Data Link Layer Chapter Outline

Random access is a method where stations take turns to send data.

Wireless Local Area Networks (WLANs) use dynamic medium access control.

PPP protocol is an example of a point-to-point protocol.

Ethernet uses static channelization for media sharing.

Satellite transmission is an example of dynamic medium access control.

Token ring is an example of a random access method.

Aloha is an example of a random access method.

In multiple access communications, stations communicate using dedicated channels allocated to each user.

In the pure ALOHA Protocol, a station waits for a specific time slot to transmit its data.

If multiple frames are transmitted at the same time in the pure ALOHA Protocol, they do not interfere with each other.

The receiver station in the pure ALOHA Protocol sends an acknowledgement (ACK) to the transmitter station if it receives a frame correctly.

In the pure ALOHA Protocol, the retransmission of a frame occurs if the transmitter station does not receive an ACK within a random backoff time B.

The vulnerability period in the pure ALOHA Protocol is equal to the sum of Tfr and 2tprop.

In pure ALOHA, after the first transmission, a station waits for a fixed backoff period B before attempting retransmission.

The timeout period in the pure ALOHA Protocol is defined as tprop + B.

In pure ALOHA, if a station did not receive an ACK within the timeout period, it directly retransmits the frame without any backoff time.

Each station monitors the medium during frame transmission to see if the transmission was successful.

If a collision happens between stations A and C, station A will detect the collision first.

The Ethernet frame size should be restricted for CSMA/CD to function properly.

If the last bit of a frame is sent without detecting a collision, the station will keep a copy of the frame.

The maximum throughput case results in 92 frames received out of 500 sent in a Slotted ALOHA network.

If network stations send 300 frames per second, then G = 300 x Tfr = 0.30 for Slotted ALOHA protocol.

In Slotted ALOHA, stations transmit frames in the first slot after frame arrival.

The vulnerable period in Slotted ALOHA protocol is from t0 + Tfr + 2tprop to t0 + Tfr + 3tprop.

The throughput S in Slotted ALOHA is defined as the ratio of successfully received frames to the number of frame transmission attempts.

For Slotted Aloha protocol, S = G x e-G where G is the number of transmission attempts per frame transmission time.

If the number of frame transmission attempts per second is 200 and Tfr is 0.05, then G = 200 x 0.05 = 10.

In Slotted Aloha protocol, the time-out period occurs after B time slots.

The throughput in the Pure Aloha protocol is defined as the ratio of successfully received frames to the number of frame transmission attempts.

For Pure Aloha protocol, the throughput (S) is calculated by S = G x e-2G.

In Pure Aloha, a higher number of transmission attempts per frame transmission time results in a higher throughput.

If a Pure Aloha network transmits 200-bit frames on a shared channel of 200 kbps, the frame transmission time Tfr is 1 ms.

If a network sends 1000 frames per second in Pure Aloha, the throughput (S) will be 0.135 (13.5%).

For a Pure Aloha network sending 500 frames per second, the throughput (S) will be 0.184 (18.4%).

The number of successfully received frames for a network sending 500 frames per second in Pure Aloha will be 250 frames.

In Pure Aloha, a lower throughput value indicates a higher efficiency of the network.