Phones OFF Please

1. Phones OFF Please The Network Layer Parminder Singh Kang Home: www.cse.dmu.ac.uk/~pkang Email: pkang@dmu.ac.uk

2. Circuit Switching: • A route is set up through a number of exchanges to the destination. • the communication channels used are dedicated for the duration of the call • channels are unusable by others until the call is terminated • tariff is based on duration of call (not amount of data transferred) • Example: the old (non digital) telephone system, i.e. when A and D have set up a • call B and C cannot use line.

3. Advantages and disadvantages?

4. Advantages: • Fixed delays • Guaranteed continuous delivery • Secure and dedicated line • Disadvantages: • Poor resource utilisation. • Difficult to support variable data rates. • Charging is based on duration of call. • Circuit switching requires a call set-up during which resources are not utilized. • If messages are much shorter than the call set-up time then circuit switching is not economical (or even Practical) • More of a problem in high-speed networks

5. 2. Packet Switching: • Technique used with digital connections to allow multiple calls to exist on • same circuit. •  Data broken into small blocks ("packets") •  Packet includes extra information in Header (serial number, • destination address...etc). • Each packet is routed to its destination individually •   Packets re-assembled into original message when destination reached. • Data tend to be in bursts and charging can be based on the amount of data • transferred not duration of call.

7. 3. ADVANTAGES OF PACKET SWITCHING • Communication channels can support many calls simultaneously. • Short messages not delayed by long messages. • More efficient than circuit switching. • DISADVANTAGES • Performance drops when many users share same network. • Sophisticated protocol architecture needed for data transfer. • Packet can arrive out of order. • variable delay. • packet overhead. • Today the majority of communication systems use packet switching. • Other Technologies? (ATM: cell swiching)

8. 3 The Network Layer:

9. The Network Layer manages the virtual link between two host computers. • The main Functions are: • splits data into packets • routes them over the virtual line • corrects for lost packets •  reassembles packets back into messages • Layer 1, 2 and 3 are known as communications subnet. • i.e. separating the pure communications aspects of the network from the higher • layers looked after by the hosts.

10. The design issues of layer 3 are: • subnet-host interface, i.e. interface between station A and router 1 • routing of packets, i.e. from station A via nodes 1-3-4-7-9-10 to station B • resolving of congestion and deadlocks. • opting new routes in case of congestion. • Load balancing. • Shortest path.

11. 4. Packet Switching Networks • The earlier and simpler networks tended to be message switching in that the • complete message from layer 4 was sent as a single entity through the subnet.  • Layer 3 of modern complex networks tend to be packet switching: • message is broken up into a number of packets which are transmitted separately over the subnet. • a packet is a group of binary bits (including data and control information) that • is switched as a whole (the information is arranged in a specified format). • packet switching data transfer; • by source and destination address. • Communication channel is occupied for the duration of the transmission only.

12. 4.1 Packet size • Effect of Packet size; big or small. • It can be seen that although the amount of data transferred is the same: •  that the smaller the packets the quicker the complete message arrives at • the destination. • if a packet has an error and has to be retransmitted the shorter the packet • the better. • Problem: • Header overhead. • packets need to be reassembled at destination. • Buffer space needed.

14. 4.2 Packet Switching Services (Virtual Circuit and Datagram) • 4.2.1 Virtual Circuit (Connection Oriented): • Transport layer provide dedicated channel for communication. • Hence,  no errors, messages delivered in same order as supplied. • Call set up; call set up in the form of Virtual Circuit. • Data Transfer; all message follow defined route. • Disconnect; Terminate virtual circuit after data transmission.

15. 4.2.2 Virtual Circuit (Connectionless): • No Concept of call setup. • Messages are routed independently from source to destination. • Hence,   sequence of packets can potentially follow different routes through the network. • each packet must carry the full destination address. • Problems; • Lost Packets. • sequence may be Out of order. • Applicable to short data transfers. •  Transport layer must add error and sequence control.

16. Is sequence control required? Consider two applications: • copying a disk image with the message identifying the sector. • user logged on to another machine. • Is error checking required, e.g. for lost frames? Consider two applications: • telephone call using digitised real time speech. If a packet is lost should: • a bank sending account information gains a bit in a 16 bit word changing • 1 into 4097. • In general most user applications require a virtual circuit, e.g. remote logon, • file transfer, email, accessing remote databases, etc.

17. 4.3 Internal structure of the subnet: • Datagram: • each packet contains the destination address. • hence, packets are routed to destination independently. • Subsequent datagrams make take a different route. • Virtual Circuit: • route is established at connect time and is fixed for the duration • of the call. • router has a switch table which stores information on the • virtual circuits that are open.

19. Each router maintains a Routing Table and Switch Table: • Routing Table consist of corresponding output line which packets should takes to • reach the destination. • for example F’s Routing Table:

20. Switch Table each entry of which is made up of two tables: • incoming table: contains entries indicating where the circuit comes from • and its number. Associated with each entry is an entry in the outgoing table. • outgoing table: contains entries indicating the destination of the circuit • and its number. • need of switch table? • When switch table needed (Datagram or VC)?

21. 4.4 Comparison of datagrams and virtual circuits • Datagrams contain the full destination address; large overhead with small packets. • Virtual circuits require table space in the memory of the hosts and routers • For small transactions the overheads of setting up a virtual circuit can be large • If traffic conditions change the routes of successive datagrams can change • but a virtual circuit is fixed for the duration of the connection., • i.e. if a router has a ‘traffic jam’ virtual circuit packets will get stuck • but datagrams can take alternative routes.

22. 6. Examples: • Bank cash machine: use datagrams for small individual messages • File transfer program: use virtual circuits for large amount of error free data • Video conference: • Use datagrams: routed individually over fastest route, any error packet • thrown away, requires datagrams put into correct sequence at receiver. • Use virtual circuits: path set up through network so each packet is • smaller than datagrams (why?) • fast transmission • sequencing built in • problem on error when packet is retransmitted

23. 5 Functions of layer-3 • Replies on error free layer 2 transmission of packets and has the following tasks: • multiplexing – interleaving multiple calls onto one line • routing – routine packets from source to destination • flow control – so a fast transmitter does not overload slower receiver • congestion control

24. 5.1 Multiplexing • Multiplexing is the interleaving of packets from many virtual circuits onto the • layer 2 link. • i.e. a sequence of packets on the layer 2 link would be from different virtual circuits. • 5.2 Routing • In both the setting up of a virtual circuit and the sending of datagrams a route • through the network has to be chosen. • Desired properties of routing algorithms are: • correctness: the destination is correct • simplicity: for speed, low cost, etc. • robustness: if an router or line fails or there is a traffic jam the • whole network should not crash

25. stability: some algorithms may never yield a route • fairness: to ensure that all users get a fair shore of the network • optimality: depends on the property to be optimised: • minimise packet delay • maximise network throughput • Shortest Path (e.g. OSPF)

26. 5.2.1 Routing algorithms • Routing relates to finding a route across the network and is carried out when a virtual • circuit is set up and for every datagram packet. • Routing Algorithms • Static: • determined in advance. • downloaded to switches/routers. • Mostly set up manually by system • Administrator. • Adaptive • Changing routing decisions to reflect • changes in traffic levels and network • topology and how the best route should • be identified. • shortest path •  shortest number of hops •  shortest transit time •  most reliable route?

27. 5.2.1 Routing algorithms • Flooding Algorithms. • Static Routing Algorithms. • Central Routing. • Isolated Routing. • Hot Potato Algorithm. • Backward Learning Algorithm.

28. 5.3 Flow Control • Flow control can be similar to layer 2 i.e. flow control of a particular call • e.g. in X.25 it is implemented by Receiver Ready and Receiver Not Ready packets • which indicate the state of a particular DTE (Data Terminal Equipment). • 5.4 Congestion control • Congestion occurs when the total load exceeds the capacity of the network.  • Causes: • Network is under engineered, i.e. need more/faster lines, • larger buffers in routers, etc. • failure (router/line dying) reducing capacity of network • too much traffic at particular points causing overload, i.e. a ‘traffic jam’

29. 5.4.1 Reserving buffer space: • Buffer space is reserved in routers for virtual circuits as they are set up.   • Maximum transmitter window sizes must be known. • can lead to a large amount of wasted buffer space if traffic on the circuits is low.  •  In practice this is not practical and dynamic buffering is used to share resources.

30. 5.4.2. Discarding Packets • Discarded arriving packets if there is no buffer space.   • Hence, suitable for connectionless services where recover is based on higher layers.  • With connection oriented services the error recover takes place on a link by link basis • and discarding one frame may cause a number to be retransmitted: • Using a GObackN protocol.  •  Also acknowledgement frames must not be discarded as they free up buffer space.  • Technique is most suitable for connectionless services.

31. 5.4.3 Use of tokens • Tokens can be used to control number of packets in network. • Data Transmission Sequence: • Capture a token. • when a packet arrives at a destination the token must be released. • Thus the number of packets in the network is kept constant. • There are problems: • there may be free tokens in the network but may not be where nodes wish to transmit • there is no method of regenerating lost tokens • It is not very robust and is equally unsuited for connectionless and connection • oriented networks.

32. 5.4.4 Adaptive windowing • Network nodes can send congestion information to hosts to control their window size. • i.e. reducing window size reduces the number of packets that can be transmitted. • problems are: • time for message to get to hosts. • deciding when to send the information. • deciding which calls it should apply to. • not knowing if the host has responded to the information. • However, a useful technique for connectionless and connection oriented networks.

33. 6 CCITT X25 for packet switching networks • Terminology:

35. 6.1  X25 routing – see TCP/IP notes for IP datagram routing • X25:  Used in WANS which employ packet switched public facilities. • e.g. PSDN,  to pass data between different sites • Function: Defines the nature of the various packets needed to set up and maintain a • connection between a DTE and a switched network. • Mechanism: Virtual Circuit • How does it work? • 3 stages: • a) Set up the call • b) Send data and maintain the connection for the duration of the call. • c)  Close down the call.

36. Setting up the call:  • calling DTE issues a CALL REQUEST packet. • a local logical channel number (LCN) is assigned for this call and is written into • the CALL REQUEST packet. • each switch has a switch table with input and output columns.

37. When CALL REQUEST packet is sent to a switch: • Routing table is used to select the best link onwards from this switch. • A free local LCN is assigned. • This information is then written into the switch table • Each switch, from source to destination,  will contain an entry in its switch table • to define the next part of the route for all packets relating to this call. • i.e. the route is established.

39. Virtual Circuit A range of LCNs is established for each link in the circuit.

40. 2. Data Transfer • Data packet sent using correct LCN • At each switch, • switch table used to determine the next hop and the output LCN • packet header updated with new LC • the packet is then forwarded onto the next switch, flowing the route defined by • all the switch table entries for that call at each switch. • 3. Call release • CLEAR REQUEST issued by one side • Appropriate entry is deleted from the switch table at each router

41. Call Management • Uses a sliding window mechanism • Send and Receive numbers used • Piggy-back acknowledgements if possible • RR available if no packet to piggy-back onto • Assume all error recovery is handled by level 2 No error recovery strategy • such as Go-Back-N. • RESET/RESTART available to recover from network crashes etc.

42. Flow Control: • Receiver sends a Receiver Not Ready (RNR) packet requesting a pause in • transmission. • Transmitter responds by suspending trans. until explicitly told to transmit again.  • RNR only suspends data packets. • After RNR receiver must still accept all packets on the network since no recovery • mechanism such as Go-Back-N. • Receiver gives permission to send again by issuing a Receiver Ready (RR) packet. • Subsequent packets accepted.