User's Manual

VOLUME II, CHAPTER 1, SYSTEM OVERVIEW
Note that while adaptive modulation can also be used on PDH links and combined
PDH and Ethernet links, unlike Ethernet there is no QoS synergy on PDH con-
nections.
Ethernet connections enjoy real synergy through the QoS awareness on the DAC GE3
GigE switch, and the service provisioning provided by any MPLS or PBB-TE network
overlay. All high priority traffic, such as voice and video, continues to get through
when path conditions are poor. Outside these conditions 'best effort' lower priority
traffic, such as email and file transfers enjoy data bandwidths that can be up to four
times the guaranteed bandwidth.
DS1 connections by comparison are dropped in user-specified order when link capac-
ity is reduced, and restored when capacity is increased.
Coding
Modulation code options provide two sets of modulation states, one for maximum
throughput, the other for maximum gain. These apply on each of the modulation
rates (QPSK, 16 QAM, 64 QAM, 256 QAM) to provide a total of eight modulation
states.
Maximum throughput delivers maximum data throughput - at the expense of some
system gain.
Maximum gain delivers best system gain - at the expense of some throughput.
Up to four of the eight modulation states offered with ACM can be selected for use.
For example:
l With four modulation rates, each can be set for maximum throughput or
maximum gain.
l With three modulation rates, such as 16 QAM, 64 QAM, 256 QAM, one rate
(any) can be set for maximum gain and additionally for maximum
throughput, to provide four step AM operation.
l With two modulation rates, such as 16 QAM (or 64 QAM) with 256 QAM,
each can be set for maximum gain and additionally for maximum throughput,
to provide four step AM operation.
This feature provides a practical trade-off between capacity and system gain to fine-
tune link performance. It provides best balance on AM operation.
The four modulation rates support near-linear 2x, 3x, 4x capacity steps.
The coding options allow capacity/gain variations on these rates to always support
up to four steps, even when just two of the possible four modulation rates are in use,
or are permitted.
Even where just one modulation rate is required/permitted, the coding option sup-
ports two-step AM operation, one for maximum throughput, one for maximum gain.
28 AVIAT NETWORKS