Introduction to Communications and Networks

ECI-2-830

Dr. Vincent Siyau

Text Books & References• Text Books (Essential)

– Data Communications: From Basics to Broadband, 3rd Edition by William J. Beyda

or– Data Communications and Networking, 3rd Edition,

Behrouz A. Forouzan

• Reference Books (not required)– Computer Networks and Internets, with Internet Applications, 4th

Edition. Douglas E. Comer & Ralph E. Droms, 2004 by Prentice Hall.

– Data and Computer Communications, 7th Edition, William Stallings, by Prentice Hall.

Lecture Notes and Tutorials

• Please check the blackboard (BB) regularly for any updates regarding the ICN lectures week by week.

• ALL Lecture Notes are available in the BB. (in Document)• Pay attention and follow closely (Save your revision time)• Very often exam questions are revealed in the lectures!

• Sometimes, tutorials will be mixed within the lectures or immediately available after each lectures to help you to refresh and revise the important parts of each lecture.

• Sometimes, mini-test & quizzes will be given to help your understanding in this subject. So, do your revision frequently!

The OSI Model

Established in 1947, the International Standards Established in 1947, the International Standards Organization (Organization (ISOISO) is a multinational body dedicated to ) is a multinational body dedicated to worldwide agreement on international standards. worldwide agreement on international standards.

An ISO standard that covers all aspects of network An ISO standard that covers all aspects of network communications is the communications is the Open Systems InterconnectionOpen Systems Interconnection ((OSIOSI) model. ) model.

Layered Architecture based on services providedPeer-to-Peer ProcessesTransparencyEncapsulation

OSI MODEL (1)

OSI MODEL (2)• The role of the OSI model is to offer a design

template to enhance the development of network protocols and technologies.

• Each layer of the model represents a subset of the requirements for reliable source (application) to destination (application) delivery of the user message

• The process of data exchange over a network is viewed as operations managed by nodes that ‘talk’ to each other over passive links. The links are not included in the model.

OSI MODEL (3)• The analysis of the data exchange is based on identifying

services that the network must provide at each stage of the exchange.

• The ISO has identified seven groupings of services that can constitute the layers of the model.

• Each layer to layer interface of the OSI model is precisely defined. This allows freedom to developers to evolve how the layers offer the specified services. The layers are in this sense independent.

• There are many parallels between networking in other areas such as the postal service and this my be helpful on occasion to illustrate telecommunication behaviour)

Seven layers of the OSI model

{

{

Application Layers

Network Layers

7 User interface

6 Common format

5 Manages Dialogue

4 Reliable message Delivery

3 Unreliable Source to destination delivery

2 Reliable link delivery

1 Data to signal

The interaction between layers in the OSI model

An exchange using the OSI model

SegmentSegment

DatagramDatagram

FrameFrame

PDU Protocol Data UnitPDU Protocol Data Unit

SDU Service Data UnitSDU Service Data Unit

OSI layer Summary

Physical layer

Signal

The physical layer:

Defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.

Define characteristics such as type of encoding or modulation, voltage levels, synchronisation, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors.

Data link layer

The data link layer provides framing which includes :

•Reliable transit of data across a physical network link.

•Physical addressing

•Error Control

•Sequencing of frames

•Flow control.

Hop-to-hop delivery –Role of framing

Network layer

The network layer:

Defines the network (logical) address. This is used for source network layer to destination network layer delivery.

This delivery is connectionless and unreliable. Although IP, which is a network layer protocol offers ICMP - which gives the source notification when delivery is not possible.

Source-to-destination delivery using the network address

Transport layer

The transport layer:

Segments the data for transport across the network.

Responsible for making sure that the message is delivered error-free - reliable.

Reassembles data using sequence numbers – connection oriented.

Flow control generally occurs at the transport layer.

Reliable process-to-process delivery of a message

Network Address

Application Address – port number

Session layer

Sets up dialogue and may partition the process

Session layer allows two systems to enter into a dialog for communications. (half or full duplex). e,g. real-time skype or Instant Messaging (IM).

It partition the process by adding the <syn> as the check point.

Presentation layer

Presentation layer is responsible for:

•Ensure Common Format, Translation, Data Compression, Encryption

Concern about syntax and semantic info exchange btw 2 sides.

Application layer

Responsible for providing services to user and enable user to access the network

OSI layer Summary

EXTRA NOTES:

For understading basic Communications and Networks structure/format

TCP/IP Protocol Suite

PROTOCOLS AND STANDARDSPROTOCOLS AND STANDARDS

Agreed rules form the basis of harmonious data exchange between network nodes. These rules are referred to as protocols in the telecoms world.

All telecommunications technologies are underpinned by protocols that should be recognised internationally managed by established standards bodies.

A protocol has three aspects:

Syntax This is the format (the length and ordering of the fields)

Semantics The purpose/meaning/interpretation of each field.

Timing When and how fast the data should be sent

Eg. Ethernet 100Base-T

Network Model• Data exchange at the local, national and international

level is complex.

• Exchange requires, among other things; user interface, agreed formatting,

common language, dialogue, security,reliability,

ability to reach the destination, and recover from faults.

• A network model must reduce this complexity and at the same time enabling a more flexible approach to network design that can offer a differentiated approach to evolution and if we are lucky revolution.

Model of Internetworking delivery• Access Networks (LAN based)• Interconnection/Transit Networks (WAN

based)

Although in this schematic the interconnection is only via one WAN a real world internet delivery would be by multiple WANS.

Source LANSource LAN

Transit WAN (‘Backbone’)

Transit WAN (‘Backbone’) Destination

LAN

Destination LAN

Gateway

Gateway

The layers in the The layers in the TCP/IP protocol suiteTCP/IP protocol suite do not exactly do not exactly match those in the OSI model. match those in the OSI model. The original TCP/IP protocol suite was defined as The original TCP/IP protocol suite was defined as having four layers: having four layers: network attachmentnetwork attachment, , internetinternet, , transporttransport, and , and applicationapplication. . However, when TCP/IP is compared to OSI, we can However, when TCP/IP is compared to OSI, we can view the TCP/IP protocol suite as five layers: view the TCP/IP protocol suite as five layers: physicalphysical, , data linkdata link, , networknetwork, , transporttransport, and , and applicationapplication..

TCP/IP PROTOCOL SUITETCP/IP PROTOCOL SUITE

TCP/IP and OSI model

Relationship of layers and addresses in TCP/IP

Not used in this unit

Three levels of addresses are used in an Three levels of addresses are used in an internet employing the TCP/IP protocols: internet employing the TCP/IP protocols:

ADDRESSINGADDRESSING

• PhysicalPhysical / MAC / MAC – layer 2– layer 2• LogicalLogical / IP / IP – layer 3 – layer 3 • Port / Port / TCP/UDP – layer 4TCP/UDP – layer 4

MAC – Data link : MAC – Data link : Data FrameData FrameIP – Network : IP – Network : Packet Packet

TCP/UDP – Transport : TCP/UDP – Transport : SegmentSegment

The concept of TCP/UDP and Port address

will be covered later in

Transport layer lecture

Figure 2.21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the source to destination.

Extra Examples:

In the diagram below a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.

Example 2.1 from Forouzan

ignored

The diagram shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each connection.

Example 2.3 from Forouzan

Quiz at:

http://highered.mcgraw-hill.com/sites/0072967757/student_view0/chapter2/multiple_choice_quiz.html

The ENDSee you next week