3/29/14

Channel Partitioning Protocols.

Introduction.

Channel partitioning protocols are multiple access protocols used to partition a broadcast channel's bandwidth among all nodes (internet devices) sharing that channel.

Perhaps a human analogy for a broadcast channel would be a cocktail party, where many people gather in a larger room (the air providing the broadcast medium) to talk and listen. A second good analogy is a classroom - where teacher(s) and student(s) similarly share the same, single, broadcast medium. A central problem in both scenarios is that of determining who goes to talk (that is, transmit into the channel), and when.

Computers use protocols - so called multiple access protocols by which nodes regulate transmission into the shared broadcast channel.


Time division multiplexing (TDM).

 photo TDM_zps7556e93e.jpg


In telecommunications and computer networks, multiplexing (sometimes contracted to muxing) is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several telephone calls may be carried using one wire.

TDM divides time into time frames and further divides each time frame into N time slots. Each slot time is then assigned to one of the N nodes. Whenever a node has a packet to send, it transmits the packet's bits during its assigned time slot in the revolving TDM frame.

Returning to our cocktail party analogy, a TDM-regulated cocktail party would allow one partygoer to speak for a fixed period of time, then allow another partygoer to speak for the same amount of time, and so on. Once everyone had a chance to talk, the pattern would repeat.


Frequency division multiplexing (FDM).

 photo FDM_zps80adbb37.jpg


While TDM shares the broadcast channel in time, FDM divides the R bps channel into different frequencies (each with a bandwidth of R/N) and assigns each frequency to one of the N nodes. FDM thus creates N smaller channels of R/N bps out of the single, larger R bps channel.


Code Division Multiple Access (CDMA).

In a CDMA protocol, each bit being sent is encoded by multiplying the bit by a signal (the code) that changes at a much faster rate (known as the chipping rate) than the original sequence of bits.

Such encoded signals can be aggregated (summed) and transmitted via the channel. if codes are chosen carefully, each receiver can recover the data sent by a given sender out of the aggregate signal simply by using the sender's code.

For example: signal A might be multiplied by +1 and -1 for 1 and 0 bits, signal B by +2 and -2, signal C by +4 and -4, and so on.

If we transmit value 3, we know unambiguously that it's 4+1-2, and can decode bytes appropriately.


In practice it is a little more complex.


Source: [3], Wikipedia.

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