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Retransmission

Retransmission [Under Data Link Layer concepts]

One of the major functions of Layer 2 is "Error Control". Error control mean error detection and correction. Different services require different error rates. Of course, lower error rate would mean better quality. For example, voice call may not require so low error rate as required by internet browsing. Few bits corrupted in voice transmission do not cause "that much trouble" in resulting analog voice, but same percercentage of bits corrupted would result in unreadable web page ! Layer 2 has the responsibility of keeping error rate (normally measured as Bit Error Rate i.e. BER) under control.

Error correction with additional bits

Typically to detect and/or correct error, few redundant bits are added to actual information bits. Though this would result in lower effective information rate , it is must for error control. Needless to say, additional bits are calculated from original information bits. The way of calculation makes one error detection/correction technique different from another. At the receiving side, the additional bits are used in error detection and correction.

An error correction technique - popular in wireless communications - is Forward Error Correction (FEC). Details needed about FEC and other such techniques.

These type of error correction techniques are typically known as "coding schemes" in wireless communications. Well known coding schemes are Convolutional coding, Turbo coding etc. "Coding rate" is a ratio signifying the amount of added error correction. k/n coding rate imply that every k bits of information have been coded as n bits.

ARQ

Retransmission of erroneous information (packets) is another way of error correction. Error correction from retransmission is popularly known as ARQ (Automatic Repeat reQuest).

Note that retransmission is not possible for all bearer services (e.g. constant bit rate). Typically, services in which retransmission is not an option, appropriate error correction technique is chosen based on factors like allowed error rate, type of medium, modulation technique etc.

As retransmission reduce the effective or overall information rate, either we need to avoid retransmissions as much as possible or find out a way to improve overall information rate even after retransmissions.

HARQ

HARQ (Hybrid ARQ) is a retransmission technique which combine ARQ with FEC like error correction techniques. In HARQ, when error correction fail, retransmission is requested. As retransmission is done only when error correction fail, (on an average) HARQ will give better information rate than ARQ.

Note that in HARQ, erroneous packets are discarded. These discarded packets - even though erroneous - do contain "some correct information". Immediate question is: is it possible to use discarded packets ?

HARQ with soft combining

HARQ with soft combining techniques takes retransmissions a step further; it "combines" earlier received erroneous packets with retransmitted packet to come out with correct packet. If it fails, retransmission is requested again. Below is a symbolic illustration of this technique:

In HARQ with soft combining, there are two possibilities for retransmissions: retransmit the same packet or retransmit variation of it. First method is known as "HARQ with Chase Combining". Second method is known as "HARQ with Incremental Redundancy".

These two techniques are popular in 3G and near-4G wireless technologies like UMTS HSPA, WiMAX, LTE etc.

Further reading would be: exactly how the retransmitted packet is combined with earlier received errorneous packet(s).

References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming and UMTS by Sanchez and Thioune.

Copyright © Samir Amberkar 2010-11§

Part 2 - LAPD Frames « Data Link layer concepts Index .