meant by congestion avoidance and congestion recovery technique?
Congestion Avoidance: It is the procedure used at beginning stage of congestion to
minimize its effort. This procedure initiated prior to or at point A. This procedure
prevent congestion from progressing to point B.
Congestion Recovery: This procedure operates around at point B and within region of
severe congestion to prevent network collapse. Here dropped frames are reported to higherlayer and further packet delivery is stopped to recover from congestion.
22. what is the role of de in frame relay?
This bit it indicates frame priority. The DE can taken value of 0 or 1.
DE=0 means frame network element; it can be discard the frame during periods of
congestion. DE=1, for generally considered as high priority frames.
23. How does frame relay report congestion?When the particular portion of the network is heavily congestion. It is
Desirable to route packets around rather than through the area of congestion.
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24Define Qos.
Refers to the properties of a network that contribute to the degree of
satisfaction that user perceive, relative to the network performance four service
categories are typically under this term capacity, data rate, latency, delay & trafficlosses.
25. Define committed burst size
The max. amount data that the network agrees to transfer under normal
Condition over a measurement interval T, these data may or may not be contiguous.
Define excess burst size
The max amount of data in excess of BC that the network will attempt to
transfer under normal condition over a measurement interval T. these data areuncommitted
16 Marks.
1.Explain queuing analysis and the different queuing models.
Queing analysis
In queueing theory, a queueing model is used to approximate a real queueing
situation or system, so the queueing behaviour can be analysed mathematically.
Queueing models allow a number of useful steady stateperformance measures tobe determined, including:
the average number in the queue, or the system,
the average time spent in the queue, or the system,the statistical distribution of those numbers or times,
the probability the queue is full, or empty, and
the probability of finding the system in a particular state.
These performance measures are important as issues or problems caused byqueueing situations are often related to customer dissatisfaction with service or
may be the root cause of economic losses in a business. Analysis of the relevantqueueing models allows the cause of queueing issues to be identified and the
impact of any changes that might be wanted to be assessed.
Notation
Queueing modelscan be represented usingKendall's notation:
A/B/S/K/N/Disc
where:
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A is the interarrival time distribution
B is the service time distribution
S is the number of servers
K is the system capacity
N is the calling population
Disc is the service discipline assumed
Some standard notation for distributions (A or B) are:
M for aMarkovian(exponential)distribution
E for anErlang distributionwith phasesD for Deterministic (constant)
G for General distributionPH for aPhase-type distribution
Models
Construction and analysis
Queueing models are generally constructed to represent the steady state of a
queueing system, that is, the typical, long run or average state of the system. As aconsequence, these are stochastic models that represent the probability that a
queueing system will be found in a particular configuration orstate.
A general procedure for constructing and analysing such queueing models is:
1. Identify the parameters of the system, such as the arrival rate, service time, Queue
capacity, and perhaps draw a diagram of the system.
2. Identify the system states. (A state will generally represent the integer number of
customers, people, jobs, calls, messages, etc. in the system and may or may not belimited.)
3. Draw a state transition diagram that represents the possible system states andidentify the rates to enter and leave each state. This diagram is a representation of a
Markov chain.
4. Because the state transition diagram represents the steady state situation between
state there is a balanced flow between states so the probabilities of being in
adjacent states can be related mathematically in terms of the arrival and service
rates and state probabilities.5. Express all the state probabilities in terms of the empty state probability, using the
inter-state transition relationships.6.
Determine the empty state probability by using the fact that all state probabilities
always sum to 1.
Whereas specific problems that have small finite state models are often able to beanalysed numerically, analysis of more general models, using calculus, yields
useful formulae that can be applied to whole classes of problems.
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Single-server queue
Single-server queues are, perhaps, the most commonly encountered queueingsituation in real life. One encounters a queue with a single server in many
situations, including business (e.g. sales clerk), industry (e.g. a production line),
transport (e.g. a bus, a taxi rank, an intersection), telecommunications (e.g.Telephone line), computing (e.g. processor sharing). Even where there are multiple
servers handling the situation it is possible to consider each server individually as
part of the larger system, in many cases. (e.g A supermarket checkout has several
single server queues that the customer can select from.) Consequently, being able
to model and analyse a single server queue's behaviour is a particularly usefulthing to do.
Poisson arrivals and service
M/M/1// represents a single server that has unlimited queue capacity and
infinite calling population, both arrivals and service are Poisson (or random)processes, meaning the statistical distribution of both the inter-arrival times and theservice times follow the exponential distribution. Because of the mathematical
nature of the exponential distribution, a number of quite simple relationships areable to be derived for several performance measures based on knowing the arrival
rate and service rate.
This is fortunate because, an M/M/1 queuing model can be used to approximate
many queuing situations.
Poisson arrivals and general service
M/G/1// represents a single server that has unlimited queue capacity andinfinite calling population, while the arrival is still Poisson process, meaning the
statistical distribution of the inter-arrival times still follow the exponential
distribution, the distribution of the service time does not. The distribution of the
service time may follow any general statistical distribution, not just exponential.
Relationships are still able to be derived for a (limited) number of performance
measures if one knows the arrival rate and the mean and variance of the service
rate. However the derivations a generally more complex.
A number of special cases of M/G/1 provide specific solutions that give broadinsights into the best model to choose for specific queueing situations because they
permit the comparison of those solutions to the performance of an M/M/1 model.
Multiple-servers queue
Multiple (identical)-servers queue situations are frequently encountered in
telecommunications or a customer service environment. When modelling these
situations care is needed to ensure that it is a multiple servers queue, not a network
of single server queues, because results may differ depending on how the queuing
model behaves.
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One observational insight provided by comparing queuing models is that a single
queue with multiple servers performs better than each server having their own
queue and that a single large pool of servers performs better than two or more
smaller pools, even though there are the same total number of servers in the
system.
One simple example to prove the above fact is as follows: Consider a system
having 8 input lines, single queue and 8 servers.The output line has a capacity of
64 kbit/s. Considering the arrival rate at each input as 2 packets/s. So, the total
arrival rate is 16 packets/s. With an average of 2000 bits per packet, the service
rate is 64 kbit/s/2000b = 32 packets/s. Hence, the average response time of thesystem is 1/(-) = 1/(32-16) = 0.0667 sec. Now, consider a second system with 8
queues, one for each server. Each of the 8 output lines has a capacity of 8 kbit/s.
The calculation yields the response time as 1/(-) = 1/(4-2) = 0.5 sec. And the
average waiting time in the queue in the first case is /(1 -) = 0.25, while in the
second case is 0.03125.
Infinitely many servers
While never exactly encountered in reality, an infinite-servers(e.g. M/M/) modelis a convenient theoretical model for situations that involve storage or delay, such
as parking lots, warehouses and even atomic transitions. In these models there is
no queue, as such, instead each arriving customerreceives service. When viewed
from the outside, the model appears to delay or store each customerfor some time.
Queueing System Classif ication
With Little's Theorem, we have developed some basic understanding of a queueing
system. To further our understanding we will have to dig deeper into
characteristics of a queueing system that impact its performance. For example,
queueing requirements of a restaurant will depend upon factors like:
How do customers arrive in the restaurant? Are customer arrivals more during
lunch and dinner time (a regular restaurant)? Or is the customer traffic more
uniformly distributed (a cafe)?
How much time do customers spend in the restaurant? Do customers typically
leave the restaurant in a fixed amount of time? Does the customer service time
vary with the type of customer?How many tables does the restaurant have for servicing customers?
The above three points correspond to the most important characteristics of aqueueing system. They are explained below:
Arrival Process The probability density distribution that determines
the customer arrivals in the system.
In a messaging system, this refers to the message
arrival probability distribution.
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Service Process The probability density distribution that determines
the customer service times in the system.
In a messaging system, this refers to the message
transmission time distribution. Since message
transmission is directly proportional to the length of
the message, this parameter indirectly refers to themessage length distribution.
Number of
Servers
Number of servers available to service the
customers.
In a messaging system, this refers to the number of
links between the source and destination nodes.
Based on the above characteristics, queueing systems can be classified by the
following convention:
A/S/n
Where A is the arrival process, S is the service process and n is the number of
servers. A and S are can be any of the following:
M (Markov) Exponential probability density
D (Deterministic) All customers have the same value
G (General) Any arbitrary probability distribution
Examples of queueing systems that can be defined with this convention are:
M/M/1: This is the simplest queueing system to analyze. Here the arrival and
service time are negative exponentially distributed (poisson process). The system
consists of only one server. This queueing system can be applied to a wide variety
of problems as any system with a very large number of independent customers can
be approximated as a Poisson process. Using a Poisson process for service time
however is not applicable in many applications and is only a crude approximation.Refer toM/M/1 Queueing Systemfor details.
M/D/n: Here the arrival process is poisson and the service time distribution isdeterministic. The system has n servers. (e.g. a ticket booking counter with n
cashiers.) Here the service time can be assumed to be same for all customers)
G/G/n: This is the most general queueing system where the arrival and service
time processes are both arbitrary. The system has n servers. No analytical solution
is known for this queueing system.
Markovian arrival processes
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In queuing theory, Markovian arrival processes are used to model the arrival
customers to queue.
Some of the most common include thePoisson process,Markovian arrival process
and the batch Markovian arrival process.
Markovian arrival processes has two processes. Acontinuous-time Markov process
j(t), a Markov process which is generated by a generator or rate matrix, Q. The
other process is a counting processN(t), which has state space
(where is the set of all natural numbers). N(t) increases every time there is a
transition inj(t) which marked.
Poisson process
ThePoissonarrivalprocessorPoisson processcounts the number of arrivals, each
of which has aexponentially distributedtime between arrival. In the most generalcase this can be represented by the rate matrix,
Markov arr ival process
The Markov arrival process(MAP) is a generalisation of the Poisson process by
having non-exponential distribution sojournbetween arrivals. The homogeneouscase has rate matrix,
Little's law
Inqueueing theory,Little's result, theorem, lemma, or lawsays:
The average number of customers in a stable system (over some time interval), N,
is equal to their average arrival rate, , multiplied by their average time in the
system, T, or:
Although it looks intuitively reasonable, it's a quite remarkable result, as it implies
that this behavior is entirely independent of any of the detailed probability
distributions involved, and hence requires no assumptions about the scheduleaccording to which customers arrive or are serviced, or whether they are served inthe order in which they arrive.
It is also a comparatively recent result - it was first proved by John Little, an
Institute Professorand the Chair of Management Science at theMIT Sloan School
of Management,in1961.
Handily his result applies to any system, and particularly, it applies to systems
within systems. So in a bank, the queue might be one subsystem, and each of the
tellers another subsystem, and Little's result could be applied to each one, as well
as the whole thing. The only requirement is that the system is stable -- it can't be in
some transition state such as just starting up or just shutting down.
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Mathematical formalization of L ittl e's theorem
Let (t) be to some system in the interval [0, t]. Let (t) be the number ofdepartures from the same system in the interval [0, t]. Both (t) and (t) are integer
valued increasing functions by their definition. Let Ttbe the mean time spent in the
system (during the interval [0, t]) for all the customers who were in the systemduring the interval [0, t]. Let Ntbe the mean number of customers in the system
over the duration of the interval [0, t].
If the following limits exist,
and, further, if = then Little's theorem holds, the limit
exists and is given by Little's theorem,
2.Explain the effects of congestion and the dffeent congestion control
methodologies in packet switching networks.
Ideal Performance
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2
Ef fects of Congestion
Congestion -Control Mechanisms
Backpressure
Request from destination to source to reduce rate
Useful only on a logical connection basis
Requires hop-by-hop flow control mechanism
Policing
Measuring and restricting packets as they enter the network
Choke packet
Specific message back to source
E.g., ICMP Source Quench
Implicit congestion signaling
Source detects congestion from transmission delays and lost packets and reduces
flow
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Expli cit congestion signal ing
Frame Relay reduces network overhead by implementing simple congestion-
notification mechanisms rather than explicit, per-virtual-circuit flow control.
Frame Relay typically is implemented on reliable network media, so data integrity
is not sacrificed because flow control can be left to higher-layer protocols. Frame
Relay implements two congestion-notification mechanisms:
Forward-explicit congestion notification (FECN)
Backward-explicit congestion notification (BECN)
FECN and BECN each is controlled by a single bit contained in the Frame Relay
frame header. The Frame Relay frame header also contains a Discard Eligibility
(DE) bit, which is used to identify less important traffic that can be dropped during
periods of congestion.
The FECN bit is part of the Address field in the Frame Relay frame header. The
FECN mechanism is initiated when a DTE device sends Frame Relay frames into
the network. If the network is congested, DCE devices (switches) set the value ofthe frames' FECN bit to 1. When the frames reach the destination DTE device, the
Address field (with the FECN bit set) indicates that the frame experienced
congestion in the path from source to destination. The DTE device can relay this
information to a higher-layer protocol for processing. Depending on theimplementation, flow control may be initiated, or the indication may be ignored.
TheBECN bitis part of the Address field in the Frame Relay frame header. DCE
devices set the value of the BECN bit to 1 in frames traveling in the opposite
direction of frames with their FECN bit set. This informs the receiving DTE device
that a particular path through the network is congested. The DTE device then can
relay this information to a higher-layer protocol for processing. Depending on the
implementation, flow-control may be initiated, or the indication may be ignored.
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Frame Relay Discard Eligibility
The Discard Eligibility (DE) bit is used to indicate that a frame has lowerimportance than other frames. The DE bit is part of the Address field in the Frame
Relay frame header.
DTE devices can set the value of the DE bit of a frame to 1 to indicate that the
frame has lower importance than other frames. When the network becomes
congested, DCE devices will discard frames with the DE bit set before discarding
those that do not. This reduces the likelihood of critical data being dropped by
Frame Relay DCE devices during periods of congestion.
Frame Relay Error Checking
Frame Relay uses a common error-checking mechanism known as the cyclic
redundancy check (CRC). The CRC compares two calculated values to determine
whether errors occurred during the transmission from source to destination. FrameRelay reduces network overhead by implementing error checking rather than error
correction. Frame Relay typically is implemented on reliable network media, so
data integrity is not sacrificed because error correction can be left to higher-layerprotocols running on top of Frame Relay.
3.Explain traffic management in detail
Traffic Management in Congested NetworkSome Considerations
Fairness Various flows should suffer equally
Last-in-first-discarded may not be fair
Quality of Service (QoS)
Flows treated differently, based on need
Voice, video: delay sensitive, loss insensitive
File transfer, mail: delay insensitive, loss sensitive
Interactive computing: delay and loss sensitive
Reservations
Policing: excess traffic discarded or handled on best-effort basis
4.Explain frame relay congestion control
Frame Relay Congestion Control
Minimize frame discard
Maintain QoS (per-connection bandwidth)
Minimize monopolization of network
Simple to implement, little overhead
Minimal additional network traffic
Resources distributed fairly
Limit spread of congestion
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Operate effectively regardless of flow
Have minimum impact other systems in network
Minimize variance in QoS
Congestion Avoidance with Explicit Signaling
Two general strategies considered:
Hypothesis 1: Congestion always occurs slowly, almost always at egress nodes
forward explicit congestion avoidance
Hypothesis 2: Congestion grows very quickly in internal nodes and requires quick
action backward explicit congestion avoidance
Explicit Signaling Response
Network Response
each frame handler monitors its queuing behavior and takes action
use FECN/BECN bits
some/all connections notified of congestion
User (end-system) Response
receipt of BECN/FECN bits in frame
BECN at sender: reduce transmission rate
FECN at receiver: notify peer (via LAPF or higher layer) to restrict flow
Frame Relay Traffic Rate Management Parameters
Committed Information Rate (CIR)
Average data rate in bits/second that the network agrees to support for a connection
Data Rate of User Access Channel (Access Rate)
Fixed rate link between user and network (for network access)
Committed Burst Size (Bc) Maximum data over an interval agreed to by network
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Excess Burst Size (Be)
Maximum data, above Bc, over an interval that network will attempt to transfer
Relationship of Congestion Parameters
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