Computer Communication & Networks

1. Computer Communication & Networks Lecture # 03 Switching Course Instructor: Engr. Sana Ziafat

2. Communication Network Communication networks Broadcast networks End nodes share a common channel (TV, radio…) Switched networks End nodes send to one (or more) end nodes Circuit switching Dedicated circuit per call (telephone, ISDN) (physical) Packet switching Data sent in discrete portions (the Internet)

3. Figure 8.2 Taxonomy of switched networks

4. Switching Networks • Long distance transmission is typically done over a network of switched nodes • A collection of nodes and connections is a communications network • Nodes not concerned with content of data • End devices are stations • Computer, terminal, phone, etc. • Data routed by being switched from node to node

5. Nodes • Nodes may connect to other nodes only, or to stations and other nodes • Node to node links usually multiplexed • Network is usually partially connected • Some redundant connections are desirable for reliability • Two different switching technologies • Circuit switching • Packet switching

6. Simple Switched Network

7. Switching Activities • Some nodes connect only to other nodes (intermediary nodes). Sole purpose is to switch data • Some nodes have one or more stations attached. They accept from and deliver data to the attached station. • Node-to-node links are usually multiplexed • Multiple paths enhance reliability

8. Circuit Switched Networks A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.

9. Circuit switching (cnt’d) Three phases involved in the communication process: • Establish the circuit • Transmit data • Terminate the circuit If circuit not available: busy signal (congestion)

10. Note In circuit switching, the resources need to be reserved during the setup phase;the resources remain dedicated for the entire duration of data transfer until the circuit is terminated. 8.10

11. Circuit switching • A dedicated communication path (sequence of links-circuit)is established between the two end nodes through the nodes of the network • Bandwidth: A circuit occupies a fixed capacityof each link for the entire lifetime of the connection. Capacity unused by the circuit cannot be used by other circuits. • Latency: Data is not delayed at switches

12. Circuit Switching- Applications • Developed for voice traffic (phone) • Inefficient • Channel capacity dedicated for duration of connection • If no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is transparent

13. Telecom Components • Subscriber • Devices attached to network • Subscriber line • Link between subscriber and network • Also called Local Loop or Subscriber Loop • Almost all Local Loops are TPW • Range from Few km up to tens of km • Exchange • Switching center in the network • End office specific switching center that supports subscribers • Trunks • Branches between exchanges • Multiplexed

14. Circuit Establishment

15. Time diagram of circuit switching switch node 1 node 2 host 1 host 2 Delay host 1- node 1 Processing delay node 1 circuit establishment Delay host 2- host 1 data transmission DATA time

16. Example: 4 users FDM frequency time TDM frequency time Circuit Switching: FDM and TDM

17. Example Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands. Solution We shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure on next Slide. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them.

18. Example (contd.)

19. Example Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference? Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 × 100 + 4 × 10 = 540 kHz

20. Applications • AM Radio • Band 530-1700KHz • Each AM Station needs 10KHz • FM Radio • Band 88-108MHz • Each FM Station needs 200KHz • TV • Each Channel needs 6MHz

21. Switching Technique • Station breaks long message into packets • Packets sent one at a time to the network • Packets handled in two ways • Datagram • Virtual circuit

22. Figure 8.7 A datagram network with four switches (routers)

23. Figure 8.8 Routing table in a datagram network

24. Note A switch in a datagram network uses a routing table that is based on the destination address.

25. Bandwidth division into “pieces” Dedicated allocation Resource reservation Packet Switching resource contention: • aggregate resource demand can exceed amount available • congestion: packets queue, wait for link use • store and forward: packets move one hop at a time • Node receives complete packet before forwarding • each end-end data stream divided into packets • user A, B packets share network resources • each packet uses full link bandwidth • resources used as needed

26. Packet switching - Why not message switching?- node 1 node 2 host 1 host 2 propagation delay host 1 – node1 message processing & set-up delay of a message at node 1 message message time Store-and-Forward

27. Use of Packets

28. Datagram • Each packet treated independently • Packets can take any practical route • Packets may arrive out of order • Packets may go missing • Up to receiver to re-order packets and recover from missing packets

29. DatagramDiagram

30. Virtual Circuit • Preplanned route established before any packets sent • Call request and call accept packets establish connection (handshake) • Each packet contains a virtual circuit identifier instead of destination address • No routing decisions required for each packet • Clear request to drop circuit • Not a dedicated path

31. VirtualCircuitDiagram

32. Source-to-destination data transfer in a virtual-circuit network

33. Virtual Circuits vs Datagram • Virtual circuits • Network can provide sequencing and error control • Packets are forwarded more quickly • No routing decisions to make • Less reliable • Loss of a node loses all circuits through that node • Datagram • No call setup phase • Better if few packets • More flexible • Routing can be used to avoid congested parts of the network

34. Circuit vs. Packet Switching Circuit Switched • Bandwidth guaranteed • Circuit capacity not reduced by other network traffic • Circuit costs independent of amount of data transmitted, resulting in wasted bandwidth Packet Switched • Bandwidth dynamically allocated on as-needed basis • May have concurrent transmissions over physical channel • May have delays and congestion • More cost-effective, offer better performance

35. packet being transmitted (delay) packets queueing(delay) free (available) buffers: arriving packets dropped (loss) if no free buffers How do loss and delay occur? packets queue in router buffers • packet arrival rate to link exceeds output link capacity • packets queue, wait for turn A B

36. 1. Store and forward delay • store-and-forwardpacket switches introduced store and forward delay • delay is proportional to the packet's length in bits. • If a packet consists of L bits, and the packet is to be forwarded onto an outbound link of R bps, then the store-and-forward delay at the switch is L/R seconds.

37. 2. Queuing Delay • Within each router there are multiple buffers (also called queues), with each link having an input buffer (to store packets that have just arrived to that link) and an output buffer. • If packet has to wait in output buffer packets suffer output buffer queuing delays • These delays are variable and depend on the level of congestion in the network. • Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely filled with other packets waiting for transmission packet loss will occur

38. Readings Chapter 8 (B. A Forouzan) Section 8.1, 8.2, 8.3 38

39. References • Chapter 8 (Data & computer Communication by Behroz A. Forozun) • Chapter 10 ( Computer Communication by William Stallings) • Chapter 1 (Computer Networking by James K. Kurose)

40. Q & A