Carrier Sense with Multiple Access(CSMA)

CSMA is a coordinate technique which tells how the computers communicate over a shared medium.
The computers listen to the carrier (they read codes which define the status of the channel).
When the carrier channel is idle, the computer starts to communicate.

Collision Detection and Back off with CSMA/CD

Collision Detection

The phenomena in which multiple frames interfere with each other, is called collision.

Ethernet CD

For collision detection, Ethernet includes a hardware which detects transmission.
It does the following operations: 1. It monitors outgoing signals 2. Grabbled signal is interpreted as a collision

Recovery from Collision

When a collision is detected by a computer, it sends special signals to all other interfaces that a collision has happened.
Then the computers wait for the channel to be idle.
If the delay time (the waiting period) is same for both computers, then they will transmit the frames at same time and a collision will happen again.
To fix this problem, both the computers choose random delays.

Exponential Backup

Due to busy medium, collisions might still happen.
Therefore, the delays are doubled.
It reduces the chances of collision.

802.11 Wireless LANs and CSMA/CA

The IEEE 802.11 wireless LAN standard uses radio signals at 2.4 GHz.
Its data rate is 11 Mbps.
The older device uses radio signals at 900 MHz with data rate 2 Mbps.

Limited Connectivity With Wireless

Not all participants might be able to communicate with each other.
Following are the reasons: 1. It has low signal strength. 2. The propagation is blocked by walls 3. It cannot use CD to avoid interference because not all participants may hear.

Here d represents the maximum range of the signals.
According to the figure, computer 2 can communicate with both, 1 and 3 but computer 1 cannot communicate with computer 3 and vise versa.

CSMA/CA

The wireless LAN uses collision avoidance instead of collision detection.
The transmitter sends very short message to the receiver.
Then the receiver broadcasts to all transmitters, a short message reserving slot for that transmitter.

Collision

The receiver might get the request at same time.
In this scenario, there is a collision at the receiver's side and no reservation is sent to the transmitters.
The transmitters use backup and retry.
When requests are received, the receiver selects one of them and reservation for that transmitter is sent.

Local Talk

Apple invented the bus topology for the LAN.
The Mac computers have a default LAN port.
Its speed is about 230.4Kbps.
It uses CSMA/CA.

Token Ring

The sender holding token sends the message.
It travels along the ring, passes through the computers.
When it reaches the destination, the data is copied.

Using the Token

The token is like a permit which allows a computer to send data.
After the transmission is done, the token is also passed along the ring.

Token and Synchronization

There is only 1 token within the whole ring.
If the token is for some reason, lost, then it is re-generated by the hardware.
The token gives power to a computer to transmit one frame.
If all the computers are ready for transmission, then round robin access is enforced.

IDM Token Ring

It uses a special cable to connect the computers to the ring interface.
Its speed was originally 4 Mbps but now it is up to 16 Mbps.

FDDI

Fiber distributed data interconnect is another ring topology.
It uses fiber optics and sends data at about 100 Mbps.

ATM (Start Network)

Asynchronous Transferred Mode technology consists of electronic packet switches to which computers can connect

Details

1. It uses fiber optic cables.
2. It can transmit over 100 Mbps.
3. The connection per computer uses 2 fiber optics.