Local Area Networks (LANs)

LAN Topologies LANs can be organised in a number of

ways Bus

• A number of devices tap into a common shared medium

• Terminating resistances at either end prevent the disruptive reflection of signals

LAN Topologies (2) Tree

• The tree topology is an extension of the bus topology

• A tree can have numerous ‘branches’, and the ‘root’ of the tree is known as the ‘headend’

• Allows a much more complex layout than a bus topology

LAN Topologies (3) Issues with Bus and Tree Topologies

• Any signal transmitted by one station is received by all

o There must be a way to indicate who the transmission is intended for. LANs send data in frames, which contain an address field to indicate which station the frame is for

• If two stations transmit at the same time, the signals collide and disrupt each other

o There must be some means of regulating who can transmit and when. This process is known as Medium Access Control, or MAC

LAN Topologies (4) Ring

• Consists of a set of repeaters joined by point to point links that form a ring

• Stations attach through repeaters• Data is transmitted in one direction only

Ring (continued)• Data is transmitted in

frames• Stations monitor

passing frames and copy any that are addressed to them

• Frames are removed once they have circulated back to the sender

• MAC is needed so that a station doesn’t transmit while a frame is passing by. E.g. Token Ring

LAN Topologies (6) Star

• Devices connect to some central node, typically by two point to point links (send and receive)

LAN Topologies (7) Star (continued) For a star topology, there are two

alternatives for the central node• Broadcast – the node simply sends any frames

it receives to all output linkso This essentially joins all the links together and makes

them appear like one medium, i.e. a bus/treeo The node is known as a ‘hub’ or ‘repeater’

• Switching – this requires the central node to be ‘intelligent’

o The node must analyse the address of incoming frames and ‘switch’ the frame to the correct outgoing link

Medium Access Control A means of controlling access to the medium

to promote orderly and efficient use Two points to consider – where and how Where – is control of the medium distributed

or centralised?• Centralised – a device is designated to have

authority to grant access to the medium. Any other device must wait for permission before it can transmit

• Distributed – devices collectively implement a MAC function to determine who has access to the medium

Medium Access Control (2) There are advantages and disadvantages to a

centralised scheme• A centralised scheme provides a common point of failure• Can act as a bottleneck• Allows greater control of access• Allows simple access logic at each station• Avoids the problem of distributing control

How – can be categorised as synchronous or asynchronous

Synchronous – a specific capacity is dedicated to a connection, as in circuit switching and multiplexing techniques• This is generally inefficient for computer communications,

as the demands of each station are unpredictable• It is better to be able to allocate access to the medium in a

dynamic (asynchronous) fashion

Medium Access Control (3) Asynchronous – this is the most commonly

used MAC method, and can be divided into three categories• Round Robin – each station in turn is given

the chance to transmito This offer may be declined or acceptedo In a distributed round robin system, each station

passes control to its immediate neighbour when it has finished with the medium. This is commonly used with a ring topology

Medium Access Control (4) Asynchronous (continued)

• Reservation – similar to synchronous MAC in that a certain capacity is dedicated to a station

o In this case the allocation is by reservation, rather than fixed

o Reservations (time slots on the medium) can be made in a distributed or centralised fashion

• Contention – no cooperation is usedo When a station wants to transmit, it goes ahead

without waiting for permissiono This is fine when not many stations want to transmit

at once, but tends to fail under heavy load

LAN Protocol Layers A LAN uses a MAC layer to control access to

the medium. This generally sits above the physical layer

The MAC layer provides• Framing• Error detection – Cyclic Redundancy Check

(CRC)• Addressing• Controlled access to the medium

MAC Layer

Physical Layer

MAC Layer

Physical Layer

MAC Layer

Physical Layer

LAN Protocol Layers (2) MAC frames differ for different LAN

technologies, but generally they all have the following attributes

• Control – carries specific control information• Destination address – who the frame is

destined for (physical)• Source address – who sent the frame

(physical)• Data – information the frame is transporting• CRC – used to verify that the frame is correct

MACControl

DestinationMAC Address

SourceMAC Address Data CRC

MACFrame

LAN Protocol Layers (3) The MAC does not provide

• Error correction• Flow control

Thus it does not provide all of the requirements of a ‘Data Link Layer’

Some LANs use another layer, that sits above the MAC layer, to provide these remaining functions• LLC (HDLC derivative)

MAC Layer

Physical Layer

MAC Layer

Physical Layer

MAC Layer

Physical Layer

LLC Layer LLC Layer LLC Layer

LAN Protocol Layers (4) Logical Link Control (LLC)

• LLC is used to provide error correction and flow control over a MAC layer

• The MAC layer already provides CRC checking and addressing

• DSAP and SSAP – these specify the user (higher-layer protocol) the frame is for on the destination device, and the user it is from on the source device. Typically these are the same

• Information – the data that the frame is carrying

LAN Protocol Layers (5) LLC specifies three methods for transferring

data (known as ‘services’) Connection-mode service – similar to HDLC

• A ‘connection’ is established with the destination station, and information is transferred using sequence numbers for flow and error control

Unacknowledged connectionless service• There is no ‘connection’, no flow control, and no

error correction Acknowledged connectionless service

• No connection is established, but an acknowledgement is required for each frame, i.e. stop-and-wait flow control/error correction

LAN Protocol Layers (6)

The LLC layer provides an interface for higher protocol layers• Software written to interact with LLC will easily port to

devices on different types of networks, provided that there is an LLC layer

Data Link Layer

Physical Layer

LLC LayerMAC Layer

Physical Layer

Theoretical datacommunicationsprotocol stack

LAN protocolstack

MAC Header Data CRC

LLC Header Data

Data

001000101011100101010010101000010010101

Issues with Bus/Tree Topologies Due to signal attenuation, a received signal

is always weaker than when it was transmitted

If it is too weak, then it becomes error prone, or unrecoverable

This is a problem with large bus/tree LANs as any station must be able to communicate with all others, no matter what distance they are away

A B C X Y Z…

This problem is usually solved by using repeaters to divide the medium into smaller segments

Repeaters relay digital signals in both directions, making the segments appear like one medium

As repeaters recover the digital signal, they remove any attenuation

Bus/Tree LANs The most popular medium for bus/tree LANs is

coaxial cable• The physical layer can use either digital or analogue

signals. In LAN terminology these are called baseband and broadband respectively

Baseband• A LAN baseband physical layer typically uses manchester

or differential manchester encoding• Digital signals do not cope with branching very well.

Hence, baseband signalling is used with bus, rather than tree topologies

• Digital signals propagate both ways from a tap, spreading to neighbouring stations on the bus

• Length of bus is restricted to a few kilometres at most due to the attenuation of the high frequency components

• FDM (frequency division multiplexing) not possible with digital signals as signal uses entire bandwidth

Bus/Tree LANs (2) Broadband

• Uses analogue signals (hence each station has a modem)• FDM is possible• Branching is possible, so bus or tree topologies can be used• Distances up to 10 km possible• Amplifiers (as opposed to repeaters) are used for signal

regenerationo Amplifiers are unidirectional, which means that broadband

LANs have to provide for two data paths (send and receive) – typically FDM is used or there are two physically separate cables

• Broadband LANs have been used by cable TV companies to provide interactive services over existing coaxial cable networks

• Hardly ever installed from scratch these days

Issues with Ring Topologies A ring LAN consists of a number of point-to-point

links joining repeaters• Repeaters regenerate data and pass it on to the next

repeater in the ring• Each repeater also serves as a station connection point• Each repeater must be able to receive data, insert data and

remove data

Issues with Ring Topologies (2) To receive data, the repeater sends a copy of the

data to the attached station, and passes the data on to the next repeater• Repeater has knowledge of the packet format in order to

scan addresses and control information• Some control strategies may allow the repeater to modify a

bit as it passes by – can be used to acknowledge a packet

Issues with Ring Topologies (3) Sending data and removing data happen at the same

time, as the only time data has to be removed is when the station is sending a new frame

When transmitting, data may appear on the incoming link. This data could be• From the same packet the transmitter is still in the process of

sending (will happen if the ‘bit length’ of the ring is shorter than the packet)

• From some other packet (if multiple packets can be on the ring at the same time) – the repeater buffers these to transmit later

Issues with Ring Topologies (4) The distance covered and number of stations can be

greater in a ring than a simple baseband bus, as each link regenerates the digital signal and thus attenuation problems are avoided

However, a problem known as ‘timing jitter’ restricts this• Each station aligns its clock to the received data stream and uses

this clock for sending data• The next station does the same, but any small error gets passed

from station to station, getting magnified (or diminished)• This means that the clock ‘jitters’ and this can cause bits to be lost• The more stations in the ring, the greater the effect

Other problems with rings• A break anywhere in the ring brings down the entire LAN• The ring has to be broken to insert a new station

Issues with Star Topologies Star topologies are

common when twisted pair is used for the medium• twisted pair has poor noise

immunity and thus tapping it as a bus results in very poor signals

• Twisted pair is usually used for point to point connections – hence the ‘star’ topology

• A hub or repeater joins the twisted pair so that logically they appear to be all one medium

Issues with Star Topologies (2) Why use twisted pair when it has poor

capabilities compared to coaxial cable?• It is already installed in many buildings for telephone

purposes• When used for a LAN, saves the significant cost of

installing special cable

Recently, intelligent ‘switches’ have been used instead of hubs• A switch determines which station is on each connection

and sends frames to the appropriate output line• At the same time, other unused lines can be used for

switching other traffic• This significantly improves the efficiency of the LAN

under heavy load and has made star topologies popular

Issues with Star Topologies (3) Star topologies can be arranged

hierarchically, with a number of hubs or switches

Bridging There is often a need to communicate with

machines that are not attached to the same network.• An example might be two campuses that wish to

exchange data. As there are a wide variety of network technologies the two campuses may not necessarily have the same type of network

• This means there has to be a method of ‘inter-network’ communication

• Two approaches are used for this purpose: bridges and routers

o Bridges interconnect similar LANso Routers are more general devices capable of

interconnecting a variety of LANs and WANs (wide area networks)

Bridging (2) Bridging was developed to join local area networks

There are a number of reasons for doing this• Reliability – the bridge joins networks but keeps them

physically separate; a fault on one won’t necessarily affect the other

• Performance – the performance of a broadcast LAN decreases as the number of machines on the network increases. A number of smaller LANs will perform better if they are grouped such that intra-network traffic exceeds inter-network traffic

S S S

LAN A

S S S

LAN B

Bridge

Bridging (3) Reasons (cont.)

• Security – if sensitive traffic can be kept on one network then the chances of it falling into the wrong hands are reduced

• Geography – LANs tend to have restricted range. If an organisation has two geographically separate LANs it wants to join, then two ‘half-bridges’, joined by a point to point link can be used

S S S

LAN A

S S S

LAN B

HalfBridge

HalfBridge

Functions of a Bridge The bridge reads all frames transmitted on A, and accepts

those addressed to B Using the medium access control protocol for B, it retransmits

the frames Does the same for the B-to-A direction

Functions of a Bridge (2) The bridge makes no modifications to the frames it

receives. It does not add headers or trailers – it simply copies the relevant frames from one LAN to another

The bridge should contain some buffer space so it can store frames in case that the destination LAN is busy

A bridge may join more than two LANs The bridge makes all the LANs appear to be one large

LAN, made up of machines with unique addresses The bridge must know what addresses are on what

LANs. There may be a large number of LANs joined by several bridges, in which case a ‘routing’ decision may need to be made

Bridge Protocol Architecture

Routing with Bridges When a bridge is used to join two LANs, it

makes a very simple routing decision – whether to forward the frame or not• This is done by checking the destination address

of the frame against a list of known addresses on the other LAN, and forwarding in the case of a match

• In the case where the bridge joins more than two LANs, it must also decide which LAN to forward the frame to

S S S

LAN A

S S S

LAN B

Bridge

Routing with Bridges (2) However, more

complex topologies can be implemented using a number of LANs and bridges

In this situation, routing can still be achieved if each bridge knows all of the station addresses that are on either side of it

However, there can be more than one route between LANs…

Routing with Bridges (3) In this situation there is

more than one route between LAN A and LAN E.

One method of handling this is to make each bridge aware of where all the stations are in the internet, but this is inefficient for large networks

It would be advantageous if the bridges knew the best route for a given address and were able to cope with a changing topology dynamically

Routing with Bridges (4) Routing is easier if the frame’s destination address

can be broken up into two parts – a network address and a station address• Eg if a 16 bit number is used to specify the destination

address in the MAC frame, then the first 8 bits can be used to specify the network the station is on, and the last 8 bits can be used to specify the station on that network

• This means that a bridge only has to look at the first byte of the number, and send the frame to that network

• As LANs are typically ‘broadcast’ in nature, the intended station will receive the frame as long as it appears on the LAN

0xF274

Network ‘F2’ Station ‘74’

Fixed Routing A routing table is developed that specifies the

route to use between any two LANs in the internet

Central Routing Directory

Source LAN

A B C D E F G

A - 101

102

103

107

105

106

B 101

- 102

103

104

105

106

C 102

101

- 103

107

105

106

D 101

103

102

- 104

105

106

E 107

104

102

103

- 105

106

F 102

101

105

103

107

- 106

G 102

101

106

103

107

105

-

Destination LAN

Fixed Routing (2) From the central routing table, individual directories

can be stored at each bridge – each bridge needs one table for each LAN to which it attaches• The information for each table is derived from a single

column in the central table

Bridge 104 Table

From LAN B From LAN E

Dest Next Dest Next

A - A -

C - B B

D - C -

E E D B

F - F -

G - G -

Bridge 107 Table

From LAN A From LAN E

Dest Next Dest Next

A - A A

C - B -

D - C A

E E D -

F - F A

G - G A

Spanning Tree Routing Fixed routing is widely used due to its simplicity and

minimal requirements. However, in a complex internet, where many bridges may be added or removed and failures must be allowed for, it is limited.

Spanning tree routing – allows bridges to automatically develop a routing table and update it in response to topology changes• It consists of three mechanisms – frame forwarding, address

learning, and loop resolution

Frame forwarding – the bridge maintains a database based on MAC addresses. Each entry consists of a MAC address, a ‘port’ number, and an ‘aging’ time

Spanning Tree Routing (2) The ‘port’ number indicates which LAN to transmit on

to reach the associated MAC address. Every time a frame is received, the following algorithm is followed

Frame receivedwithout error onport x

DA found infiltering DB?

Outbound port= port x?

Forward frameon outboundport

Forward frameon all portsexcept x

No

Yes No

Yes

Bridge forwarding

DA = destination address

Spanning Tree Routing (3) To add entries to the database the bridge

employs a method known as address learning:

A timer value of 300 seconds is commonly used. When it expires, the entry is removed. This allows the database to handle dynamic changes in the network

Bridge learning

SA found infiltering DB?

Update directionand timer Finished

Add SA todatabase withdirection andnew timer

Yes

No SA = source address

Spanning Tree Routing (4) This method of address learning can lead to ‘loops’ in networks

where there is more than one possible path between LANs• In the situation below both bridges will see frames from station A on

LAN Y, and erroneously update their tables to indicate that station A is in that direction

Spanning Tree Routing (5) The preceding problem can be avoided by

determining a ‘spanning tree’ for the network• This is a collection of bridges that connect all

LANs in the internet once only (no closed loops)

• It involves assigning bridges with unique identifiers and ‘costs’ for each bridge port, so that between them they can determine a hierarchy that spans the internet, and ensures that only one bridge will forward a frame in a given situation