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ELEC3030 (EL336) Computer Networks
S Chen
ELEC3030 Computer Networks
Professor Sheng Chen:
Building 86
,^ Room 1021
E-mail: sqc@ecs.soton.ac.ukDownload lecture slides from: http://www.ecs.soton.ac.uk/
∼sqc/EL336/
or get them from Course Office (
ECS Student Services
Reading Text
:^ A.S. Tanenbaum,
Computer Networks
, 4th edition, Prentice Hall,
- 2nd Reading Text
:^ L. Chappell and E. Tittel,
Guide to TCP/IP
, 2nd edition,
Thomson, 2004.I will cover half of the unit, including
-^ Introduction
Physical layer
-^ Data link layer -^ Network layer
S Chen
Overview
-^ Two most important aspects of computer networks are
hardware
and^ software
-^ Hardware:
as communication is a primary concern in a network, we are dealing with both computers and
communication technologies
-^ In terms of scale, historically computer networks are classified as^ – Local area networks
(LAN): within room, building or campus, and size from
10 m up to a few km – Metropolitan area networks
(MAN): size in tens km and may cover a city
(WAN): within a country or even whole continent, and
size from 10 km to 100 km – Internetworks
: deal with how to connect different kinds of networks
→^ resulting
the Internet which really covers the whole Planet • Software: what actually makes computer networks is software. With “software” weare not talking computer codes, but design thinking,
methodology
and^ framework
S Chen
The Hardware
-^ Transmission technology: there are two basic methods^ – Broadcast
: transmission is broadcasted to and received by all, as in broadcast networks such as packet radio, satellite and LANs – Point-to-point
:^ transmission goes from sender to receiver possibly via some intermediate switching nodes, as in switched networks such as WANs and Internet • Are wireless networks
WIFI
broadcast or point-to-point link based?
-^ LANs: use broadcast transmission technologyTwo typical topologies for LANs are bus andtoken ring
Cable^
Computer(b)
Computer (a)
-^ MANs: are bigger versions of LANsTwo examples are the distributed queue, dualbus on right and air interface for fixed broadbandwireless access system (IEEE 802.16)
WMAX
1
2
3
N
Bus A
Direction of flow on bus B
Head end Direction of flow on bus A Computer^ Bus B
...
S Chen
The Hardware (continue)
-^ LANs and MANs do not have any
switching
nodes: the wire (or wireless) does all
the work
→^ This makes them extremely efficient but harder to get bigger
-^ WANs: hosts are connected to a subnet, whichcontains routers (switching nodes) and trunksRouters generally adopt a store-and-forward(packet-switched) principle
Subnet^
Router
Host LAN
-^ Internetwork: many networks exist with very different hardware and software, andinterconnecting different networks is called internetwork^ –^ Connect a collection of different LANs within a department^ –^ Connect different LANs through a WAN, with WAN acting as a subnet^ –^ Connect different WANs to each other by means of gateways
→^ the Internet
-^ Subnet
: collection of routers and trunks;
Network
: combination of a subnet and
its hosts;
Internetwork
: distinct networks interconnected together
S Chen
Services
-^ There are two different types of services:
connection-oriented
and^ connectionless
:^ the user first establishes a connection, then uses the connection to
communicate, finally releases the connection (think this as using telephone) – Connectionless
: each data unit has the complete destination address and is routed through the network to the destination independently (think this as posting a letter) • Each service offers certain
quality
: e.g. whether ordered delivery and how reliable
-^ A^ service
is^ specified
by^ a
set^ of
primitives
(operations) available to a user to access the service^ –^ Four classes of service primitives are:
request
entity wants the service to do some work indication
entity is to be informed about an event response
entity wants to response to an event confirm
response to earlier request has come back
-^ Consider a simplest connection-oriented service with 8 primitives^ 1.^
CONNECT.request – request a connection to be established
(dial a phone number)
2.^ CONNECT.indication – signal the called party
(phone rings)
3.^ CONNECT.response – used by the callee to accept/reject calls
(pick up the phone)
4.^ CONNECT.confirm – tell the caller whether the call was accepted
(caller hears ringing stop)
5.^ DATA.request – request that data be sent
(say something)
6.^ DATA.indication – signal the arrival of data
(callee hears you)
7.^ DISCONNECT.request – request that a connection be released
(caller hangs up)
8.^ DISCONNECT.indication – signal the release of the connection
(callee hears busy tone)
S Chen
OSI 7-layer Reference Model
-^ Recall
that
software
makes
computer networks and layeredstructure is a basic principle inorganising
computer
network
software • In^ design,
one^
would^
like^ to
have a
framework
for dividing network
software
into^
several
layers, defining what each layerdoes,^
and^ finally
providing
implementation
specifications →^ This kind of framework iscalled^ reference model • A well known example is the OSI7-layer reference model specifiedby ISO
Layer^ Application^7 InterfacePresentation^6 Interface^5 Session Transport^43 Network Data link^2 Physical^1 Host A
Name of unitexchangedAPDU PPDU SPDU TPDU Packet Frame Bit ApplicationPresentationSession Transport Network Data link PhysicalHost B
Network^
Network Data link^
Data link Physical^
Physical Router^
Router Application protocolPresentation protocolSession protocolTransport protocolInternal subnet protocol Communication subnet boundary Network layer host-router protocolData link layer host-router protocolPhysical layer host-router protocol
S Chen
TCP/IP Reference Model
-^ This is where Internet started: used to be a wild cowboy’s world but now is better standardised1.^ Application layer
: does similar things as OSI application layer2. Transport layer
:^ does similar things as OSI transport layerTwo end-to-end protocols are defined:TCP – transmission control protocol(for reliable connection-oriented) andUDP – user datagram protocol (forunreliable connectionless)3. Internet layer
: similar in functionality to OSI network layer
TCP/IP OSI ApplicationPresentationSessionTransportNetworkData linkPhysical 7 6 5 4 3 2 1
Application TransportInternetHost-to-network
Not presentin the model
4.^ Host-to-network layer
: anything below the internet layer, not very well defined
-^ Two reference models represent two different views of the world:
telecommunication camp and
computer camp. OSI camp views the world (i.e. the network) as rigid, well defined and organised,TCP/IP camp historically views the world as hostile and chaotic New standards
are now often defined with
best^ of
both^
reference models
S Chen
Comparison of OSI and TCP/IP Models
-^ Three important concepts,
services
,^ interfaces
and^ protocols
, are well defined in
OSI model, but not in original TCP/IP model • Transport
layer does the hardest job, dealing with end-to-end “connection”
-^ Data link
layer is also very important:
An end-to-end “connection” consists of
many “links”, and each possibly noisy link need to be made reliable • On the other hand, no one really knows precisely what
session
layer does
-^ It may also be argued that a separate
presentation
layer is not strictly necessary
-^ Based on comparison, we will adoptthe hybrid
5-layer reference model
This^
is^ a^
good^
framework
for
discussion of computer networks
5 Application layer 4 Transport layer 3 Network layer 2 Data link layer 1 Physical layer
