Slotted Aloha has double the capacity of an Aloha system. In slotted Aloha, time is divided into discrete intervals, each interval corresponding to one frame. This approach requires the users to agree on slot boundaries.
In slotted Aloha, as against in pure Aloha, a computer is not permitted to send whenever a carriage return is typed. Instead. it is required to wait for the beginning of the next slot. Thus the continuous pure Aloha is turned into a discrete one.
Since the vulnerable period is now half. the probability of no other traffic during the same slot as out test frame is e--G, which leads to S = Ge G.
The slotted Aloha peak at G = 1. shown in Figure 6.6, has a throughput of S = 1/e or about 0.368, ... twice that of pure Aloha.
Operating at higher values of G reduces the number of empties but increases the number of collisions exponentially.The probability that a frame will not encounter any collision is e G, the probability that all other users are silent in that slot. The probability of a collision is then just 1 - e -G . The probability of a transmission requiring exactly k attempts (that is, k - 1 collisions followed by one success) is:
The expected number of transmissions, E, per carriage return type is then:
As a result of the exponential dependence of E upon G, small increases in the channel load can drastically reduce its performance.
Carrier Sense Multiple Access (CSMA) Protocols
Protocols in which stations listen for a carrier (transmission) and act accordingly are called carrier sense protocols. The first carrier sense protocol is 1-persistent CSMA. When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment. If the channel is busy, the station waits until it detects an ideal channel. When the station detects an ideal channel, it transmits a frame. If collision occurs, the station waits a random amount of time and starts all over again. The protocol is called 1-persistent because the station transmits with a probability of 1, whenever it finds the channel ideal.
A second carrier sense protocol is non-persistent CSMA. In this protocol, a conscious (knowing what is going around) attempt is made to be less greedy than in the previous one. Before sending, a station senses the channel. If no one else is sending; the station begins doing so itself. However, if the channel is already in use, the station does not continuously sense it for the purpose of seizing it immediately upon detecting the end of the previous transmission. Instead, it waits a random period of time and then repeats the algorithm. Intuitively, this algorithm should lead to better channel utilization and longer delays than 1-persistent CSMA.
The last protocol is p-persistent CSMA. It applies to slotted channels and works as follows: when a station becomes ready to send, it senses the channel. If it is ideal, it transmits with a probability p, with a probability q = 1 - p, it differs until the next slot. If the slot is also ideal, it either transmits or defers again, with probabilities p and q. This process is repeated until either the frame has been transmitted or another station has begun transmitting. In the latter case, it acts as if there had been a collision (it waits a random time and starts again). If the station initially senses the channel busy, it waits until the next slot and applies the above algorithm.
CSMA with Collision Detection (CSMA/CD)
Persistent and non-persistent CSMA protocols are clearly an improvement over Aloha because they ensure that no station begins to transmit when it senses the channel busy. Another improvement is for stations to abort their transmissions as soon as they detect a collision.
The interference between two signals is called a collision. Although a collision does not harm the hardware, it produces a garbled transmission that prevents either of the two frames from being received correctly. To ensure that no other computer transmits simultaneously, the Ethernet standard requires a sending station to monitor signals on the cable. If the signal on the cable differs from the signal that the station is sending, it means that a collision has occurred. Whenever a collision is detected, a sending station immediately stops transmitting. Technically, monitoring a cable during transmission is known as collision detection (CD), and the Ethernet mechanism is known as Carrier Sense Multiple Access with Collision Detection (CSZfA/CD).
In other words, if two stations sense the channel to be ideal and begin transmitting simultaneously, they will both detect the collision almost immediately. Rather than finish transmitting their frames, they should abruptly stop transmission as soon as the collision is detected. Quickly terminating damaged frames save time and bandwidth. This protocol, known as CSIVIA/CD, is widely used on LA-Ns in the MAC sublayer. The access mechanism used in an Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD standardized in IEEE 802.3).
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