Transcript Data Networks Second Edition

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)
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Inside subnet, nodes & communication
links.
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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)
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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
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In the future, data network, voice
networks, cable networks will be
integrated more.
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ISDN(integrated services digital network)
Broadband ISDN : greater data rates.
Section 1.2
Messages and Switching
1.2.1 Messages and Packets
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message:
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Airline reservation system : data, flight
no# …
Email : document
File system : file
Image transmission system : image
…
1.2.1 Messages and Packets
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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
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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
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In larger transaction : a message
sequence is called a session.
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Requires many messages over a
considerable time period.
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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
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Messages within a session are triggered
by events.
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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
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Detailed characteristics for
applications.
1.
2.
3.
4.
Message arrival rate and variability of
arrivals.
Session holding time.
Expected message length and length
distribution.
Allowable delay : 10ms ~ 1ms
Sessions
5.
6.
Reliability : error-free 、 occasional loss.
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
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Circuit switching
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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
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Each communication link on this path allocates
a portion of rs of its total transmission
capacity.(done by TDM or FDM multiplexing)
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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
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Why inefficient?
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:message arrival rate
1
:expected
interarrival
rate b/w
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1
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messages for a given session
x :expected tx time of a message
L :expected length of messages
Circuit Switching
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So
x  L / rs
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Note
x to 1/   fractionof timeto S is busy
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 x  1
, idle time 
Circuit Switching
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Fig.1.5
Circuit Switching
If allowable expecteddelay T
x  P  T ; x: tx time, P: Propagation delay
 λx  λ T
If λ T  1 (i.e. T  1/λ )
Utilization λ x 
.e.
i rs must be large enough tomeet
allowable delay
Circuit Switching
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Session for which T  1 are referred
to “bursty” sessions
For interactive terminal sessions,
T  0.01,  utilization : 1%
Link costs become less important
wasted capacity of circuit switching:less
important
Circuit Switching and Storeand-Forward Switching
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Store-and-forward Switching
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Without making reservation/allocation
Using full transmission rate of the link on
packet/message basis
Advantage
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Fully utilized , whenever has traffic to send
Disadvantage
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Queuing delay,hard to control,overloaded
nodes => need to be slowed down
Store-and-Forward Switching
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Message Switching
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Store-and-forwarding,messages basis
Packet Switching
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Store-and-forwarding,packets basis
Store-and-Forward Switching
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Virtual Circuit routing
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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
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Store-and-forwarding,packets find its own
path according to current information
available at nodes visited
Section 1.3
Layering
Laying
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Hierarchical modularity
Laying(Fig 1.7)
Laying
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OSI(Open System Interconnection)
model by ISO(International Standards
Organization)
Laying(Fig 1.8)
1.3.1 Physical Layer
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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
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At Rx end,map signals back to bits
Modem(digital data Modulator and
demodulator) : broadly referred here
Physical Layer
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compare
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Synchronous bit pipe
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Intermittent synchronous bit pipe
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1bit per t second
DLC module supplies bits at a synchronous rate when
has data
Asynchronous characters
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Map into fixed-length bit strings and transmitted
asynchronously as they are generated
Physical Layer
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Interface between DLC
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Module on one end might be temporarily
inoperable
Some initialization is required
For synchronous operation , must provide
timing
Physical Layer
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RS-232-C & physical layer of X.21
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DCE: Data Communication Equipment
DTE: Data Terminal Equipment
DTE sends a signal to DCE “request-tosend”.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
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To convert unreliable bit pipe at layer 1 into
higher-layer
Sending packets asynchronously but errorfree
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Variable delay b/w packet into DLC exit from the
other end
Need to correct errors
Overhead control bits
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Header
trailer
Data Link Control (DLC) Layer
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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
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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
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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
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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
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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
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For datagram network,each packet is
routed individually
Service offered
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Using VC
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Packet in order,connection - oriented
Using datagram
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Packets out of order,connectionless service
Network Layer
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Flow control
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Congestion control
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Avoid sending data too fast
Avoid congestion within subnet
Solution
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Good route
Good buffer management
Control flow of packet into network s.t. congestion
control
Network Layer
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Connection – oriented service
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Possible to negotiate => guarantee service
at setup
Connectionless service
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No opportunity for negotiate
Network Layer
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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
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e.g. malfunctioning user could dump enough data
into network quickly to cause congestion => still
need some regulatry rule
Network Layer
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Routing & flow control
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Primarily for WAN
For LAN , routing is not a major
problem,congestion is possible
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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
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Note
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Network layer delivering every packet may
be reliable or might be unreliable.
The higher layer might have to recover
errors
Internet Sublayer
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To connect different subnetworks together
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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
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Bridges interface at DLC layer.For LAN , routing &
flow control are done in MAC.
1.3.4 Transport layer
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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
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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
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Data encryption
Data compression
Code conversion
1.3.7 Application layer
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Consideration variation in service
offered by various layers
Sometimes , not conform to OSI model
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e.g.ATM ( Asynchronous Transfer Mode ) ,
broadband ISDN
Section 1.4
A Simple Distributed Algorithm
problem