EE5412 Telecommunications Network

EE5412 LAN
LanQA0a.doc
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Difference in throughput performance between the pure ALOHA and the slotted ALOHA LAN access scheme.

Assume packet length equal to propagation delay equal to one slot time.
The best channel utilization that can be achieved is around 18% for pure ALOHA and 36% for slotted ALOHA. In pure ALOHA the vulnerable period for collision last for two times the slot time. It is the vulnerable period that is largely affect the throughput performance of the network. With slotted ALOHA, station only transmit at the beginning of a timeslot and the vulnerable is reduced. With assumption on packet length and propagation delay, the performance of slotted ALOHA is twice that of the pure ALOHA.

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Back-off mechanism used in IEEE802.3 and its effect on performance and stability
Mechanism that maintains the LAN throughput at various loads [light load and heavy load] to the network.

Truncated Binary exponential back-off is used in IEEE802.3.
Packets are transmitted into the network in random. On detecting collision, the transmitting station will stop transmission for a random time interval before trying to transmit again. Delay is computed in each individual node using an algorithm designed to minimize collision. Packet is retransmitted after the delay. If a second collision is detected, the station will double its mean random delay time and back-off again. This is called binary exponential back-off. The station will attempt to transmit for up to 16 times before reporting an error and give up retransmission.
n= number of retransmission attempt
k= min(n,10)
The delay is an integer of multiple of slot time. The number of slot times to delay before the n th retransmission is chosen as a uniform distributed random integer r.
0 <= r <= 2k

Re-transmission has the effect of increasing the input load to the network. As the load increase, the probability of packet collision also increases. At heavy load the chance of successful transmission is small resulting in very small throughput and very high re-transmission. The system will lockup and become unstable.


Mechanism maintaining the LAN throughput at various load
The binary exponential back-off algorithm has the effect of keeping the system stable.

Under light load, delay time is small and re-transmission does not have any effect.
As load increase, probability of collision increase, number of retransmission will also increase. Backoff delay time will also increase exponentially.

Heavy load -
The increase in backoff time has the effect of reducing the input to the network. This will in turn reduce the probability of collision. The net effect is to maintain the network throughput and give a chance to the network to clear the backlog.
Since excessive input causing overload is only a temporary phenomenon, the network will recover and maintain its stability.

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Maximum throughput that can be achieved from a pure ALOHA channel
Ans: 0.184

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Meaning of the following terms associated with a CSMA/CD bus network:
a. Slot time
b. Jam sequence
c. Truncated binary exponential backoff


Slot Time -
a. Due to signal propagation delay of the transmission medium, there is a small but finite time interval between a DTE connected to one end of the transmission medium starting to transmit a frame and another DTE connected to the other end receiving the signal. This is known as worst case propagation delay time of the cable and during this interval two DTEs may initiate a transmission on the assumption that the transmission medium is idle.

In the worst case, the corrupted signal will take the same time interval to propagate back to the first DTE before it detects the collision. This total time, twice the worst-case propagation delay, is known as the slot time and is represented in terms of bits at the particular bit rate being used. With thick-wire coaxial cable the maximum cable length is limited to 2.5km and at 10 Mbps, the slot time is 512 bits. This means that a DTE must transmit a minimum of 512 bits before it knows whether a collision has occurred or not.

Jam sequence-
The actual period of collision can be very short.

To ensure the two (or more) DTEs involved in a collision detect its occurrence, it is necessary for both stations to continue transmitting for a short time interval after first detecting the collision has occurred. On detecting a collision, therefore, each DTE sends a further random bit pattern known as the jam sequence.

Truncated Binary exponential backoff -
On detecting a collision, the two (or more) DTEs involved will try to retransmit the frames that have been corrupted. Hence to avoid both DTEs again starting at the same time, each DTE waits a random time internal before attempting to transmit its frame. This is known as the random backoff interval.

In the event of a second collision occurring, the previous (random) time interval is doubled before attempting again. This procedure then repeats for each subsequent collision up to a maximum number of tries before abandons trying to transmit the frame.
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Operation of p-persistent CSMA protocol
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Modification made to CSMA protocol in order to improve its performance

Collision detection – After sending a packet, the sending station checks if there is any collision. If yes, it can send a jamming signal to notify other stations.
Collision avoidance – mechanism to claim the channel before transmission.

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Even though the Slotted_ALOHA protocol is more efficient than Pure-ALOHA, it also has some disadvantages. Following are the problems.

Slotted-Aloha requires that common clocks be established at each station. The clocks should be synchronized to send traffic at specific periods. Slotted-Aloha works with a time division multiplexing slot. The slot is fixed in length. If user does not use the slot, the slot becomes empty and channel capacity will be wasted.

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Two parameters that describe the performance of a LAN -

The key performance considerations for a LAN are the throughput or channel utilization and the delay characteristics. Throughput is a measure of the successful traffic or the total rate of data being transmitted between nodes.

This is expressed in terms of the offered load which is the actual message load or traffic demand presented to the network. Delay is a measure of the average interval of time between when a packet is ready for transmission from a source station until its successful reception at the destination.

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IEEE802.3 standard access scheme with the aid of a flow chart
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Reason for collision in random access scheme
Due to signal propagation delay through the network, carrier signal can not be detected by the host before packet transmission and hence collision.
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