Saturday, December 27, 2008

Putting Data on the Cable

Access Methods
The 4 major methods
Carrier Sense Multiple Access Methods
1. with collision detection (CSMA/CD)
2. with collision avoidance (CSMA/CA)

Token passing that allows only a singe opportunity to send data

A Demand Priority method

Carrier Sense Multiple Access with Collision Detection. (CSMA/CD)
1. Computer senses that the cable is free.
2. Data is sent.
3. If data is on the cable, no other computer can transmit until the cable is free again.
4. If a collision occurs, the computers wait a random period of time and retransmit.
o Known as a contention method because computers compete for the opportunity to send data. (Database apps cause more traffic than other apps)
o This can be a slow method
o More computers cause the network traffic to increase and performance to degrade.
o The ability to "listen" extends to a 2,500 meter cable length => segments can't sense signals beyond that distance.

Carrier Sense Multiple Access with Collision Avoidance  (CSMA/CA)
in CSMA/CA, the computer actually broadcasts a warning packet before it begins transmitting on the wire. This packet eliminates almost all collisions on the network because each computer on the network does not attempt to broadcast when another computer sends the warning packet.
All other computers wait until the data is sent.
The major drawback of trying to avoid network collisions is that the network traffic is high due to the broadcasting of the intent to send a message.

Token Passing
Special packet is passed from computer to computer.
A computer that wants to transmit must wait for a free token.
Computer takes control of the token and transmits data. Only this computer is allowed to transmit; others must wait for control of the token.
Receiving computer strips the data from the token and sends an acknowledgment.
Original sending computer receives the acknowledgment and sends the token on.
the token comes from the Nearest Active Upstream Neighbor  and when the computer is finished, it goes to the Nearest Active Downstream Neighbor
uses "beaconing" to detect faults => this method is fault tolerant
NO contention => equal access to all computers on the network
NO collisions

Demand Priority
0. 100 Mbps standard called 100VG-AnyLAN. "Hub- based".
1. Repeaters manage network access by performing cyclical searches for requests to send from all nodes on the network. The repeater or HUB is responsible for noting all addresses, links and end nodes and verifying if they are all functioning. An "end node" can be a computer, bridge, router or switch.
2. Certain types of data are given priority if data reaches the repeater simultaneously. If two have the same priority, BOTH are serviced by alternating between the two.

Advantages over CSMA/CD 
1. Computers Uses four pairs of wires which can send and receive simultaneously.
2. Transmissions are through the HUB and are not broadcast to all other computers on the network.
3. There is only communication between the sending computer, the hub and the destination computer.
Other methods  
The cabling system for an AppleTalk network is called LocalTalk.
LocalTalk uses  CSMA/CA
AppleTalk has a dynamic network addressing scheme.
During bootup, the AppleTalk card broadcasts a random number on the network as its card address. If no other computer has claimed that address, the broadcasting computer configures the address as its own. If there is a conflict with another computer, the computer will try to use different IP combinations until it finds a working configuration.

ARC Net uses a token passing method in a logical ring similar to Token Ring networks.
However, the computers in an ARC Net network do not have to be connected in any particular fashion.
ARC Net can utilize a star, bus, or star bus topology.
Data transmissions are broadcast throughout the entire network, which is similar to Ethernet.
However, a token is used to allow computers to speak in turn.
The token is not passed in a logical ring order because ARCNet does not use the ring topology; instead the token is passed to the next highest numerical station
Use DIP switches to set the number (the Station Identifier) of the workstations, which you want to be beside each other so the token is passed to the next computer efficiently.
ARC Net isn't popular anymore => ARC Net speeds are a mere 2.5 Mbps.

Most important ARC Net facts for you to know:
ARC Net uses RG-62 (93 ohms) cabling;
it can be wired as a star, bus, or star bus; and
it uses a logical-ring media access method.

Summary Chart  

Type of Communication
CSMA/CD - Broadcast-based
CSMA/CA - Broadcast-based
Token Passing-Token-based
Demand Priority-Hub-based

Type of Access Method
CSMA/CD - Contention
CSMA/CA -Contention
Token Passing-Non-Contention
Demand Priority-Contention

Type of Network
CSMA/CD -Ethernet
CSMA/CA -Local Talk
Token Passing-Token Ring ARCnet
Demand Priority-100VG-AnyLAN

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