CSCD 218 : DATA COMMUNICATIONS AND NETWORKING 1

LECTURE 2INTRODUCTION AND BASIC CONCEPTSCIRCUIT SWITCHING,PACKET SWITCHING,

FRAME RELAY AND NETWORK TOPOLOGIES

LECTURER : FERDINAND KATSRIKU (PhD)

Simple Switching Network

Switched Communications Networks

Long distance transmission between stations (called “end devices”) is typically done over a network of switching nodes.

Switching nodes do not concern with content of data. Their purpose is to provide a switching facility that will move the data from node to node until they reach their destination (the end device).

A collection of nodes and connections forms a communications network.

In a switched communications network, data entering the network from a station are routed to the destination by being switched from node to node.

Switching NodesNodes may connect to other nodes, or to some

stations.Network is usually partially connected– However, some redundant connections are desirable for

reliabilityTwo different switching technologies– Circuit switching– Packet switching

Circuit SwitchingCircuit switching:– There is a dedicated communication path between two stations

(end-to-end)– The path is a connected sequence of links between network

nodes. On each physical link, a logical channel is dedicated to the connection.

Communication has three phases :– Establish– Transfer– Disconnect

Must have switching capacity and channel capacity to establish connection

Must have intelligence to work out routing

Circuit Switching (Cont)

Circuit Switching (Cont)The telephone message is sent all together; it is not

broken up.The message arrives in the same order that it was

originally sent. In modern circuit-switched networks, electronic signals

pass through several switches before a connection is established.

During a call no other network traffic can use those switches.

The resources remain dedicated to the circuit during the entire data transfer and the entire message follows the same path.

Circuit switching can be analog or digital.

Circuit Switching (Cont)With the expanded use of the Internet for voice

and video, analysts predict a gradual shift away from circuit-switched networks.

A circuit-switched network is excellent for data that needs a constant link from end-to-end, for example, real-time video.

Circuit Switching (Cont)AdvantagesCircuit is dedicated to the call – no interference, no sharingGuaranteed the full bandwidth for the duration of the callGuaranteed quality of service

Disadvantages Inefficient – the equipment may be unused for a lot of the

call; if no data is being sent, the dedicated line still remains open.

It takes a relatively long time to set up the circuit. During a crisis or disaster, the network may become

unstable or unavailable. It was primarily developed for voice traffic rather than data

traffic.

Packet SwitchingProblem of circuit switching

» designed for voice service» Resources dedicated to a particular call» For data transmission, much of the time the connection

is idle (say, web browsing)» Data rate is fixed

oBoth ends must operate at the same rate during the entire period of connection

Packet switching is designed to address these problems.

Basic Operations of Packet Switching

• Data are transmitted in short packets– Typically at the order of 1000 bytes– Longer messages are split into series of packets– Each packet contains a portion of user data plus some control info

• Control info contains at least– Routing (addressing) info, so as to be routed to the intended

destination– Recall the content of an IP header!

• store and forward – On each switching node, packets are received, stored briefly

(buffered) and passed on to the next node.

Packet Switching (Cont)In packet-based networks, the message gets broken

into small data packets.These packets are sent out from the computer and

they travel around the network seeking out the most efficient route to travel as circuits become available.

This does not necessarily mean that they seek out the shortest route.

Each packet may go a different route from the others.

Packet Switching (Cont)Each packet is sent with a ‘header address’ which

tells it where its final destination is, so it knows where to go.

One packet also contains details of how many packets should be arriving so that the recipient computer knows if one packet has failed to turn up.

If a packet fails to arrive, the recipient computer sends a message back to the computer which originally sent the data, asking for the missing packet to be resent.

Model of Packet Switching Network

Packet Switching Techniques• A station breaks long message into packets• Packets are sent out to the network sequentially,

one at a time• How will the network handle this stream of packets

as it attempts to route them through the network and deliver them to the intended destination?– Two approaches• Datagram approach• Virtual circuit approach

Datagram• Each packet is treated independently, with no

reference to packets that have gone before.– Each node chooses the next node on a packet’s path.

• Packets can take any possible route.• Packets may arrive at the receiver out of order.• Packets may go missing.• It is up to the receiver to re-order packets and

recover from missing packets.• Example: Internet

DatagramDiagram

Virtual Circuit• In virtual circuit, a preplanned route is established

before any packets are sent, then all packets follow the same route.

• Each packet contains a virtual circuit identifier instead of destination address, and each node on the pre-established route knows where to forward such packets.– The node need not make a routing decision for each

packet.• Example: X.25, Frame Relay, ATM

VirtualCircuit

Diagram

A route between stations is set up prior to data transfer.

All the data packets then follow the same route.

But there is no dedicated resources reserved for the virtual circuit! Packets need to be stored-and-forwarded

Virtual Circuit vrs Datagram• Virtual circuits

– Network can provide sequencing (packets arrive at the same order) and error control (retransmission between two nodes).

– Packets are forwarded more quickly• Based on the virtual circuit identifier• No routing decisions to make

– Less reliable• If a node fails, all virtual circuits that pass through that node fail.

• Datagram– No call setup phase

• Good for busty data, such as Web applications– More flexible

• If a node fails, packets may find an alternate route• Routing can be used to avoid congested parts of the network

Advantages of Packet Switching• 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– On each node, packets with higher priority can be forwarded first. They will

experience less delay than lower-priority packets.

• Availability – no waiting for a direct connection to become available

• During a crisis or disaster, when the public telephone network might stop working, e-mails and texts can still be sent via packet switching

Disadvantages of Packet Switching• Under heavy use there can be a delay• Data packets can get lost or become corrupted• Protocols for packet switching are typically more complex• Not so good for some types data streams (e.g. real-time

video streams can lose frames due to the way packets arrive out of sequence)

Questions

• Describe circuit switching and Packet Switching

• Compare and contrast packet switching with circuit switching

• Give a simple example of packet switching application

Frame RelayA packet-switching protocol for connecting devices

on a Wide Area Network (WAN)” quoted from Webopedia.

Frame Relay (FR) is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model.

Originally designed for use across ISDN interfacesFrames carry data between user devices called data

terminal equipment (DTE), and the data communications equipment (DCE) at the edge of the WAN.– It does not define the way the data is transmitted within the service provider’s

Frame Relay cloud

Advantages and Disadvantages of FR The principal potential disadvantage of frame relay, compared to

X.25, is that we have lost the ability to do link-by-link flow and error control. Although frame relay does not provide end-to-end flow and error control, this is easily provided at a higher layer. With the increasing reliability of transmission and switching facilities, this is not a major disadvantage.

The advantage of frame relay is that we have streamlined the communications process. The protocol functionality required at the user-network interface is reduced, as is the internal network processing. As a result, lower delay and higher throughput can be expected. Studies indicate an improvement in throughput using frame relay, compared to X.25, of an order of magnitude or more. The ITU-T Recommendation I.233 indicates that frame relay is to be used at access speeds up to 2 Mbps. However, frame relay service at even higher data rates is now available.

NETWORK TOPOLOGIES

TopologyTopology defines how the devices (computers,

printers..etc) are connected and how the data flows from one device to another.

There are two conventions while representing the topologies :● Physical Topology : defines how the devices are physically wired.● Logical Topology : defines how data flows from one device to another.

Topologies are broadly categorized into :i) Bus ii) ii ) Ring iii) iii) Star iv) iv) Mesh

Network Topology

Bus Topology• A bus topology is multipoint.• Here one long cable act as a backbone to link all the

devices are connected to the backbone by drop lines and taps.

• Drop line- is the connection b/w the devices and the cable.

• Tap- is the splitter that cut the main link.• This allows only one device to transmit at a time.• There is a limit on the number of taps a bus can

support and on the distance between those taps.• Every computer (node) shares the networks total

bus capacities

A bus topology model

Bus Topology

Bus Topology

Bus Topology

A device want to communicate with other device on the network sends a broadcast message onto the wire all other devices see.

But only the intended devices accepts and process the message.

Terminators stop signals after reaching end of wire

Bus Topology

Advantages:1. Ease of installation2. Less cabling

Disadvantages:3. Difficult reconfiguration and fault isolation.4. Difficult to add new devices.5. Signal reflection at top can degradation in quality.6. If any fault in backbone can stops all transmission.

Star Topology• Here each device has a dedicated point-to-point

link to the central controller called “Hub”(Act as a Exchange).

• There is no direct traffic between devices.• The transmission are occurred only through the

central “hub”.• When device 1 wants to send data to device 2;

First sends the data to hub. Which then relays the data to the other connected device.

Star Topology

Star Topology

Star Topology• Advantages:1. Less expensive then mesh since each device is

connected only to the hub.

2. Installation and configuration are easy.

3. Less cabling is need then mesh.

4. Robustness.(if one link fails, only that links is affected. All other links remain active)

5. Easy to fault identification & to remove parts.

6. No disruptions to the network when connecting(or) removing devices.

Star TopologyDisadvantages:1. Even it requires less cabling then mesh when

compared with other topologies it still large.(Ring or bus).

2. Dependency(whole network dependent on one single point(hub). When it goes down. The whole system is dead.

Ring Topology• Here each device has a dedicated connection with

two devices on either side.• The signal is passed in one direction from device to

device until it reaches the destination and each device have repeater.

• When one device received signals instead of intended another device, its repeater then regenerates the data and passes them along.

• To add or delete a device requires changing only two connections.

Ring Topology

Ring Topology

Advantages:1. Easy to install.

2. Easy to reconfigure.

3. Fault identification is easy.

Disadvantages:4. Unidirectional traffic.

5. Break in a single ring can break entire network.

Mesh Topology• Here every device has a point to point link to every

other device.• Node 1 node must be connected with n-1 nodes. • A fully connected mesh can have n(n-1)/2 physical

channels to link n devices.• It must have n-1 I/O ports.

Mesh Topology

Mesh Topology

Advantages:1. They use dedicated links so each link can only

carry its own data load. So traffic problem can be avoided.

2. It is robust. If any one link get damaged it cannot affect others.

3. It gives privacy and security.(Message travels along a dedicated link)

4. Fault identification and fault isolation are easy.

Mesh Topology• Disadvantages:1. The amount of cabling and the number of I/O ports

required are very large. Since every device is connected to each devices through dedicated links.

2. The sheer bulk of wiring is larger then the available space.

3. Hardware required to connect each device is highly expensive.

Assignment II1. List seven network components/devices and give

the functions of each.2. Briefly describe any four types of network.3. List and explain any four types of network servers.4. State and explain five importance of a computer

network.

(10points each)