Data Networks Second Edition

munchdrabNetworking and Communications

Oct 30, 2013 (3 years and 11 months ago)

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Data Networks

Second 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 ordering

e.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 r
s
(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 r
s

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 r
s

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

Circuit Switching


So




Note



Circuit Switching


Fig.1.5

Circuit Switching

Circuit Switching


Session for which are referred
to

bursty


sessions


For interactive terminal sessions,



Link costs become less important


wasted capacity of circuit switching:less
important

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
Mo
dulator and
dem
odulator) : 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