COMPUTER NETWORKS (ECS 601) MAHUA S. MAITY CSE DEPTT.

COMPUTER NETWORKS(ECS 601)

MAHUA S. MAITYCSE DEPTT.

UNIT 1

Introduction Concepts: Goals and Applications of Networks, Network structure and architecture, The OSI reference model, services, Network Topology Design - Delay Analysis, Back Bone Design, Local Access Network Design. Physical Layer Transmission

Media, Switching methods, ISDN, Terminal Handling.

Introduction

• Data Communication• Networks• Protocols and Standards• Standard Organizations

Uses of Computer Networks

1. Network Goals & Application

• Resource Sharing

• High Reliability.

• Saving Money.

• Powerful Communication Medium

1. LAN

2. MAN

3. WAN

2. Networks for Companies (1-tier, 2-tier, 3-tier)

Client-server model

CONTT.

2. Networks for Companies

Client-server model

Concurrent server vs. Iterative server

Stateful server vs. Stateless server

3.Networks for People

•Person-to-person communicationElectronic mail, ICQ (I seek you), Videoconference

•Interactive entertainmentVideo-on-Demand, Games

•Access to remote informationWorld Wide Web

4. Social Issues

PrivacyCopyrightPornographyAnonymityfreedom of speech vs. censorshipresponsibility of the service providers

Data Communication System Components

Basic Concepts

• Line Configuration• Topology• Transmission Mode• Categories of Networks• Internetworks

Point-to-Point Line Configuration

continued

Point-to-Point Line Configuration

continued

Point-to-Point Line Configuration

Multipoint Line Configuration

Mesh Topology

Star Topology

Tree Topology

Bus Topology

Ring Topology

Hybrid Topology

Simplex

Half-Duplex

Full-Duplex

OSI Model(Open systems interconnection references model)

• The model• Functions of the layers

OSI Model

7 Application (Network Services like email, file transfer)

Presentation (formatting, encryption and compression)

5 Session (setup and management of end-to-end conversion )

4 Transport (end - to – end delivery of messages )

3 Network (end - to – end transmission of packets)

2 Data Link (transmission of packets on one given link)

1 Physical (transmission of bits)

Design Issues for the Layers

•A mechanism for identifying senders and receivers (naming and addressing)•rules of transfer (simplex, half-duplex, full-duplex)•error control (error correction and error detection)•ordering and sequencing•flow control, congestion control•message or packet size (disassembling and reassembling)•multiplexing and de-multiplexing•routing•security

OSI Layers(The interaction between layers in the OSI model )

The OSI Reference Model (Encapsulation)

applicationapplication

presentation

session

transport

network

data link

physical

7

6

5

4

3

2

1

data

dataAH

dataAHPH

dataAHPHSH

dataAHPHSHTH

dataAHPHSHTHNH

dataAHPHSHTHNH

bit streams

DT

H: headerT: trailEach may be empty.

DH

An Exchange Using the OSI Model

Physical Layer

Physical Layer

The physical layer is also concerned with the following:

• Physical characteristics of interfaces and medium

• Representation of bits.

• Data rate.

• Synchronization of bits.

• Line configuration.

• Physical topology.

• Transmission mode.

Figure 3-5

Data Link Layer

Data Link Layer

The Data Link Layer is also concerned with the following:

• Framing

• Physical addressing.

• Flow control.

• Error control.

Data Link Layer Example

Network Layer

Network Layer

The Network Layer is also concerned with the following:

• Logical Address

• Routing

Network Layer Example

continuedNetwork Layer Example

Transport Layer

Transport Layer

The transport layer include the following:• Service-point addressing.

• Segmentation and reassembly.

• Connection control

• Flow control

Transport Layer Example

continued

Transport Layer Example

Session Layer

Session Layer

The session layer include the following:

• Dialog control(half-duplex or full duplex)

• Synchronization

Presentation Layer

Presentation Layer

Specific responsibilities of the presentation layer include the following:

• Translation.

• Encryption.

• Compression.

Application Layer

Reference Models

The TCP/IP Reference Model (Transmission Control Protocol/Internet Protocol

The TCP/IP Reference Model

A Comparison of the OSI and TCP/IP Reference Model

SIMILARITIES

The main similarities between the two models include the following:

• They share similar architecture. -    Both of the models share a similar architecture.  This can be illustrated by the fact that both of them are constructed with layers.

• They share a common application layer.- Both of the models share a common "application layer".  However in practice this layer includes different services depending upon each model.

• Both models have comparable transport and network layers.- This can be illustrated by the fact that whatever functions are performed between the presentation and network layer of the OSI model similar functions are performed at the Transport layer of the TCP/IP model.

•Knowledge of both models is required by networking professionals.- According to article obtained from the internet networking professionals "need to know both models".

•Both models assume that packets are switched.- Basically this means that individual packets may take differing paths in order to reach the same destination.

DIFFERENCES •The main differences between the two models are as follows:TCP/IP Protocols are considered to be standards around which the internet has developed.  The OSI model however is a "generic, protocol- independent standard.”

•TCP/IP combines the presentation and session layer issues into its application layer.

•TCP/IP combines the OSI data link and physical layers into the network access layer.

•TCP/IP appears to be a more simpler model and this is mainly due to the fact that it has fewer layers.

•TCP/IP is considered to be a more credible model- This is mainly due to the fact because TCP/IP protocols are the standards around which the internet was developed therefore it mainly gains creditability due to this reason.  Where as in contrast networks are not usually built around the OSI model as it is merely used as a guidance tool.

•The OSI model consists of 7 architectural layers whereas the TCP/IP only has 4 layers

A Critique of the OSI Model and Protocols

1. Bad timing2. Bad technology3. Bad implementation4. Bad politics

A Critique of the OSI Model and Protocols

Badtiming

Local Area Networks

Ethernet

Token Ring

Local Area Networks

Standardization Body

IEEE (Institute of Electric and Electronic Engineers) 802 group

For example:802.3: CSMA/CD (Carrier Sense Multiple Access with Collision Detection) (Ethernet is one of them.)802.4: Token Bus802.5: Token Ring

Local Area Network

continuedLocal Area Network

Metropolitan Area Networks

DQDB: Distributed Queue Dual Bus (IEEE 802.6 standard)

Wide Area Networks

Wide Area Networks

Network topologies

Metropolitan Area Network (Example)

store-and-forward network

A

B

CA sends a message to C through B.

B must store this message until B is sure that C has received it.

Store first, then forward. But when to start forwarding?

Wide Area Networks

A

B

CA sends a message to C through B.

When to starting forwarding?

1. After the message is completely received2. Start forwarding after a fixed amount of information(bits) received3. Start forwarding immediately after receiving data (cut-through)

store-and-forward network

Wide Area Networks

A

B

CA sends a message to C through B.

If a message takes 1 minute to travel a link:

(1) A to B, then B to A: 2 minutes(2) message is decomposed into 4 parts: 1.25 minutes (each part is called a packet)

0 m10.25 m2 m10.5 m3 m2 m10.75 m4 m3 m21.0 m4 m31.25 m4

A B C

Contt.

Wireless Networks

The fast-growing segment of the industry:•notebook computers•personal digital assistants•cellular phones

Before long, we would have:•palmtop computers•wristwatch computers

Wireless Networks

Wide Area Network

Figure 2-19

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Internetwork (Internet)

Switching

A

B

C

D

E

F

G

H

(1) circuit switching (in telephone)(2) packet switching(3) message switching

Current network practice: store-and-forward packet switching

Wide Area Networks:Dod: ARPANET in 1960sIBM: SNA in 1974DEC: DECNET in 1975CCITT X.25 in 1970s

Wide Area Networks

Switching Networks

• Long distance transmission is typically done over a network of switched nodes

• Nodes not concerned with content of data

• End devices are stations

– Computer, terminal, phone, etc.

• A collection of nodes and connections is a communications network

• Data routed by being switched from node to node

Simple Switched Network

Circuit Switching• Dedicated communication path between two

stations• Three phases

– Establish

– Transfer

– Disconnect

• Must have switching capacity and channel capacity to establish connection

• Must have intelligence to work out routing

Circuit Switching - Applications

• Inefficient– Channel capacity dedicated for duration of connection

– If no data, capacity wasted

• Set up (connection) takes time• Once connected, transfer is transparent• Developed for voice traffic (phone)

Public Circuit Switched Network

Telecomms Components

• Subscriber– Devices attached to network

• Subscriber line– Local Loop– Subscriber loop– Connection to network– Few km up to few tens of km

• Exchange– Switching centers– End office - supports subscribers

• Trunks– Branches between exchanges– Multiplexed

Circuit Establishment

Circuit Switching Concepts• Digital Switch

– Provide transparent signal path between devices• Network Interface• Control Unit

– Establish connections• Generally on demand• Handle and acknowledge requests• Determine if destination is free• construct path

– Maintain connection– Disconnect

Packet Switching Principles

• Circuit switching designed for voice

– Resources dedicated to a particular call

– Much of the time a data connection is idle

– Data rate is fixed

• Both ends must operate at the same rate

Basic Operation• Data transmitted in small packets

– Typically 1000 octets

– Longer messages split into series of packets

– Each packet contains a portion of user data plus some control info

• Control info

– Routing (addressing) info

• Packets are received, stored briefly (buffered) and past on to the next node

– Store and forward

Use of Packets

Advantages

• Line efficiency– Single node to node link can be shared by many packets over time

– Packets queued and transmitted as fast as possible

• Data rate conversion– Each station connects to the local node at its own speed

– Nodes buffer data if required to equalize rates

• Packets are accepted even when network is busy– Delivery may slow down

• Priorities can be used

Disadvantages

Disadvantages:• Protocols for packet switching are typically more complex.

• It can add some initial costs in implementation.

• If packet is lost, sender needs to retransmit the data.

• Another disadvantage is that packet-switched systems stillcan’t deliver the same quality as dedicated circuits inapplications requiring very little delay - like voiceconversations or moving images.

Message Switching

• In message switching there is no need to establish a dedicatedpath between two stations.

• When a station sends a message, the destination address isappended to the message.

• The message is then transmitted through the network, in itsentirety, from node to node.

• Each node receives the entire message, stores it in its entiretyon disk, and then transmits the message to the next node.

• This type of network is called a store-and-forward network.

Message Switching

Advantages:• Channel efficiency can be greater compared to circuit-switched systems, because more devices are sharing thechannel.

• Traffic congestion can be reduced, because messages may be temporarily stored in route.

• Message priorities can be established due to store-and-forward technique.

• Message broadcasting can be achieved with the use ofbroadcast address appended in the message.

Message Switching

Disadvantages:

• Message switching is not compatible with interactive applications.

• Store-and-forward devices are expensive, because theymust have large disks to hold potentially long messages.

• Guided - wire

• Unguided - wireless

• Characteristics and quality determined by

• medium and signal

• For guided, the medium is more important

• For unguided, the bandwidth produced by the

• antenna is more important

• Key concerns are data rate and distance

Transmission Media(Overview)

Design Factors

• Bandwidth

- Higher bandwidth gives higher data rate

• Transmission impairments

- Attenuation

- Order of losses: Twisted pair, coaxial then fibre

• Interference

- Overlapping of frequencies in unguided medium

- Emanations from adjacent cables in guided. (Use screening)

• Number of receivers

- In guided media

- More receivers (multi-point) introduce more attenuation

Guided Transmission Media

• Twisted Pair

• Coaxial cable

• Optical fiber

Twisted Pair

Twisted Pair - Applications

• Most common medium

• Telephone network

- Between house and local exchange (subscriber loop)

• Within buildings

- To private branch exchange (PBX)

• For local area networks (LAN)

- 10Mbps or 100Mbps

Twisted Pair - Pros and Cons

• Cheap

• Easy to work with

• Low data rate

• Short range

Twisted Pair - TransmissionCharacteristics

• Analog

- Amplifiers every 5km to 6km

• Digital

- Use either analog or digital signals

- repeater every 2km or 3km

• Limited distance

• Limited bandwidth (1MHz)

• Limited data rate (100MHz)

- 1 Ghz at short distances & new encoding schemes

• Susceptible to interference and noise

UTP(Unshielded Twisted Pair ) Categories

• Cat 3• up to 16MHz• Voice grade found in most offices• Twist length of 7.5 cm to 10 cm

• Cat 4• up to 20 MHz

• Cat 5• up to 100MHz (1 GHz using 4 pair & compression)• Data grade cable• Commonly pre-installed in new office buildings• Twist length 0.6 cm to 0.85 cm

• Cat 6• 200 MHz to 250MHz• 1 Ghz uncompressed: 4 x 250 Mhz

Coaxial Cable

Coaxial Cable Applications

• Most versatile medium

• Television distribution– Ariel to TV

– Cable TV

• Long distance telephone transmission– Can carry 10,000 voice calls simultaneously

– Being replaced by fiber optic

• Short distance computer systems links

• Local area networks

Coaxial Cable - TransmissionCharacteristics

• Analog– Amplifiers every few km

– Closer if higher frequency

– Usuable spectrum up to 500MHz

• Digital– Repeater every 1km

– Closer for higher data rates

Optical Fiber - Benefits

• Greater capacity– Data rates of hundreds of Gbps

• Smaller size & weight

• Lower attenuation

• Electromagnetic isolation

• Greater repeater spacing– 10s of km at least

Optical Fiber - Applications

• Long-haul trunks– 1500km, 20 – 60k voice channels

• Metropolitan trunks– 12 km, 100k channels

• Rural exchange trunks– 40 – 160Km, 5k voice channels

• Subscriber loops– Voice data cables leased by corporate clients

• LANs– 100Mbps – 1 Ghz

Optical Fiber - Applications

• Long-haul trunks– 1500km, 20 – 60k voice channels

• Metropolitan trunks– 12 km, 100k channels

• Rural exchange trunks– 40 – 160Km, 5k voice channels

• Subscriber loops– Voice data cables leased by corporate clients

• LANs– 100Mbps – 1 Ghz

Delay Analysis

1. Processing Delay(header)

2. Queuing Delay

3. Transmission Delay(1st come 1st serve)

4. Propagation Delay(physical medium)

Integrated Services Digital Network

Networking Devices

• NIC

• Hub

• Bridge

• Switch

• Router

• Gateway

NIC

Hub(Figure :4-port Ethernet Hub)

Switch

Router

Gateway