Understanding Network Architecture

• Access Methods– CSMA/CD

• Carrier Sense Multiple Access/Collision Detect

– CSMA/CA• Carrier Sense Multiple Access/Collision Avoid

– Token Passing– Demand Priority

• Can’t buy this type of hardware any more

Access Methods

• Carrier Sense Multiple Access with Collision Detection– Ethernet

– Senses when the cable is connected (Carrier Sense)

– Senses when no traffic is passing to transmit (multiple access)

– Senses possible collisions (Collision Detect)

– Computers contend for access on the wire

– C03dem01, 02, 03, 04, 05, 06

Packet Traffic

Network Cable

Collisions

Signal 1

Signal 2

0

50

5

0

10

Sum of 1 and 2

Ethernet threshold

CSMA/CA

• Carrier Sense Multiple Access Collision Avoidance– The computer announces that it wants to

transmit.– Other computers wait until that computer

transmits it’s data– Creates more traffic on the wire– c03dem07

Token Ring

• Special packets called tokens circulate in a ring from computer to computer.– If a free token is detected then a computer will

take control and send it’s data– If a token is destined for a specific computer

that computer takes the token– One one computer at a time can use the token

so no collisions take place.– C03dem08, 09, 10, 11 ,12

Demand Priority

• 100VG-Any LAN (obsolete)– Hubs (repeaters) manage the packet passing by

searching for requests– Similar to CSMA/CD except that the hubs be assign

priorities.– More efficient since broadcasts are minimized across

the network.– Was made obsolete by Ethernet– Vendors don’t sell 100VG NICs anymore– c03dem13,14

Packet Contents

• Contain data for passing between computers– Have timing, begin, and end markers– Have addressing (source/dest) information– Have error detection/correction– Have type identifiers so computers can filter

out packets at a low level.– Contain requests for how to pass the packet

Actual Ethernet Frame

• 62 bits of preamble (alternating 1’s and 0’s)

• 2 bits Start of Frame Delimiter (2 one’s)

• Destination Ethernet Address

• Source Ethernet Address

• Length or Type

• Data (if less than 46 bytes then pad to 46)

• 4 bytes (32 bits) Frame Check Sequence

Ethernet destination Address (first 32 bits)

Ethernet Dest (last 16) Ethernet Src (16 bits )

Ethernet source Address (last 32 bits)

Type Code or Length of packet (16 bits)

Length from 512 to 4096 bytes

Ethernet Checksum

IP header, TCP header, then your data

Ethernet Packet

Network Traffic

• All of the computers in you local area see all of the packets on the wire and have to decode them and see if a particular packet is addressed to this computer.

• Each NIC has a unique address assigned to it.• When a packet is sent that unique address is build in to the

packet.• Before connection is established the source computer sends

out a broadcast (all 1’s in the data portion of the destination address.

• The destination system (responds to a specific type of request) then replies with it’s address to finish the connection setup

Network Installations

• Cross connects– Blocks that connect wall jacks to hubs– May changes/removals easier

• Patch panels– Allow changes from one network to another– Allow changes to wiring if a hubs fails

Hub Advantages

• Standard Telephone Wiring

• Standard Punch Blocks and Cross Connects

• Ability to disable a single port

• Easier to monitor traffic patterns

• Visible Indication of Link Status

• If a wire is cut only one port is affected – Unless the backbone feed is cut!

5-4-3 Repeater Rule 10MB

• 3 repeater rule

Repeater

Repeater

Repeater

Repeater

System 1

Sys 2 Sys3

System 1 can talk to Sys 2 but not Sys3

5-4-3 Rule

• 5 segments (between repeaters or work groups)

• 4 repeaters

• 3 segments can have computers attached.

• Violation of this rule means one end of the network can’t talk to the other end.

Network Diameter (100MB)

• 205 Meter Limit

Repeater

Repeater

100M

5M

100M

10MB diameter is 500M

Segmentation of the Network

• Segmentation can control broadcast traffic on a network

• Segmentation can enhance security on the network (limited traffic monitoring)

• Segmentation can be implemented with bridges (2 port) and switches (multi ports)

• If more segmentation is needed then a routed network may need to be installed.

Switches and Bridges

Repeater

Repeater

Repeater

RepeaterB

ridge

Bridge Insides

Interface

Interface

Filter based on ethernet address

LAN A LAN B

Switch Insides

A

B

C

D

E

F

G

Switching based onethernet address

100baseT to 10baseTHub/Bridge10baseT 100baseT

10baseT hub

100baseT hub

100baseTto10baseTBridgeModule

10MB Ethernet Rules

• 3 repeater (populated) limit

• 5 repeater per segment limit

• 7 bridges per ethernet lan

• Cable lengths

• 1024 stations per ethernet lan

100 MB Ethernet Rules

• 2 hubs no more than 5 meters apart

• Total length of network segment is 205 meters.

• Must use Cat 5 or better cable

• Punch blocks must be Cat 5 certified

How to get around limits?Hubs

Switch

ROUTER

Other Lans

To the Internet(switchingbased onIP address)

Firew

all

Token Ring

Mic (Media Interface Connector at eachcomputer)

4 or 16 MB/s

MAU or MSAU

• Multistation Access Unit

``Ring in Ring out

Computer Ports

The MAU allows the ring to be installed in a star topologyThe MAU isolates a port if the attached computer fails

Token ring operation

• First computer on the network sends a token• The token travels around the ring• If a computer wants to transmit it takes control of the token• That computer sends it’s data and the frame goes around

the ring to the destination computer.• The destination computer takes the token and marks the

token as received.• The token continues until it arrives at the source computer

who acknowledges the data. The sending computer then removes the token from the ring and sends a new token.

More token passing

• One 1 token can be on the ring at one time.• The token passing only goes in one

direction.• Transfer rates are either 4 or 16 MBS• IBM POS (Point of Sale) systems still use

token ring networks• Routers can handle mixed token ring to

ethernet networks.

Appletalk

• LocalTalk– 230 Kbps, single twisted pair, obsolete

• EtherTalk– uses ethernet

• TokenTalk• AppleShare

– File Sharing

• Zones– Sort of like subnets or workgroups

ARCnet

• ARCnet hubs and nics are still available • Uses token bus access method• Can handle up to 255 network numbers per

network.• Uses a single coax or twisted pair cable per

computer• The cable is different than ethernet cable• The cable distances are longer than ethernet• 2.5 Mbps speed