1. CSCD 218 : DATA COMMUNICATIONS AND NETWORKING 1
LECTURE 2
INTRODUCTION AND BASIC CONCEPTS
CIRCUIT SWITCHING,PACKET SWITCHING, FRAME RELAY AND NETWORK TOPOLOGIES
LECTURER : FERDINAND KATSRIKU (PhD)

2. Simple Switching Network

3. 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.

4. Switching Nodes Nodes may connect to other nodes, or to some stations.
Network is usually partially connected
However, some redundant connections are desirable for reliability
Two different switching technologies
Circuit switching
Packet switching

5. Circuit Switching Circuit switching: Communication has three phases :
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
Communication via circuit switching implies that there is a dedicated communication path between two stations. That path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection. Communication via circuit switching involves three phases:
Circuit establishment - Before any signals can be transmitted, an end-to-end (station-to-station) circuit must be established.
Data transfer - Data can now be transmitted through the network between these two stations. The transmission may be analog or digital, depending on the nature of the network. As the carriers evolve to fully integrated digital networks, the use of digital (binary) transmission for both voice and data is becoming the dominant method. Generally, the connection is full duplex.
Circuit disconnect - After some period of data transfer, the connection is terminated, usually by the action of one of the two stations. Signals must be propagated to the intermediate nodes to deallocate the dedicated resources.
Circuit switching can be rather inefficient. Channel capacity is dedicated for the duration of a connection, even if no data are being transferred. For a voice connection, utilization may be rather high, but it still does not approach 100%. For a client/server or terminal-to-computer connection, the capacity may be idle during most of the time of the connection. In terms of performance, there is a delay prior to signal transfer for call establishment. However, once the circuit is established, the network is effectively transparent to the users.

6. Circuit Switching (Cont)

7. 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.
Communication via circuit switching implies that there is a dedicated communication path between two stations. That path is a connected sequence of links between network nodes. On each physical link, a logical channel is dedicated to the connection. Communication via circuit switching involves three phases:
Circuit establishment - Before any signals can be transmitted, an end-to-end (station-to-station) circuit must be established.
Data transfer - Data can now be transmitted through the network between these two stations. The transmission may be analog or digital, depending on the nature of the network. As the carriers evolve to fully integrated digital networks, the use of digital (binary) transmission for both voice and data is becoming the dominant method. Generally, the connection is full duplex.
Circuit disconnect - After some period of data transfer, the connection is terminated, usually by the action of one of the two stations. Signals must be propagated to the intermediate nodes to deallocate the dedicated resources.
Circuit switching can be rather inefficient. Channel capacity is dedicated for the duration of a connection, even if no data are being transferred. For a voice connection, utilization may be rather high, but it still does not approach 100%. For a client/server or terminal-to-computer connection, the capacity may be idle during most of the time of the connection. In terms of performance, there is a delay prior to signal transfer for call establishment. However, once the circuit is established, the network is effectively transparent to the users.

8. 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.

9. Circuit Switching (Cont)
Advantages
Circuit is dedicated to the call – no interference, no sharing
Guaranteed the full bandwidth for the duration of the call
Guaranteed 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.

10. Packet Switching Problem 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
Both ends must operate at the same rate during the entire period of connection
Packet switching is designed to address these problems.

11. 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.

12. 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.

13. 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.

14. Model of Packet Switching Network

15. 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
If a station has a message to send through a packet-switching network that is of length greater than the maximum packet size, it breaks the message up into packets and sends these packets, one at a time, to the network. A question arises as to how the network will handle this stream of packets as it attempts to route them through the network and deliver them to the intended destination. Two approaches are used in contemporary networks: datagram and virtual circuit.

16. 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

17. Datagram Diagram
In the datagram approach, each packet is treated independently, with no reference to packets that have gone before. This approach is illustrated in Stallings DCC8e Figure 10.9, which shows a time sequence of snapshots of the progress of three packets through the network. Each node chooses the next node on a packet's path, taking into account information received from neighboring nodes on traffic, line failures, and so on. So the packets, each with the same destination address, do not all follow the same route, and they may arrive out of sequence at the exit point. In this example, the exit node restores the packets to their original order before delivering them to the destination. In some datagram networks, it is up to the destination rather than the exit node to do the reordering. Also, it is possible for a packet to be destroyed in the network. For example, if a packet-switching node crashes momentarily, all of its queued packets may be lost. Again, it is up to either the exit node or the destination to detect the loss of a packet and decide how to recover it. In this technique, each packet, treated independently, is referred to as a datagram.

18. 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

19. Virtual Circuit 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
In the virtual circuit approach, a preplanned route is established before any packets are sent. Once the route is established, all the packets between a pair of communicating parties follow this same route through the network. This is illustrated in Stallings DCC8e Figure Because the route is fixed for the duration of the logical connection, it is somewhat similar to a circuit in a circuit-switching network and is referred to as a virtual circuit. Each packet contains a virtual circuit identifier as well as data. Each node on the pre-established route knows where to direct such packets; no routing decisions are required. At any time, each station can have more than one virtual circuit to any other station and can have virtual circuits to more than one station.
So the main characteristic of the virtual circuit technique is that a route between stations is set up prior to data transfer. Note that this does not mean that this is a dedicated path, as in circuit switching. A transmitted packet is buffered at each node, and queued for output over a line, while other packets on other virtual circuits may share the use of the line. The difference from the datagram approach is that, with virtual circuits, the node need not make a routing decision for each packet. It is made only once for all packets using that virtual circuit.

20. 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
If two stations wish to exchange data over an extended period of time, there are certain advantages to virtual circuits. First, the network may provide services related to the virtual circuit, including sequencing and error control. Sequencing refers to the fact that, because all packets follow the same route, they arrive in the original order. Error control is a service that assures not only that packets arrive in proper sequence, but also that all packets arrive correctly. Another advantage is that packets should transit the network more rapidly with a virtual circuit; it is not necessary to make a routing decision for each packet at each node.
One advantage of the datagram approach is that the call setup phase is avoided. Thus, if a station wishes to send only one or a few packets, datagram delivery will be quicker. Another advantage of the datagram service is that, because it is more primitive, it is more flexible. For example, if congestion develops in one part of the network, incoming datagrams can be routed away from the congestion. With the use of virtual circuits, packets follow a predefined route, and thus it is more difficult for the network to adapt to congestion. A third advantage is that datagram delivery is inherently more reliable. With the use of virtual circuits, if a node fails, all virtual circuits that pass through that node are lost. With datagram delivery, if a node fails, subsequent packets may find an alternate route that bypasses that node. A datagram-style of operation is common in internetworks.

21. 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, s and texts can still be sent via packet switching

22. 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)

23. Questions Describe circuit switching and Packet Switching
Compare and contrast packet switching with circuit switching
Give a simple example of packet switching application

24. Frame Relay
A 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 interfaces
Frames 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
Today's networks employ reliable digital transmission technology over high-quality, reliable transmission links, many of which are optical fiber. In addition, with the use of optical fiber and digital transmission, high data rates can be achieved. In this environment, the overhead of X.25 is not only unnecessary but degrades the effective utilization of the available high data rates. Frame relay is designed to eliminate much of the overhead that X.25 imposes on end user systems and on the packet-switching network. The standards for frame relay matured earlier than those for say ATM. Accordingly, there is a large installed base of frame relay products. The key differences between frame relay and a conventional X.25 packet-switching service are:
• Call control signaling, which is information needed to set up and manage a connection, is carried on a separate logical connection from user data. Thus, intermediate nodes need not maintain state tables or process messages relating to call control on an individual per-connection basis.
• Multiplexing and switching of logical connections takes place at layer 2 instead of layer 3, eliminating one entire layer of processing.
• There is no hop-by-hop flow control and error control. End-to-end flow control and error control are the responsibility of a higher layer, if they are employed at all.
Thus, with frame relay, a single user data frame is sent from source to destination, and an acknowledgment, generated at a higher layer, may be carried back in a frame. There are no hop-by-hop exchanges of data frames and acknowledgments.

25. 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.

26. NETWORK TOPOLOGIES

27. Topology
Topology 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 :
Bus
ii ) Ring
iii) Star
iv) Mesh

29. Network Topology

30. 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

31. A bus topology model

32. Bus Topology

33. Bus Topology

34. 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

35. Bus Topology Advantages: Ease of installation Less cabling
Disadvantages:
Difficult reconfiguration and fault isolation.
Difficult to add new devices.
Signal reflection at top can degradation in quality.
If any fault in backbone can stops all transmission.

36. 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.

37. Star Topology

38. Star Topology

39. Star Topology Advantages:
Less expensive then mesh since each device is connected only to the hub.
Installation and configuration are easy.
Less cabling is need then mesh.
Robustness.(if one link fails, only that links is affected. All other links remain active)
Easy to fault identification & to remove parts.
No disruptions to the network when connecting(or) removing devices.

40. Star Topology Disadvantages:
Even it requires less cabling then mesh when compared with other topologies it still large.(Ring or bus).
Dependency(whole network dependent on one single point(hub). When it goes down. The whole system is dead.

41. 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.

42. Ring Topology

43. Ring Topology Advantages: Easy to install. Easy to reconfigure.
Fault identification is easy.
Disadvantages:
Unidirectional traffic.
Break in a single ring can break entire network.

44. 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.

45. Mesh Topology

46. Mesh Topology Advantages:
They use dedicated links so each link can only carry its own data load. So traffic problem can be avoided.
It is robust. If any one link get damaged it cannot affect others.
It gives privacy and security.(Message travels along a dedicated link)
Fault identification and fault isolation are easy.

47. Mesh Topology Disadvantages:
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.
The sheer bulk of wiring is larger then the available space.
Hardware required to connect each device is highly expensive.

48. Assignment II
List seven network components/devices and give the functions of each.
Briefly describe any four types of network.
List and explain any four types of network servers.
State and explain five importance of a computer network.
(10points each)