2021/22 COMP3234B & ELEC3443B
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Introduces basic terminologies and concepts
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Describes the building blocks that composite nowadays networks
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Introduces the idea of a network architecture
Computer and Communication Networks
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[ILO1 - Concepts] be able to explain the terminologies of computer
networks; be able to describe the services and functions provided by
each layer in the Internet protocol stack
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Computer and Communication Networks
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Recommended Readings
 Chapter 1 of Computer Networking – A Top-Down Approach Featuring the Internet, 7th
edition by J. Kurose et. al
 Sections 1.1, 1.5
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A communication service enables the exchange of information
between users at different locations.
 Communication services & applications are everywhere.

Web browsing
File transfer
E-mail
Smart
device
Information
feed
server
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A communication service enables the exchange of information
between users at different locations.
 Communication services & applications are everywhere.

Internet phone
P2P
Messenger
Network games
Video meeting
smart
device
smart
device
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Networking is about transmitting messages from senders (sources) to
receivers (sinks) over a communication network.

A communication network is a set of devices connected by
communication links, which may be constructed out of different
physical media
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A device on the network
 Sometimes, we refer a device as a node in the network
 Broadly speaking, we have two kinds of devices/nodes
 End-systems devices and we usually call them - the hosts
 Connecting devices
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End-systems or hosts
 These are the smart computing devices in which network applications are running
 Nowadays, we have billions of end-systems attached to the Internet

Communication Link
 Refer to the physical media by which messages are moving around
 There are many types of communication link
 Point-to-point link – physical link connects up only a pair of nodes
 guided media – Wire (copper, optic fiber, . . )
 unguided media – Wireless (Satellite, . . )
 multiple access link – allows more than two nodes sharing a single physical link and everyone
attached to the links sees the messages
 (Fast) Ethernet, Token ring, Fibre Channel
 WiFi, Bluetooth
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It is not possible to have every end-system directly connect to each other
by a dedicated communication link

End-systems are connected together by a network of communication links
and connecting devices

 These nodes – connecting devices – are attached to at least two links and run software that
forwards data received on one link out on another link

We call this set of links and nodes
- Switched networks
 Two types
 Circuit-switched networks
 Packet-switched networks
 Datagram packet-switched networks
 Virtual circuit packet-switched networks
Computer and Communication Networks
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A network can be defined recursively as two or more networks
connected by connecting devices

 A set of independent networks are interconnected to form an internetwork
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Internet is an extremely complex system
 Internet consists of many pieces




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Nodes: Computers, laptops, mobiles, switches, routers . .
Technologies: Ethernet, FC, WiFi, Bluetooth, WiMAX, 4G, 5G, . . .
Applications: Email, BT, WWW, FTP, Whatsapp, Skype, Youtube, . . .
Protocols: HTTP, SMTP, DNS, FTP, SSH, TCP, IP, STCP, RTP, SIP, RIP, OSPF, . . .
Is there any way of organizing or structuring such a complex system?
 Modern networks are structured in a layer design
 Each layer defines a collection of conceptually similar functions / services usually distinct
from those of the other layers
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Layer i+1
Layer i
Layer i-1
A layered architecture enables us to divide a large and complex system into
several smaller and manageable parts

 So would be easy for people to focus on a well-defined, specific part of the whole system
 Each layer provides some services to the layer above and uses the services from the layer just
below
 Each layer uses abstractions to hide the complexity (of design and implementation)
 Separate the services (functions) from the implementation
 This simplifies design, implementation, and testing as well as modifying and evolving
 Services in each layer can be designed separately from those in other layers
 Can have multiple services (abstractions) at each layer
 Each abstraction provides a different service to higher layers but building on the same low-level
abstractions
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Protocol
 Is the realization of a service (abstraction) of a layer
 Each layer of the layered architecture contains one or more protocols that offer services
characteristics for that layer

The Internet and computer networks make extensive use of protocols
 Different protocols are used to accomplish different communication tasks
“Mastering the field of computer networking is equivalent to understanding the what,
why, and how of networking protocols.”
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Each protocol has two different interfaces
 service interface
 operations or services that this protocol offers to the layer above
 peer-to-peer interface
 To implement the communication service, a set of rules that defines
 the message format in the message exchanged with peer of the same layer
 order of messages sent and received among peers (of the same layer)
 actions taken on messages transmission and/or receipt of messages
service
interface
•
•
•
•
Layer K+1
Layer K+1
Layer K
peer-to-peer interface
Layer K
Layer K-1
Layer K-1
•
•
•
•
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
Most “peer-to-peer” communication is “indirect”
 The messages exchanged between two peers of a protocol is carried by or passed to lower-level
protocol
A low-level protocol at level N does not interpret/understand the content
of messages given by a high-level protocol at level N+1

 It just view the message passed from upper layer as “data”
 We call this the payload of the protocol N
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Encapsulation
 Each protocol has the payload field that carries the data message passed from the layer above
 Each protocol adds a header (and/or trailer) that carries protocol specific information
 addresses, protocol numbers, length, etc. . .
 will be used by the receiving-side peer protocol object
 Header + Payload – that forms the Protocol Data Unit (PDU) of this protocol layer (and will pass
this PDU to immediate lower-level layer)
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We call the PDU of application layer – Message;
the PDU of transport layer – Segment;
the PDU of network layer – Datagram;
the PDU of link layer – Frame.
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Protocol Stack
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Internet Protocol Stack/Suite
 Protocols of various layers of the network making up the network architecture
 Defined by Internet Engineering Task Force (IETF)
 Hourglass Design
Application
Network applications run on this layer.
• HTTP, SMTP, DNS, FTP, RTP, SNMP, Telnet, MSN
Transport
Support transferring of data between endpoints (processes)
• TCP, UDP, SCTP, DCCP, QUIC
Network
Routing of datagrams from source (device) to destination (device)
• IP (and routing protocols)
Link
Physical
Data transfer between directly-linked (adjacent) nodes
• WiFi, Ethernet, DSL, PPP, FDDI, Bluetooth
Bits on the wire or physical media
Computer and Communication Networks
Defined as
one layer in
RFC 1122
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Purpose of the OSI reference model was to
provide a framework for the development of
protocols
 Physical:
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 Definition & specification of the physical aspects of a
communications link
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Session:
(not that clearly defined)
 Define the dialogue discipline, e.g., half-duplex mode,
etc.
 Support the recovery of transport-level connection if
failure
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Presentation: (not that clearly defined)
 Provides services to allow applications to interpret
meaning of data
Network:
 Routing and Forwarding.
 Internetworking is part of network layer and consider
about how to transfer packets across multiple
possibly dissimilar networks
Transport:
 Transfers data between end-points. Reliable stream
transfer or quick-and-simple (no guarantee) singleblock transfer.
Data link:
 Groups bits into frames.
 Error detection and control to improve reliability.
 Define medium access control for multiple-access
network
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
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Data formats and coding. e.g. compression, encryption.
Application:
 Provides specific functions related to an application
service
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A network architecture is the layered structure of hardware and
software that supports the exchange of data between end systems.

At each layer of the network architecture, one or more common
protocols are implemented. Each protocol provides a set of rules for the
exchange of data between systems.
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