Echo cancellation – Nortel Networks NN43001-563 User Manual

Page 67

Dialing plans

numbers of varying lengths to terminate at a destination. Flexibility of the
number of digits which can be dialed is achieved using Special Numbers
(SPNs).

Electronic Switched Network (ESN5) network signaling

IP Trunk 3.01 (and later) and ITG Trunk 2.x support a mixed network of
remote nodes with ESN5 and standard (that is, non-network) signaling.
ESN5 is an extension of MCDN signaling which can be used by IP Trunk
3.01 (and later), ITG Trunk 2.x, and IP Peer ( CS 1000M).

ESN5 inserts the Network Class of Service (NCOS) prefix ahead of the
dialed numbers. Make sure that, if ESN5 is to be used, it is provisioned on
both the IP trunk cards and the Route Data Block (RDB) for that node. If
ESN5 is provisioned for an IP Trunk 3.01 (and later) node, all remote ITG
2.x and IP Trunk 3.01 (and later) node must have that node provisioned as
"SL1ESN5" in the dialing plan. If this is not done, a default NCOS is inserted
by the ESN5 node receiving the call from the non-ESN5 VoIP gateway. Fore
more information, see

"ESN5 network signaling" (page 229)

Echo cancellation

All telephony voice services now in use reflect some level of echo back
to the user. The term "echo" refers to the return of a signal’s reflection
to the originator.

Packet voice networks introduce sufficient latency to cause what a caller
would consider an audible echo. The echo path is round-trip. Any speech
coding, packetization, and buffering delays accumulate in both directions of
transmission, increasing the likelihood of audibility.

Echo cancellation reduces feedback sounds and background noise for
clearer voice quality. Some less advanced IP telephony products do not
include echo cancellation circuitry, resulting in voice quality of a level below
business communications standards. Without echo cancellation, the talking
parties can hear varying levels of echo as they speak.

Echo canceller tail delay

Early versions of ITG Trunk DSPs and DSP firmware had a maximum echo
canceller (ECAN) tail delay of 32 ms. More recent cards and firmware
support higher tail delays, with the ITG-P and the Media Card 32-port card
supporting up to 128 ms. However, when the capability was added, the
default in TM 3.1 remained unchanged at 32 ms, even though the ECAN
performance was significantly better with 128 ms. This problem has been
resolved in TM 3.1, but ITG Trunk and IP Trunk nodes defined by customers
with the original TM 3.1 software still use the incorrect default value.

Nortel Communication Server 1000

IP Trunk Fundamentals

NN43001-563

01.01

Standard

Release 5.0

30 May 2007