DSP
DSP is a specialized chip advised accurately for agenda arresting processing.
DSPs accredit Cisco platforms to calmly action agenda articulation traffic. DSPs on a router
provide stream-to-packet arresting processing functionality that includes articulation compression,
echo cancellation, and tone- and voice-activity detection.
A media ability is a software-based or hardware-based article that performs mediaprocessing
functions on the abstracts streams to which it is connected. A few examples are
media-processing functions that accommodate bond assorted streams to actualize one output
stream (conferencing), casual the beck from one affiliation to accession (media termination
point), converting the abstracts beck from one compression blazon to accession (transcoding),
echo cancellation, signaling, abortion of a articulation beck from a TDM ambit (coding/
decoding), packetization of a stream, and alive audio (annunciation).
The agreement “DSP” and “media resource” are generally acclimated interchangeably in some
documentation.
The four above functions of DSPs in a articulation aperture are as follows:
■ Transcoding: Transcoding is the absolute digital-to-digital about-face from one codec
to another. Transcoding compresses and decompresses articulation streams to bout endpoint-
device capabilities. Transcoding is appropriate back an admission articulation beck is
digitized and aeroembolism (by agency of a codec) to save bandwidth, but the local
device does not abutment that blazon of compression. Ideally, all IP telephony devices
would abutment the aforementioned codecs, but this is not the case. Rather, altered devices
support altered codecs.
Transcoding is candy by DSPs on the DSP farm. Sessions are accomplished and managed
by Cisco Unified Communications Manager. Cisco Unified Communications
Manager additionally refers to transcoders as accouterments MTPs.
If an appliance or account can handle alone one specific codec type, which is usually
G.711, a G.729 alarm from a alien armpit charge be transcoded to G.711. This can be
done alone via DSP resources. Because applications and casework are generally hosted in
main sites, DSP transcoding assets are best accepted in axial sites.
■ Articulation termination: Articulation abortion applies to a alarm that has two alarm legs, one leg
on a TDM interface and the added leg on a VoIP connection. The TDM leg charge be
terminated by accouterments that performs coding/decoding and packetization of the
stream. DSPs accomplish this abortion function. The DSP additionally provides answer cancellation,
voice action detection, and jitter administration at the aforementioned time it performs
voice termination.
■ Media Abortion Point (MTP): An MTP is an article that accepts two full-duplex
voice streams application the aforementioned codec. It bridges the media streams and allows them to
be set up and broken bottomward independently. The alive abstracts accustomed from the input
stream on one affiliation is anesthetized to the achievement beck on the added connection,
and carnality versa. In addition, the MTP can be acclimated to transcode a-law to mu-law and
vice versa, or it can be acclimated to arch two access that advance altered packetization
periods. MTPs are additionally acclimated to accommodate added processing of a call, such as
RFC 2833 support.
■ Audio Conferencing: In a acceptable circuit-switched articulation network, all articulation traffic
goes through a axial accessory (such as a PBX system), which provides audio conferencing
services as well. Because IP phones address articulation cartage anon between
phones, a network-based appointment arch is appropriate to facilitate multiparty
conferences.
A appointment arch is a ability that joins assorted participants into a distinct call. It
can acquire any cardinal of access for a accustomed conference, up to the maximum
number of streams accustomed for a distinct appointment on that device. A one-to-one correspondence
exists amid media streams affiliated to a appointment and participants
connected to the conference. The appointment arch mixes the streams together
and creates a altered achievement beck for anniversary affiliated party. The achievement stream
for a accustomed affair is the blended of the streams from all affiliated parties minus
their own ascribe stream. Some appointment bridges mix alone the three loudest talkers
on the appointment and administer that blended beck to anniversary actor (minus
their own ascribe beck if they are one of the talkers).
Hardware appointment bridges are acclimated in two environments. They can be acclimated to
increase the conferencing accommodation in a axial armpit after putting an added load
on Cisco Unified Communications Manager servers, which can host software-based
conference bridges. More important is the use of accouterments appointment bridges in
remote sites. If no remote-site appointment assets are deployed, every conference
will be baffled to axial resources, consistent in sometimes-excessive WAN usage.
In addition, DSP-based appointment bridges can mix G.711 and G.729 calls, appropriately supporting
any call-type book in multisite environments. In contrast, software-based
conference bridges deployed on Cisco Unified Communications Manager servers can
mix alone G.711 calls.
Other accessible uses for MTPs accommodate the following:
■ Repacketization: An MTP can be acclimated to transcode a-law to mu-law and carnality versa,
or it can be acclimated to arch two access that advance altered packetization
periods.
■ H.323 Added Services: MTPs can be acclimated to extend added services
to H.323 endpoints that do not abutment the H.323v2 OpenLogicalChannel and
CloseLogicalChannel appeal appearance of the Empty Capabilities Set (ECS). This
requirement occurs infrequently. Cisco H.323 endpoints abutment ECS, and most
third-party endpoints accept abutment as well. Back needed, an MTP is allocated and
connected into a alarm on account of an H.323 endpoint. After insertion, the media
streams are affiliated amid the MTP and the H.323 device, and these connections
are present for the continuance of the call. The media streams affiliated to the
other ancillary of the MTP can be affiliated and broken as bare to implement
features such as authority and transfer.
When an MTP is appropriate on an H.323 alarm and none is available, the alarm will proceed
but will not be able to adjure added services.
Chapter 2: Considering VoIP Design Elements 93
Note Implementations above-mentioned to Cisco Unified Communications Manager Release 3.2
required MTPs to accommodate added casework for H.323 endpoints, but Cisco Unified
Communications Manager Release 3.2 and after no best crave MTP assets to provide
this functionality.