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Router

NetworkNetwork

Data LinkData Link

PhysicalPhysical A router routes with the network address (such as the IP address)

Data LinkData Link

PhysicalPhysical A bridge routes withthe MAC address

PhysicalPhysical A repeater boosts thesignal

Repeater

Bridge

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BACKBONE1

BACKBONE2

Local network

Local network

Local network

Local network Local network

BACKBONE3

LAN BLAN B

LAN ALAN A LAN DLAN DLAN CLAN C

LAN ELAN E LAN FLAN F

Bridge orrouter

Bridge orrouter

Bridge orrouter

Bridge orrouter

2.1 Interconnection of networks

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Centralserver

Bus network

Ring network

Star network

2.2 Network topologies

Star NetworkAdvantages:Since the data rate is relatively low between central server and the node, a low-specification twisted-pair cable can be used connect the nodes to the server. A fault on one of the nodes will not affect the rest of the network. Typically, mainframe computers use a central server with terminals connected to it.Disadvantages:Network is highly dependent upon the operation of the central server. If it were to slow significantly then the network becomes slow. In addition, if it were to become un-operational then the complete network would shut down.

Star NetworkAdvantages:Since the data rate is relatively low between central server and the node, a low-specification twisted-pair cable can be used connect the nodes to the server. A fault on one of the nodes will not affect the rest of the network. Typically, mainframe computers use a central server with terminals connected to it.Disadvantages:Network is highly dependent upon the operation of the central server. If it were to slow significantly then the network becomes slow. In addition, if it were to become un-operational then the complete network would shut down.

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Concentrator (or hub)

Concentrator (or hub)

Network backbone

2.3 Tree topology

Tree topologyResources should be grouped logically when attached to hubs, such as local file servers, printers, and so on.

Advantages:Nodes local to a hub can communicate with each other without the data traffic going onto other network segments.A fault on a single computer or a hub does not bring the whole network down.Communication between the hub and the computer has relatively low data transfers, as opposed to the transfer connected hubs to hubs.Easy to connect and disconnect to.

Disadvantages:Network may suffer from slow data transfer if the network is not planned to reflect tree topology. Typically network is created with workgroups.

Most networking technologies now use hubs to connect to.Typically for Ethernet and Token Ring networks.

Tree topologyResources should be grouped logically when attached to hubs, such as local file servers, printers, and so on.

Advantages:Nodes local to a hub can communicate with each other without the data traffic going onto other network segments.A fault on a single computer or a hub does not bring the whole network down.Communication between the hub and the computer has relatively low data transfers, as opposed to the transfer connected hubs to hubs.Easy to connect and disconnect to.

Disadvantages:Network may suffer from slow data transfer if the network is not planned to reflect tree topology. Typically network is created with workgroups.

Most networking technologies now use hubs to connect to.Typically for Ethernet and Token Ring networks.

Workgroup- with printer- file server- etc

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2.4 Star Network

Centralserver

Star NetworkAdvantages:Since the data rate is relatively low between central server and the node, a low-specification twisted-pair cable can be used connect the nodes to the server. A fault on one of the nodes will not affect the rest of the network. Typically, mainframe computers use a central server with terminals connected to it.Disadvantages:Network is highly dependent upon the operation of the central server. If it were to slow significantly then the network becomes slow. In addition, if it were to become un-operational then the complete network would shut down.

Star NetworkAdvantages:Since the data rate is relatively low between central server and the node, a low-specification twisted-pair cable can be used connect the nodes to the server. A fault on one of the nodes will not affect the rest of the network. Typically, mainframe computers use a central server with terminals connected to it.Disadvantages:Network is highly dependent upon the operation of the central server. If it were to slow significantly then the network becomes slow. In addition, if it were to become un-operational then the complete network would shut down.

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2.5 Bus network

Common bus

All computers have access toa common bus at the same time

Ethernet hub

Bus networkUses a multi-drop transmission medium.All nodes on the network share a common bus and all share communications. This allows only one device to communicate at a time. A distributed medium access protocol determines which station is to transmit. data frames contain source and destination addresses, where each station monitors the bus and copies frames addressed to itself.

Twisted-pair cables give data rates up to 100Mbps, whereas, coaxial and fibre optic cables give higher bit rates and longer transmission distances. Gigabit Ethernet is now available (1Gbps).

A typical bus network is Ethernet 2.0.

Bus networkUses a multi-drop transmission medium.All nodes on the network share a common bus and all share communications. This allows only one device to communicate at a time. A distributed medium access protocol determines which station is to transmit. data frames contain source and destination addresses, where each station monitors the bus and copies frames addressed to itself.

Twisted-pair cables give data rates up to 100Mbps, whereas, coaxial and fibre optic cables give higher bit rates and longer transmission distances. Gigabit Ethernet is now available (1Gbps).

A typical bus network is Ethernet 2.0.

Advantages:Good compromise over the other two topologies as it allows relatively high data rates. If a node goes down, it does not affect the rest of the network.

Disadvantages:Requires a network protocol to detect when two nodes are transmitting at the same time. Does not cope well with heavy traffic rates.

Advantages:Good compromise over the other two topologies as it allows relatively high data rates. If a node goes down, it does not affect the rest of the network.

Disadvantages:Requires a network protocol to detect when two nodes are transmitting at the same time. Does not cope well with heavy traffic rates.

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Nodes can only transmit data whenthey capturethe token

Electronic tokenis passed from node to node

2.6 Token passing ring network

Token RingOrderly access to ring.Single electronic token passes from one computer to the next around the ring. Computer can only transmit data when it captures the token. Each link between nodes is a point-to-point link and allows the usage of almost any type of transmission medium.

Typically, twisted-pair cables allow a bit rate of up to 16Mbps, but coaxial and fibre optic cables are normally used for extra reliability and higher data rates.

A typical ring network is IBM Token Ring and FDDI.

Token RingOrderly access to ring.Single electronic token passes from one computer to the next around the ring. Computer can only transmit data when it captures the token. Each link between nodes is a point-to-point link and allows the usage of almost any type of transmission medium.

Typically, twisted-pair cables allow a bit rate of up to 16Mbps, but coaxial and fibre optic cables are normally used for extra reliability and higher data rates.

A typical ring network is IBM Token Ring and FDDI.

Advantages:All nodes on the network have an equal chance of transmitting data.

Disadvantages:If one of the nodes goes down then the whole network may go down.Token may get lost, or many tokens are generated.Difficult to add and delete nodes to/from the ring.

Advantages:All nodes on the network have an equal chance of transmitting data.

Disadvantages:If one of the nodes goes down then the whole network may go down.Token may get lost, or many tokens are generated.Difficult to add and delete nodes to/from the ring.

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2.7 Token Ring (example data exchange)

ControlToken(a)

DataFrame(b)

ControlToken

(d)

Ack.A

D

C

B

(c)

Token RingControl token rotates round the ring.Node wishing to transmit data captures the token.Node captures token and transmits a data frame.Data frame rotates round network.All nodes read the frame and determine if the data is for them.Destination node reads the data, and sets an acknowledgement flag.Data frame continues round the network, until the source node receives it.Source node puts the control token back on the ring.

Token RingControl token rotates round the ring.Node wishing to transmit data captures the token.Node captures token and transmits a data frame.Data frame rotates round network.All nodes read the frame and determine if the data is for them.Destination node reads the data, and sets an acknowledgement flag.Data frame continues round the network, until the source node receives it.Source node puts the control token back on the ring.

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2.8 CSMA/CD CSMA/CDEthernet uses carrier sense, multiple access with collision detection (CSMA/CD). Nodes monitor the bus (or Ether) to determine if it is busy. A node wishing to send data waits for an idle condition then transmits its message. Collisions can occur when two nodes transmit at the same time, thus nodes must monitor the cable when they transmit. When a collision occurs, both nodes stop transmitting frames and transmit a jamming signal. This informs all nodes on the network that a collision has occurred. Each of the nodes involved in the collision then waits a random period of time before attempting a re-transmission. As each node has a random delay time then there can be a prioritization of the nodes on the network.

CSMA/CDEthernet uses carrier sense, multiple access with collision detection (CSMA/CD). Nodes monitor the bus (or Ether) to determine if it is busy. A node wishing to send data waits for an idle condition then transmits its message. Collisions can occur when two nodes transmit at the same time, thus nodes must monitor the cable when they transmit. When a collision occurs, both nodes stop transmitting frames and transmit a jamming signal. This informs all nodes on the network that a collision has occurred. Each of the nodes involved in the collision then waits a random period of time before attempting a re-transmission. As each node has a random delay time then there can be a prioritization of the nodes on the network.

Two nodes transmitat the same time

1

Nodes detect therehas been a collision

2

Nodes transmit ajamming signal

3

Nodes wait a randomperiod before retransmitting

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Common bus

All computers have access toa common bus at the same time

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2.9 Routers, Bridges and Repeaters

Router

NetworkNetwork

Data LinkData Link

PhysicalPhysical A router routes with the network address (such as the IP address)

Data LinkData Link

PhysicalPhysical A bridge routes withthe MAC address

PhysicalPhysical A repeater boosts thesignal

Repeater

Bridge

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RepeaterRepeater

Network segment (repeater extends the network segment)

RouterRouter

Bridge only forwards if thestation (or MAC) address is not on the connected network segment that it originated from. Broadcasts are also passed over.

BridgeBridge

Router only forwards if the network address is on anothernetwork. It does not forward broadcasts.

Network segment bounded by a router or a bridge

2.10 Repeaters, bridges and routers

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NapierNapier

Hub(or switch)

Hub(or switch)

RouterRouter

Hub(or switch)

Hub(or switch)

EaStMANnetwork

EaStMANnetwork

UoEUoE

Stirling University

Stirling University

Heriot-Watt

Heriot-Watt

SuperJANET

SuperJANET

LonMANLonMAN

BWE MAN

BWE MAN AbMANAbMAN InternetInternet

Edinburghand StirlingMAN

Connect toall the Universitiesin the UK

2.13 Example connection (UK Universities)

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Wiring closet

Backbone cabling(vertical cabling).Typically fiber opticcable (to preventground loops andTo run for long distances)

Telecommunicationsoutlet/ connector

Work areastation cable(max: 3m)

Horizontal Cable(max: 90m)

Patchcable/jumper(max: 6m)

2.11 Cabling

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200 m

HubHub

200 m

100 m

2.12 Maximum for twisted-pair