Page 287 - Optical Communications Essentials

Optical Link Design

Optical Link Design 277

achieved with a return-to-zero (RZ) code, as shown in Fig. 16.7. Here the pulse
for a 1 bit occupies only the first half of the bit interval and returns to zero in
the second half of the bit interval. No pulse is used for a 0 bit.

16.4.2. Block codes
Introducing redundant bits into a data stream can be used to provide adequate
timing and to have error monitoring features. A popular and efficient encoding
method for this is the class of mBnB block codes. In this class of codes, blocks of
m binary bits are converted to longer blocks of n m binary bits. As a result of
the additional redundant bits, the required bandwidth increases by the ratio
n/m. For example, in an mBnB code with m 1 and n 2, a binary 1 is mapped
into the binary pair 10, and a binary 0 becomes 01. The overhead for such a code
is 50 percent.
Suitable mBnB codes for high data rates are the 3B4B, 4B5B, 5B6B, and
8B10B codes. If simplicity of the encoder and decoder circuits is the main criter-
ion, then the 3B4B format is the most convenient code. The 5B6B code is the
most advantageous if bandwidth reduction is the major concern. Various ver-
sions of Ethernet use either the 3B4B, 4B5B, or 8B10B formats. Fibre Channel
employs an 8B10B code.

16.5. Forward Error Correction
For high-speed broadband networks, the data transmission reliability provided
by the network may be lower than the reliability requested by an application.
In this case, the transport protocol of the network must compensate for the dif-
ference in the bit loss rate. Two basic schemes for improving the reliability are
automatic repeat request (ARQ) and forward error correction (FEC). ARQ
schemes have been used for many years and are implemented widely. This tech-
nique uses a feedback channel between the receiver and the transmitter to
request message retransmission in case errors are detected at the receiver.
Since each such retransmission adds at least one round-trip time of latency,
ARQ may not be feasible for applications requiring low latency. Among such
low-latency applications are voice and video services that involve human inter-
action, process control, and remote sensing in which data must arrive within a
certain time in order to be useful.
Forward error correction avoids the shortcomings of ARQ for high-bandwidth
optical networks requiring low delays. FEC is a mathematical signal processing
technique that encodes data so that errors can be detected and corrected. In
FEC techniques, redundant information is transmitted along with the original
information. If some of the original data are lost or received in error, the redun-
dant information is used to reconstruct them. Typically the amount of redun-
dant information is small, so the FEC scheme does not use up much additional
bandwidth and thus remains efficient.

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