lans no. 1 seattle pacific university small local area networks: single collision-domain networks...

LANs No. 1Seattle Pacific University

Small Local Area Networks:Single Collision-Domain Networks

Kevin BoldingElectrical Engineering

Seattle Pacific University


Local Area Networks• General Definition(s) of a LAN

• Small (<100 stations, single building)

• Trusted users

• Single ownership

• Fast

• Single Collision Domain networks

• All devices connected to the network share the network at the same time

• Only one device may send data on the network at a time

• Data is sent to all receivers; no routing occurs


Network Topologies

• Network topology

• Describes the physical layout of a network

• How are the computers and devices connected?

• Basic shape of design

• Bus (line)

• Ring

• Star


Bus Topology

• One cable connects all components

• Cable may be straight or “snake” through the building

• Individual devices are connected on branches of the backbone

12 37 13 09

To: 37 Data:xxxx

• Each message is sent to all computers

• Only the addressed computer responds to the message

XX


10Base-2 Bus LANs• Most Bus LANs are connected

using 10Base-2 Coaxial Cable

• 10Mbps, baseband transmission, 200m max

• 50, RG-58 A/U

• BNC-type connections

• Use a ‘T’ to make a spur to connect a workstation

• Terminators

• All cable ends must be terminated (50) to prevent reflections


Bus Issues

• There’s only one cable system

• Any breaks will isolate part of the network• It’s even worse!

• Unplugging a single workstation creates an un-terminated end in the network• Communication in the whole network is stopped

• Like trying to talk in an echo chamber

• The Bus is a passive system

• Signals are simply sent through the wires

• No active components refresh the signals

• Repeaters can be used if necessary


Bus Pros and Cons

• Simple

• “Plug and play”

• Many single-point failure modes

• Broken backbone

• Any unplugged computer that isn’t terminated

Advantages Disadvantages

• Cheap

• Mostly all cables

• Cables are relatively inexpensive

• Linear arrangement

• Just run your backbone in a path that goes near the workstations

• Difficult to trace errors

• All one big “wire”

• Problem could be anywhere

• Limits on cable length seriously constrain size

• Repeaters needed


Star (Hub) topologies

• A Hub is a multi-port repeater

• Reads input from any one of its ports

• Repeats it to all other ports

• Systems built around a hub usually have a star topology

• Usually use 10Base-T or 100Base-T

• 10/100Mbps, Baseband, UTP

• RJ-45 Connectors

HP ProCurve Compact 10Base-T Hub 8

Standard Ports

Crossover Port

• Logically, the same as a bus

• Medium is still shared by all stations at all times


Star (Hub) Pros and Cons

• Termination is no longer an issue

• An un-terminated line only effects communication to one device

• Can’t bring the network down by unplugging your network connection

• Single-point failure

• If the hub goes down, all is lost

Advantages Disadvantages

• Expansion is easier

• Active hubs reduce loading limitations

• Easier troubleshooting

• Isolated connections

• More cable needed

• All cables must reach from workstations to the hub


Star-Star Multi-Hub LANs

• Connect multiple stars (hubs) together in a bus or star

• Isolates individual groups from each other

• Hierarchical• Logically, still one single shared medium

Star-Star

A Star-Star hub network is logically the same as a bus network


Connecting Hubs

HP ProCurve Compact 10Base-T Hub 8

Crossover Port

RxTx

TxRxHub NIC

RxTx

TxRxHub Hub

TxRx

• Connection from hub-to-hub requires a crossover port or crossover cable


Ethernet Physical Media and Signals• Most widely-accepted Ethernet standards use the same medium

and connectors

• 4-pair UTP – Cat 5e is the most common today

• RJ-45 connectors

• Cable length of up to 100m


Ethernet – 10BaseT

• 10BaseT

0 1 0 1 1 1 0 1 0 0 0 1

• Uses Manchester coding at 10Msps to send 10Mbps

• Uses only 2 of the 4 pairs – one for each direction


Ethernet – 100BaseTX

• 100BaseTX

0 1 0 1 1 1 0 1 0 0 0 1 0 1

• For error correction, uses a 4B/5B code – sends 5 bits to represent 4 actual bits (80% efficient)

• Uses MLT-3 3-level coding (similar to AMI) at 125M symbols/s

• 125M symbols/s x 4/5 bits/symbol = 100Mbps

• Uses only 2 of the 4 pairs – one for each direction


Ethernet – 1000BaseT• 1000BaseT

00 01 11 10 11 10 00 01 00 11 00 10 00 11

000111

1011

• Each UTP pair uses a 5-level coding (2-bits per symbol) at 125M symbols/s to send 250Mbps• Warning – this reduces the tolerable SNR by 6dB, which increases the error rate

• Uses all four pairs at the same time – 4 x 250Mbps = 1Gbps in one direction

• Uses simultaneous bi-directional signaling (echo cancellation) to send signals at the same time in both directions on the wire – 1Gbps in both directions

• Uses a complex coding scheme (Trellis coding) to decrease the error rate. This is equivalent to adding back 6dB to the SNR


Ethernet – 10GBaseT• 10GBaseT

• 10Gbps over twisted pair

• 55m over Cat-6 UTP

• 100m over Cat-6a UTP

• 16-level encoding (4 bits/symbol) * 800M symbols/s = 3.2 Gb/s per pair

• Encoding loses a few bits 2.5 Gb/s per pair

• 4 pairs, simultaneous bi-directional 10Gb/s in both directions

• Fiber – Several standards using single- or multi-mode fiber

• Up to 80 km!

lans no. 1 seattle pacific university small local area networks: single collision-domain networks...

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