Data Link Layer

Data link layer can be characterized by two types of layers:

1. Medium Access Layer (MAL)
2. Logical Link Layer

Aloha Protocols

History

The Aloha protocol was designed as part of a project at the University of Hawaii. It provided data transmission between computers on several of the Hawaiian Islands using radio transmissions.

• Communications was typically between remote stations and a central sited named Menehune or vice versa.
• All message to the Menehune were sent using the same frequency.
• When it received a message intact, the Menehune would broadcast an ack on a distinct outgoing frequency.
• The outgoing frequency was also used for messages from the central site to remote computers.
• All stations listened for message on this second frequency.

Pure Aloha

Pure Aloha is an unslotted, fully-decentralized protocol. It is extremely simple and trivial to implement. The ground rule is - "when you want to talk, just talk!". So, a node which wants to transmits, will go ahead and send the packet on its broadcast channel, with no consideration whatsoever as to anybody else is transmitting or not.

One serious drawback here is that, you dont know whether what you are sending has been received properly or not (so as to say, "whether you've been heard and understood?"). To resolve this, in Pure Aloha, when one node finishes speaking, it expects an acknowledgement in a finite amount of time - otherwise it simply retransmits the data. This scheme works well in small networks where the load is not high. But in large, load intensive networks where many nodes may want to transmit at the same time, this scheme fails miserably. This led to the development of Slotted Aloha.

Slotted Aloha

This is quite similar to Pure Aloha, differing only in the way transmissions take place. Instead of transmitting right at demand time, the sender waits for some time. This delay is specified as follows - the timeline is divided into equal slots and then it is required that transmission should take place only at slot boundaries. To be more precise, the slotted-Aloha makes the following assumptions:

• All frames consist of exactly L bits.
• Time is divided into slots of size L/R seconds (i.e., a slot equals the time to transmit one frame).
• Nodes start to transmit frames only at the beginnings of slots.
• The nodes are synchronized so that each node knows when the slots begin.

If two or more frames collide in a slot, then all the nodes detect the collision event before the slot ends.

In this way, the number of collisions that can possibly take place is reduced by a huge margin. And hence, the performance become much better compared to Pure Aloha. collisions may only take place with nodes that are ready to speak at the same time. But nevertheless, this is a substantial reduction.