PRICIPLES OF TELECOMMUNICATION
AND PACKET NETWORKS MSc MODULE
EEM.ptn 2007-08
Lecture Component:
Packet Networks and Open Systems
Lectures 4-5
Prof. George Pavlou
Centre for Communication Systems Research
http://www.ee.surrey.ac.uk/CCSR/Networks/
G.Pavlou@surrey.ac.uk
Tel: 01483 689480
A2. 2
INTRODUCTION TO PACKET NETWORKS
AND OPEN SYSTEMS
A2. 3
INTRODUCTION
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Packet networks were introduced to connect computers together.
• Also referred to as computer or data networks.
„
Typical services offered are:
• Electronic mail, file transfer, login to remote computers, file access to
central server computers, printer sharing, remote database access,
electronic directory services, hypertext document access (the web),
etc.
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Multimedia services are also possible over packet networks,
“packetising” audio and video streams.
The multi-service networks of the future will be based on packet
technology.
Future: Higher speeds, QoS, mobility and multi-services
A2. 4
PACKET NETWORK ENVIRONMENT
Computer
B
Computer
A
Application
Process
Communication
Subsystem
User-to-user
communications
Computer-to-computer
communications
Computer-to-network
communications
Data Communications Network
A2. 5
Application
Process
Communication
Subsystem
TYPES OF PACKET NETWORKS
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Local Area Networks (LANs) - typically used inside a building or
buildings situated close together.
Metropolitan Area Networks (MANs) - employ LAN-like
technologies over distances of some kilometres e.g. in a town/city.
• LANs and MANs employ typically a “broadcast” technology i.e.
each transmission is received by all computers on the network.
Wide Area Networks (WANs) - from distances of several
kilometres upwards.
• WANs are typically “mesh” type networks i.e. nodes (switches /
routers) connected in a graph fashion with computers attached
to “edge” nodes.
A2. 6
PACKET NETWORK EXAMPLES (1)
LAN 1
LAN 2
Bridge
Bridge
Local / Metropolitan
Area Networks
Backbone MAN
Bridge
LAN 3
Bridge
LAN 4
A2. 7
PACKET NETWORK EXAMPLES (2)
Computer/Host
Core Node
Edge Node
Mesh-type Wide Area Network
A2. 8
STANDARDISATION BODIES
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International Standards organisation (ISO).
• National bodies contribute to ISO e.g. British Standards Institute
(BSI) in the UK, American National Standards Institute (ANSI) in
the US, etc.
Telecommunications:
• International Telecommunications Union - Telecommunications
Standardisation Sector (ITU-T).
ISO and ITU-T have aligned most of the Open Systems standards
Internet:
• Internet Engineering Task Force (IETF).
Others:
• Institute of Electrical and Electronic Engineers (IEEE).
• ...
A2. 9
PACKET NETWORK STANDARDISATION
„
Packet network standards come mainly from two different sources,
which express different philosophies.
• ISO / ITU-T have produced standards based on the Open Systems
Interconnection (OSI) reference model; these standards adhere to a
“telecommunications culture” and put emphasis on Quality of Service
(QoS) - X.25, Frame Relay and ATM are relevant network technologies.
• The Internet Engineering Task Force (IETF) produces “standards” for the
Internet, which provided initially a best-effort service though efforts to add
QoS are under way - the Internet Protocol (IP) is the key network
technology.
A2. 10
THE ISO/ITU-T OSI REFERENCE MODEL
Switching Node
Computer B
PDU: Protocol Data Unit
Process B
Computer A
Process A
Application PDU (APDU)
Application layer (7)
Presentation layer (6)
Session layer (5)
Transport layer (4)
Network layer (3)
Link layer (2)
Physical layer (1)
Presentation PDU (PPDU)
Session PDU (SPDU)
Transport PDU (TPDU)
Packets
Frames
Bits
Network layer
Link layer
Physical layer
Packets
Frames
Bits
Physical Data flow
Communications channel
A2. 11
Application layer (7)
Presentation layer (6)
Session layer (5)
Transport layer (4)
Network layer (3)
Link layer (2)
Physical layer (1)
SOME CLARIFICATIONS
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Switching/Routing Nodes run only layers 1-3 of the OSI-RM
model.
There are two ways to organise internally the network layer,
which are both supported by the OSI model:
• connection-oriented, reliable.
• connectionless, unreliable.
As we move from layer 1 upwards, the various layers are
implemented in the following fashion:
• hardware.
• firmware.
• software running in the operating system kernel e.g. the
network and transport layers.
• software linked with the applications e.g. the upper layers.
A2. 12
COMPLETE OSI PDU STRUCTURE
LPDU
NPDU
TPDU
SPDU
PPDU
APDU
LH NH TH SH PH AH
LH: Link Header
NH: Network Header
TH: Transport Header
SH: Session Header
PH: Presentation Header
Message
FCS
FCS: Frame Check Sequence
(link trailer)
A2. 13
OSI-RM LAYER FUNCTIONALITY (1)
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Physical layer (1):
• modulation/demodulation.
Link layer(2):
• framing and transparency (bit stuffing).
• connection management, error and flow control
(in connection-oriented reliable service).
• medium access control (MAC) for shared media, e.g. LANs,
wireless – MAC is the lower part sub-layer of the link layer
Network layer (3):
• network addressing and routing.
• connection management, error / flow control
(in connection-oriented service).
Transport layer (4):
• end-to-end TPDU transfer, fragmentation.
• connection management, error control and flow control
(in connection-oriented reliable service).
A2. 14
OSI-RM LAYER FUNCTIONALITY (2)
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Session Layer (5):
• dialog management for half-duplex services.
• synchronisation control for checkpointing and recovery.
Presentation Layer (6):
• data structure encoding/decoding.
• data compression and security.
Application Layer (7):
• a set of generic Application Service Elements supporting
applications.
• specific applications such as file transfer, e-mail, directory
services, network management, remote database access, etc.
Layer 3 has both connection-oriented (VC) and connectionless
(datagram) modes of operation while higher-layers are
predominantly connection-oriented targeting data services
A2. 15
SOME OSI PROTOCOLS
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Link Layer: HDLC + variations, LLC.
Link/Network Layer: Frame Relay, ATM.
Network Layer: X.25, CLNP, ES-IS / IS-IS routing.
Transport Layer: COTP classes 0 to 4.
Session Layer: OSI Session Protocol.
Presentation Layer: OSI Presentation Protocol, ASN.1, BER.
Application Layer: FTAM, X.400 Mail, X.500 Directory, X.700
Management.
A2. 16
THE INTERNET REFERENCE MODEL
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There is no Internet RM in the same fashion as the OSI-RM but we can
retrofit the Internet protocols to a “reference model”.
It would be similar to the OSI-RM but with 5 layers:
• no session and presentation layers, such functionality is provided
when needed directly by applications.
The equivalent to the Link Layer is called Host-to-Network or Network
Access layer.
The Physical Layer exists in the sense that bits need to be transmitted
over a physical medium but there are no Internet physical layer
specifications.
The Network Layer has adopted a connectionless only approach
through the Internet Protocol (IP).
The Transport and Application Layers support both connection-oriented
and connectionless modes of operation.
•
Connectionless transport and application services without error and flow
control are good for real-time packetised audio and video.
A2. 17
THE INTERNET REFERENCE MODEL (cont’d)
Application
Program
Application
Program
Application
Application packets
Application
Transport
Segments
Transport
Network
Datagrams
Network
Datagrams
Network
Network Access
Frames
Network Access
Frames
Network Access
Bits
Physical
Physical
Host
Router
A2. 18
Bits
Physical
Host
LAYERING PRINCIPLES (1)
Open
System A
Open
System B
Open
System C
•
•
•
•
•
Highest Layer
(N+1)–Layer
(N)–Layer
(N-1)–Layer
(N)–Layer
(N)–Service
(N)–Service Access Point
(N)–Protocol
(N)–Protocol Data Unit
Lowest Layer
Physical Media
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Each layer has a well defined interface to the layer above and below.
Peer-to-peer protocol: Is a set of rules to exchange information
between corresponding layers.
A2. 19
LAYERING PRINCIPLES (2)
User
Service Layer
Correspondent
User
CON.req
User
(N+1)–Layer
Req
(N)–Layer
Conf
Service
Entity
Service
Provided
Service
Used
Correspondent
User
Resp
Ind
CON.ind
CON.resp
CON.conf
Peer Service
Entity
Time
DATA.req
DATA.ind
DATA.resp
(N-1)–Layer
DATA.conf
PDUs
DISC.req
DISC.ind
A2. 20