Phones OFF Please

The Network LayerParminder Singh Kang

Home: www.cse.dmu.ac.uk/~pkang

Email: pkang@dmu.ac.uk

1. 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.

Advantages and disadvantages?

• 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

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.

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)

3 The Network Layer:

• 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.

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.

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.

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.

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.

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.

• 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.

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.

• 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:

Host H1 H2 H3 H4

Line to C to C to K to K

• 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.

    A’s table      C’s table      F’s table      K’s table L’s tableH1 1 C 1 A 1 F 1 C 1 K 1 F 1 L 1 K 1 H4 1

H1 2 C 2 A 2 H2 1          

H1 3 C 3 A 3 F 2 C 2 K 2 F 2 H3 1

H2 4 F 3 C 3 K 3 F 3 L 2 K 2 H4 2

H3 2 L 3 K 3 H4 3

• need of switch table?

• When switch table needed (Datagram or VC)?

4.4 Comparison of datagrams and virtual circuits

1.Datagrams contain the full destination address; large overhead with small packets.

2.Virtual circuits require table space in the memory of the hosts and routers

3. For small transactions the overheads of setting up a virtual circuit can be large

4. If traffic conditions change the routes of successive datagrams can change

5.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.

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

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

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

• 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) 

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?

5.2.1 Routing algorithms

• Flooding Algorithms.

• Static Routing Algorithms.

• Central Routing.

• Isolated Routing.

• Hot Potato Algorithm.

• Backward Learning Algorithm.

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’

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.

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.

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.

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. 

6 CCITT X25 for packet switching networks

• Terminology:

DTE Data Terminal Equipment is the user computer or terminal which is linked to a DCE.

DCE Data Circuit Terminating Equipment which provides the interface to the network

PAD Packet Assembly/Disassembly Interface enables dumb terminals or PCs to link to an X25 networks via an RSR232 serial interface.

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.

1. 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.

Input OutputLink LCN Next hop New LCN

• 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.

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

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

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.

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.