Data Networks Second Edition Dimitri Bertsekas / Robert Gallager

Data NetworksSecond Edition

Dimitri Bertsekas / Robert Gallager

Chapter 1

Introduction and Layered Network Architecture

Section 1.1

Historical Overview

1.1 Historical Overview

Historical Overview(60’s)

Historical Overview(70’s)

Historical Overview(70’s) Inside subnet, nodes &

communication links. IMP(Interface message processors) :

to route message through subnets. – called also switches.

wide area network local area network

Historical Overview(80’s)

Historical Overview(80’s) 1980‘s, more and more networks

connected via gateways and bridges

Each subnet has its own conventions and control algorithms (protocols) for handling data – gateways and bridges must deal with this inhomogeneity.

Historical Overview In the future, data network, voice

networks, cable networks will be integrated more. ISDN(integrated services digital

network) Broadband ISDN : greater data rates.

Section 1.2

Messages and Switching

1.2.1 Messages and Packets

message: Airline reservation system : data,

flight no# … Email : document File system : file Image transmission system : image …

1.2.1 Messages and Packets

A message is usually a string of binary symbols, 0 or 1 (bit).

Sender → message → recipient

compression Compression can reduce expected

length of representation.

Messages and Packets Control overhead : ensure reliable

communication route control congestion, etc.

Usually broken into shorter bit strings(packets)※ transmit long messages is harmful ,

(e.g. delay, buffer management, congestion control)

1.2.2 Sessions In larger transaction : a message

sequence is called a session. Requires many messages over a

considerable time period. Setup procedure(similar to setting up a

call) A connection

In other networks, no such setup is required. Each message is treated independently → connectionless

Sessions Messages within a session are

triggered by events. Message initiation times are arbitrary,

unpredictable. Model messages / packets arrival for

a given session as a random process. Poisson process On/Off flow model Application are rapidly changing →

model complex

Sessions Detailed characteristics for

applications.1. Message arrival rate and variability

of arrivals.2. Session holding time.3. Expected message length and

length distribution.4. Allowable delay : 10ms ~ 1ms

Sessions

5. Reliability : error-free 、 occasional loss.

6. Message and packet orderinge.g. file transfer : message arrival rate :

10w delay requirement

: relaxed reliability : high

1.2.3 Circuit Switching and Store-and-Forward Switching Circuit switching

Inefficient utilization When a session s is initialed,

allocated a given transmission rate rs

(bits per second) A path is created from transmitting

site through the subnet to destination site.

Circuit switching Each communication link on this path

allocates a portion of rs of its total transmission capacity.(done by TDM or FDM multiplexing)

Note : sum of rates for all sessions cannot exceed total capacity of links, otherwise, new session is rejected.

Guaranteed transmission rate rs similar to telephone network.

But, in a data network, required transmission rates are different and vary over a wide range

Circuit Switching Why inefficient?

:message arrival rate :expected interarrival rate b/w

messages for a given session :expected tx time of a message :expected length of messages

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Circuit Switching and Store-and-Forward

Switching Store-and-forward Switching

Without making reservation/allocation Using full transmission rate of the link

on packet/message basis Advantage

Fully utilized , whenever has traffic to send

Disadvantage Queuing delay,hard to control,overloaded

nodes => need to be slowed down

Store-and-Forward Switching

Message Switching Store-and-forwarding,messages basis

Packet Switching Store-and-forwarding,packets basis

Store-and-Forward Switching

Virtual Circuit routing Store-and-forwarding,but a particular

path is set up when a session initiated using a fixed path

Capacity is allocated on a demand basis

Dynamic routing Store-and-forwarding,packets find its

own path according to current information available at nodes visited

Section 1.3

Layering

Laying Hierarchical modularity

Laying(Fig 1.7)

Laying OSI(Open System Interconnection)

model by ISO(International Standards Organization)

Laying(Fig 1.8)

1.3.1 Physical Layer Provide a virtual link for transmitting

a sequence of bits between any pair of nodes

Map incoming bits from the next higher layer into signals for the channel At Rx end,map signals back to bits

Modem(digital data Modulator and demodulator) : broadly referred here

Physical Layer compare

Synchronous bit pipe 1bit per t second

Intermittent synchronous bit pipe DLC module supplies bits at a synchronous rate

when has data Asynchronous characters

Map into fixed-length bit strings and transmitted asynchronously as they are generated

Physical Layer Interface between DLC

Module on one end might be temporarily inoperable

Some initialization is required For synchronous operation , must

provide timing

Physical Layer RS-232-C & physical layer of X.21

DCE:Data Communication Equipment DTE: Data Terminal Equipment

DTE sends a signal to DCE “request-to-send”.DCE replies with “clear-to-send”

The interchange is a very simple example of a protocol or distributed algorithm

1.3.2 Data Link Control (DLC) Layer

To convert unreliable bit pipe at layer 1 into higher-layer

Sending packets asynchronously but error-free Variable delay b/w packet into DLC exit from

the other end Need to correct errors

Overhead control bits Header trailer

Data Link Control (DLC) Layer

Some request retransmissions when error occur

For some LAN,multi-access may take place.

The signal received is a function of the signals from a multiplicity of transmitting nodes

MAC(Medium Access Control) Sublayer

Considered as lower sublayer of layer 2

Allocate multi-access channel , so that each node can successfully transmit its frame without interference from other nodes

MAC(Fig 1.10)

1.3.3 Network Layer Implementing routing and flow

control for its network Use packet header along with

stored information to accomplish these functions

Transport layer also provides additional information as a set of parameters in accordance with interface protocol

Network Layer(Fig 1.11)

Network Layer Along with transmit packets from lower

layer and new packets from higher layer,the network layer generates its own control packets

For virtual circuit routing Select a route when VC being

generated(distributed way or by source node)

Ensure each packet of the session follows the assigned route(by placing enough information in the header)

Network Layer For datagram network,each packet

is routed individually Service offered

Using VC Packet in order,connection - oriented

Using datagram Packets out of order,connectionless

service

Network Layer Flow control

Avoid sending data too fast Congestion control

Avoid congestion within subnet Solution

Good route Good buffer management Control flow of packet into network s.t.

congestion control

Network Layer Connection – oriented service

Possible to negotiate => guarantee service at setup

Connectionless service No opportunity for negotiate

Network Layer High link capacities in the future will make

it possible to operate network economically with low utilization and make flow control unnecessary

Unfortunately,as link capacity increase,access rate into networks also increase e.g. malfunctioning user could dump enough

data into network quickly to cause congestion => still need some regulatry rule

Network Layer Routing & flow control

Primarily for WAN For LAN , routing is not a major

problem,congestion is possible Could be dealt with in MAC sublayer Major functions of network layer are

accomplished in MAC sublayer. Thus,connectionless service is common

here.

Network Layer Note

Network layer delivering every packet may be reliable or might be unreliable.

The higher layer might have to recover errors

Internet Sublayer To connect different subnetworks together

Solution: create a new sublayer:internet sublayer top part of network layer

A gateway connecting two subnets will interface

Internet modules also : routing & flow control

Note Bridges interface at DLC layer.For LAN , routing

& flow control are done in MAC.

1.3.4 Transport layer Break messages into packets , and

reassembles packets Might multiplex several low-rate

sessions all from same source and going to same destination

Might split one high-rate session into multiple sessions(flow control)

If network layer is unreliable,achieve reliable end-to-end connection

End-to-end flow control

1.3.5 Session layer Provide transport layer with

information needed to establish the session

Achieve load sharing b/w many processors

Access rights in setting up sessions Who pay for the service Handle interaction b/w 2 end points

1.3.6 Presentation layer Data encryption Data compression Code conversion

1.3.7 Application layer Consideration variation in service

offered by various layers Sometimes , not conform to OSI

model e.g.ATM ( Asynchronous Transfer

Mode ) , broadband ISDN

Section 1.4

A Simple Distributed Algorithm problem