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