Implementing SBC Transcoding and Codec Restriction

Implementing SBC Transcoding and Codec
Restriction

Transcoding is the process of translating a media stream encoded using one codec into a media stream
encoded using another codec. For example, translating a media stream encoded as Pulse Code
Modulation u-law (PCMU) into one encoded as G.726-32.
The primary reason for transcoding configurations is to configure the capabilities of external media
transcoding devices when these devices cannot be discovered automatically. In-band auto-discovery of
transcoder capabilities is currently not supported. Therefore, this step must be done when configuring
all connections to all current remote transcoding devices.

Note Transcoding configurations can be skipped altogether if the described reason does not apply.

Media gateways are allowed to connect whether or not configuration has been supplied for them. To help
avoid configuration errors, the signaling border element (SBE) logs a warning if an incoming connection
is received from a media gateway that is not a data border element (DBE) and does not have transcoding
configured.

Note For a complete description of commands used in this chapter, refer to the SBC Transcoding Commands
on Cisco IOS XR Software module of Cisco IOS XR Software Session Border Controller Command
Reference. To locate documentation for other commands that appear in this chapter, use the command
reference master index, or search online.

Feature History for Implementing SBC Transcoding and Codec Restriction

Release Modification
Release 3.3.0 This feature was introduced on the Cisco XR 12000 Series Router.
Release 3.4.0 No modification.
Release 3.4.1 Restricting Codecs feature was supported.

Cisco IOS XR Session Border Controller Configuration Guide
OL-15854-01 SBC-353

Implementing SBC Transcoding and Codec Restriction
Contents

Contents
• Prerequisites for Implementing SBC Transcoding and Codec Restriction, page SBC-354
• Restrictions for Codecs, page SBC-354
• Information About SBC Transcoding, page SBC-357
• Information About Codec Restriction, page SBC-359
• How to Implement SBC Transcoding, page SBC-359
• How to Implement Codec Restriction, page SBC-362
• Configuration Example for Transcoding, page SBC-366
• Configuration Examples for a Codec Restriction, page SBC-366
• Additional References, page SBC-368

Prerequisites for Implementing SBC Transcoding and Codec
Restriction
The following prerequisites are required to implement SBC transcoding:
• You must be in a user group associated with a task group that includes the proper task IDs for SBC
commands being used. For detailed information about user groups and task IDs, see the Configuring
AAA Services on Cisco IOS XR Software module of Cisco IOS XR System Security Configuration
Guide.
• You must install and activate the package installation envelope (PIE) for the SBC software.
For detailed information about PIE installation, refer to the Upgrading and Managing Cisco IOS XR
Software module in Cisco IOS XR Getting Started Guide.
• The SBC must already be created. See the procedures described in the SBC Configuration
Prerequisites module.
• Transcoding must be configured before you can restrict codecs.
• All SBE and DBE configurations required to make simple calls must already be configured.
Transcoding configurations follow these configurations.

Restrictions for Codecs
• The media packet forwarder on the DBE polices the bandwidth consumed by each media stream, but
it cannot police the type of codecs or the packetization periods.
• Unrecognized codecs cannot be configured as members of the codec whitelist.
• Active calls are not released if there is a change in the codec whitelist during the call.
• If a codec whitelist is configured, SBC removes any unlisted codecs from the call setup flow and
media gate allocation.
• Multiple codec whitelists can be configured on a call admission control (CAC) policy basis. For
example, the list of codecs allowed for calls from “SipAdj1” can be different than the list of codecs
allowed for calls from “SipAdj2”.

SBC-354 OL-15854-01


• If a codec whitelist has not been configured, all recognized codecs (see Table 32 – Table 35) are
allowed for all calls.
• You must use the textual value of the codec description that appears on the session description
protocol (SDP) to configure the codec whitelist, for example “PCMU” or “telephone-event”.
• Disallowing all codecs is not supported. However, you can set a bandwidth limit of zero to achieve
the same result.
• Codec lists are not applied to media-bypass calls (those in which the SBC does not reserve media
resources).
• The format of the codec name is the same as the string used to represent it in SDP, for example
PCMU or VDVI. All recognized codec names are listed in Table 32 – Table 35.
• A single codec can be added to each list only once, with a single packetization period.
• For each codec on a list, CAC restricts the signaled packetization period for any stream using that
codec to be greater than or equal to the packetization period configured along with the codec in the
list. If a stream uses more than one codec in the list, the greatest of all the packetization periods
configured for each codec in the list is applied to the stream.

Note The bandwidths listed in the tables below are the bandwidths without the transport layer overheads.
Therefore, the actual bandwidths reserved by the SBC are higher than the listed values.

Table 32 Sample-Based Audio Codecs with Packetization Time 10 ms

Payload Type Codec Name Clock Rate (Hz) Sample Size (bits) Channels
0 PCMU 8000 8 1
5 DVI4 8000 4 1
6 DVI4 16000 4 1
8 PCMA 8000 8 1
10 L16 44100 16 2
11 L16 44100 16 1
15 G728 8000 2 1
16 DVI4 11025 4 1
17 DVI4 22050 4 1
— G726-40 8000 5 1
2 G726-32 8000 4 1
— G726-24 8000 3 1
— G726-16 8000 2 1
— L8 8000 8 1
— DAT12 8000 12 2
— L20 44100 10 2
— L24 44100 24 2

OL-15854-01 SBC-355


Table 33 Non-Sample-Based Audio Codecs

Payload Type Codec Name Packetization Time (ms) Allocated Bandwidth (bits/s)
3 GSM 10 13200
7 LPC 10 5600
9 G722 10 64000
13 CN 10 400
18 G729 10 8000
4 G723 30 6400
12 QCELP — 13300
14 MPA — 131072
— G729D 10 6400
— G729E 10 11800
— GSM-EFR 10 12400
— VDVI 10 25000
— AMR 10 12500
— AMR-WB 10 24420
— dsr-es201108 10 4800
— EVRC 10 8550
— EVRC0 10 8550
— mpa-robust 10 327680
— G7221 10 32000
— MP4A-LATM 10 131072
— SMV 10 8550
— SMV0 10 8550
— G729AB 10 8000

Table 34 Video Codecs

25 CelB — 524228
26 JPEG — 524228
28 nv — 524228
31 H261 — 524228
32 MPV — 524228
33 MP2T — 524228
34 H263 — 524228
— BMPEG — 524228
— BT656 — 170000000

SBC-356 OL-15854-01

Information About SBC Transcoding

Table 34 Video Codecs (continued)

— DV — 1500000000
— H263-1998 — 524228
— H263-2000 — 524228
— MP1S — 1600000
— MP2P — 524228
— MP4V-ES — 524228
— raw — 1500000000
— SMPTE292M — 1500000000

Table 35 Other Codecs

Codec Name Packetization Time (ms) Allocated Bandwidth (bits/s)
telephone-event 20 1600
tone 20 1600
RED 20 1
parityfec 20 1
t140 100 80
pointer 20 1600
H224 20 6560
T.38 — 15500
X-NSE 20 1600

Transcoding requires specialized digital signal processor (DSP) hardware, which is not available within
SBC itself. A Cisco MGX 8880 device is used to provide transcoding function for one or more SBCs.
SBC automatically brings the transcoding device into use for any call requiring transcoding between
these codecs, as long as the call admission control (CAC) policy configuration does not preclude the
transcoder service from being supplied for the call. When a call that requires transcoding is set up, the
signaling border element (SBE) goes through the following steps:
1. Receives an initial Signaling request from the calling endpoint. This triggers SBE to perform initial
call setup on the incoming and outgoing DBEs. Then, SBE forwards the request to the called
endpoint.
2. Receives a response from the called endpoint that indicates that none of the codecs in the initial
request are acceptable. This triggers SBE to bring a transcoder into the call, which is inserted in the
media path between the incoming and outgoing DBEs. A new request is sent to the called endpoint,
indicating the new codec type generated by the transcoder.

OL-15854-01 SBC-357


3. SBE may have to iterate through the list of codecs the transcoder supports until it finds one that is
acceptable to the called endpoint. When this is done, the call is connected and media transmission
begins.
Figure 17 shows where the transcoder sits in the network, and the path taken by the media in a transcoded
call.

Figure 17 Transcoding Configuration

H.248 Control
Associations
SBE

Transcoder

G.711 Media path
G.711 G.729
V
Telephone
DBE
G.729
V

158012
DBE Telephone

Note Although Figure 17 shows two data border elements (DBEs), transcoding is possible with a single DBE.
With a single DBE, the media flows through the DBE twice, once on its way from the sending endpoint
to the transcoder and a second time as it flows from the transcoder to the receiving endpoint.

For SBC to program the transcoder, it must be registered. The transcoding device acts as an H.248 media
gateway, so it needs to be configured with the IP address and port of the SBE or SBC to connect to. The
SBE or SBC acts as an H.248 media gateway controller. (See the documentation for the transcoder device
for notes on how to do this. The documentation for the Cisco MGX 8880 is found at
http://www.cisco.com/en/US/products/hw/gatecont/ps3869/products_configuration_guide_book09186
a0080535937.html.)
In addition, the SBE must have the following specific configurations:
• An H.248 control address and port must be configured by using the control address h248 command.
By default, this is on port 2944, and it is the address and port to which the transcoder must connect.
• An explicit media gateway needs to be configured by using the media-gateway command. The
explicit media gateway must have its list of supported codecs defined so that the SBC knows which
codecs the transcoder can translate between, and it must be identified as a transcoder by using the
codecs and transcoder commands.
• The show services sbc sbe media-gateway-associations command is used to check that the
transcoder has correctly registered with the SBE. If this has happened, the transcoder should appear
in the list of known media gateways with an active association.

SBC-358 OL-15854-01

Information About Codec Restriction

Information About Codec Restriction
A compressor-decompressor (codec) is a device or program that performs a transformation on a data
stream or signal. The SBC is hardcoded with a set of recognized codecs (see Table 32 – Table 35),
including all commonly used voice and video codecs. The default behavior is to allow all recognized
codecs on all calls. Any other codec present in call signaling is removed by the SBC.
The Codec Restriction feature allows you to restrict which codecs a particular call can use and to
configure a minimum permissible packetization period for each permitted codec.

How to Implement SBC Transcoding
This section contains the following procedure on how to implement SBC transcoding:
• Configuring Transcoding, page SBC-359
In this configuration area, you supply a configuration for a list of remote media gateways. By default,
media gateways are allowed to connect whether or not configuration has been supplied for them. To help
avoid configuration errors, the SBE logs a warning if an incoming connection is received from a media
gateway that is not a DBE and does not have transcoding configured.
The following basic steps for implementing transcoding are listed:
1. Configure the IP address, port, and transport protocol for an H.248 media gateway controller on
SBC. This step may not be required if the media gateway controller has already been configured.
2. Configure the media gateway IP address.
3. Configure the codecs to be transcoded (for example, between G.711ulaw and G.729A).
4. Specify the media gateway as a transcoder.
5. Activate SBE.

Configuring Transcoding
Perform this task to configure transcoding for SBC.

SUMMARY STEPS

1. configure
2. sbc service-name
3. service-location preferred-active {node-id} [preferred standby node-id]
4. sbe
5. control address h248 [ipv4 ipv4_IP_address]
6. control address h248 [port port-number]
7. control address h248 [transport [tcp | udp]]
8. media-gateway {ipv4 ipv4_IP_address}
9. codecs {codec-list}
10. transcoder
11. exit

OL-15854-01 SBC-359


12. activate
13. end
or
commit

DETAILED STEPS

Command or Action Purpose
Step 1 configure Enters global configuration mode.

Example:
RP/0/0/CPU0:router# configure
Step 2 sbc service-name Enters the configuration mode of an SBC service.
• Use the service-name argument to define the name of
Example: the service.
RP/0/0/CPU0:router(config)# sbc mysbc
RP/0/0/CPU0:router(config-sbc)#
Step 3 service-location preferred-active node-id Enables a service card to run SBC function as a primary
[preferred standby node-id] location, and optionally, secondary location.

Example:
preferred-active 0/2/CPU0 preferred-standby
0/4/CPU0
Step 4 sbe Enters the configuration mode of the signaling border
element (SBE) function of the SBC.
Example:
RP/0/0/CPU0:router(config-sbc)# sbe
RP/0/0/CPU0:router(config-sbc-sbe)#
Step 5 control address h248 [ipv4 ipv4_IP_address] Configures an SBE to use a given IPv4 H.248 control
address when acting as a media gateway controller.
Example: • Use the ipv4 keyword to specify the IPv4 H.248 control
RP/0/0/CPU0:router(config-sbc-sbe)# control address.
address h248 ipv4 88.88.133.21
Step 6 control address h248 [port port-number] Configures an SBE to use a given port for H.248
communications when acting as a media gateway controller.
Example: • Use the port keyword to specify the listening port
RP/0/0/CPU0:router(config-sbc-sbe)# control number. The range is from 1 to 9999. Port 2944 is the
address h248 port 2944 default H.248 port. (If using the default port, this step
can be skipped.)
Step 7 control address h248 [transport [tcp | udp]] Configures an SBE to use a given transport for H.248
communications when acting as a media gateway controller.
Example: • Use the udp keyword to configure the UDP transport
RP/0/0/CPU0:router(config-sbc-sbe)# control for H.248 signaling.
address h248 transport udp
• Use the tcp keyword to configure the TCP transport for
H.248 signaling.

SBC-360 OL-15854-01


Step 8 media-gateway {ipv4 ipv4_IP_address} Configures the media gateways on an SBE and enters SBE
media gateway configuration mode.
Example: • Use the ipv4 keyword to configure the IPv4 address for
RP/0/0/CPU0:router(config-sbc-sbe)# the allowed media gateway.
media-gateway ipv4 10.0.0.1
RP/0/0/CPU0:router(config-sbc-sbe-mg)#
Step 9 codecs {codec-list} Configures the codecs supported by the media gateway.
• Use the codec-list argument as a description of the
Example: quoted and comma-separated list of the codecs
RP/0/0/CPU0:router(config-sbc-sbe-mg)# codecs supported.
“m=audio 6000 RTP/AVPR 4,a=rtpmap:0 PCMU/8000”
Step 10 transcoder Configures the media gateway with transcoder support.

Example:
RP/0/0/CPU0:router(config-sbc-sbe-mg)#
transcoder
Step 11 exit Exits the current configuration mode.

Example:
RP/0/0/CPU0:router(config-sbc-sbe-mg)# exit
Step 12 activate Initiates the SBC service after all SBE address
configuration has been successfully committed.
Example:
RP/0/0/CPU0:router(config-sbc-sbe)# activate
Step 13 end Saves configuration changes.
or
• When you issue the end command, the system prompts
commit you to commit changes:
Uncommitted changes found, commit them before
Example: exiting(yes/no/cancel)?
RP/0/0/CPU0:router(config-sbc-sbe)# end [cancel]:
or
– Entering yes saves configuration changes to the
RP/0/0/CPU0:router(config-sbc-sbe)# commit running configuration file, exits the configuration
session, and returns the router to EXEC mode.
– Entering no exits the configuration session and
returns the router to EXEC mode without
committing the configuration changes.
– Entering cancel leaves the router in the current
configuration session without exiting or
• Use the commit command to save the configuration
changes to the running configuration file and remain
within the configuration session.

OL-15854-01 SBC-361

How to Implement Codec Restriction

You must configure the codecs and then apply them as described in the following tasks:
• Configuring Codecs, page SBC-362
• Configuring a CAC Policy to Use a Codec List, page SBC-364

Configuring Codecs
To restrict which codecs a particular call can use and to configure a minimum permissible packetization
period for each permitted codec, you must configure CAC with a list of codecs, provide a description for
the list, and then add any codecs to the list.
Perform this task to configure codecs.

SUMMARY STEPS

1. configure
2. sbc service-name
3. sbe
4. codec-list {list-name}
5. description {description}
6. codec {codec-name} [packetization-period value]
7. end
or
commit
8. show services sbc sbc-name sbe codec-list {list-name}

DETAILED STEPS


Example:
Example:

SBC-362 OL-15854-01


Step 4 codec-list {list-name} Create a codec list and enters SBE codec list configuration
mode.
Example: • Use the list-name argument to configure the name of
RP/0/0/CPU0:router(config-sbc-sbe)# codec-list the codec list.
my_codecs
RP/0/0/CPU0:router(config-sbc-sbe-codec-list)# Note A codec list can exist despite having no codecs.
Deleting all codes from a list does not delete the list.
Step 5 description {description} Adds a description for the specified codec list using a
readable text string format.
Example: It is also included for each codec list when a summary of all
RP/0/0/CPU0:router(config-sbc-sbe-codec-list)# codec lists is displayed.
description Legitimate codes
Step 6 codec {codec-name} [packetization-period value] Configures a description of a codec list and sets a minimum
packetization period (optional) for the codec.
Example: • Use the codec-name argument to configure the name of
RP/0/0/CPU0:router(config-sbc-sbe-codec-list)# a codec. This value must be one of the list of codecs that
codec PCMU packetization-period 20 the SBE is hardcoded to recognize. Otherwise, when
you execute this command, the SBE displays an error.
or
RP/0/0/CPU0:router(config-sbc-sbe-codec-list)# [cancel]:
end
or – Entering yes saves configuration changes to the
running configuration file, exits the configuration
RP/0/0/CPU0:router(config-sbc-sbe-codec-list)#
commit
Step 8 show services sbc sbc-name sbe codec-list Displays detailed information about the codec lists
{list-name} configured on the SBE.
If the list name is omitted, details are displayed for all codec
Example: lists on the SBE.
RP/0/0/CPU0:router# show services sbc mysbc sbe
codec-list my_codecs

OL-15854-01 SBC-363


Configuring a CAC Policy to Use a Codec List
Perform this task to configure a CAC policy to use a codec list.

SUMMARY STEPS

1. configure
2. sbc service-name
3. sbe
4. cac-policy-set {policy-set ID}
5. cac-table {table-name}
6. entry {entry-id}
7. codec-restrict-to-list list-name
8. end
or
commit
9. show services sbc sbc-name sbe cac-policy-set set-id [table name {entry entry-id | entries} |
tables]

DETAILED STEPS


Example:
Example:
Step 4 cac-policy-set {policy-set ID} Configures a specific CAC policy that is set to restrict
codecs and enters SBE CAC policy configuration mode.
Example: • Use the policy-set ID argument to define the unique
RP/0/0/CPU0:router(config-sbc-sbe)# identifier of the policy set.
cac-policy-set 1
RP/0/0/CPU0:router(config-sbc-sbe-cacpolicy)#

SBC-364 OL-15854-01


Step 5 cac-table {table-name} Identifies the CAC table and enters SBE CAC policy CAC
table configuration mode.
Example: • Use the table-name argument to identify the name of
RP/0/0/CPU0:router(config-sbc-sbe-cacpolicy)# the admission control table.
cac-table MyCacTable
RP/0/0/CPU0:router(config-sbc-sbe-cacpolicy-
cactable)#
Step 6 entry {entry-id} Identifies the specific table entry and enters SBE CAC
policy CAC table entry configuration mode.
Example: • Use the entry-id argument to identify the number of the
RP/0/0/CPU0:router(config-sbc-sbe-cacpolicy- table entry.
cactable)# entry 1
RP/0/0/CPU0:router(config-sbc-sbe-cacpolicy-cac
-table-ent)#
Step 7 codec-restrict-to-list {list-name} Configures CAC to restrict the codecs used in signaling a call
to the set of codecs provided in the named list.
Example:
-table-ent)# codec-restrict-to-list my_codecs
or
RP/0/0/CPU0:router(config-sbc-sbe-cacpolicy-cac [cancel]:
-table-ent)# end
or – Entering yes saves configuration changes to the
running configuration file, exits the configuration
-table-ent)# commit
Step 9 show services sbc sbc-name sbe cac-policy-set Displays detailed information for a specific entry in a CAC
set-id [table name {entry entry-id | entries} | policy table, including any restricted codecs.
tables]

Example:
RP/0/0/CPU0:router(config-sbc-sbe)# show
services sbc mysbc sbe cac-policy-set 1 table
standard_policy_list entry 1

OL-15854-01 SBC-365

Configuration Example for Transcoding

Configuration Example for Transcoding
This section provides the following configuration example for configuring transcoding on an SBE:
• Configuring Transcoding: Example, page SBC-366

Configuring Transcoding: Example
The following example shows how to configure transcoding on an SBE:
configure
sbc ABC
service-location preferred-active 0/3/CPU0
sbe
control address h248 ipv4 88.88.133.2
control address h248 port 2944
control address h248 transport udp
media-gateway ipv4 200.200.201.101
codecs m=audio 1234 RTP/AVP 0 18,a=rtpmap:0 PCMU/8000,a=rtpmap:18 G729/8000
transcoder
exit
activate
end

Configuration Examples for a Codec Restriction
This section provides configuration examples for configuring restrictions on codecs and configuring a
CAC policy to use a codec list:
• Configuring Codecs: Example, page SBC-366
• Configuring a CAC Policy to Use a Codec List: Example, page SBC-367

Configuring Codecs: Example
This section shows how to configure a codec restriction.
Figure 18 contains three adjacencies (A, B, and C). Any calls involving “A” need to be configured to use
only the G729 and PCMU codecs with a minimal preferred packetization period of 10 milliseconds.
However, calls between “B” and “C” can use any available codecs.

SBC-366 OL-15854-01

Configuration Examples for a Codec Restriction

Figure 18 Scenario for Configuring Codec Restriction

A B C

SBC SBC SBC

AdjB

210611
SBC
AdjA AdjC
The following example shows how to create a codec list containing the specified codecs configured with
a minimal preferred packetization period:
configure
sbc mysbc
sbe
codec-list allowable_codecs
description The set of codecs allowed on adjacency AdjA
codec g729 packetization-period 20
codec pcmu packetization-period 10
end

After configuring codec restriction, you must configure a CAC policy to use the codec list (see
“Configuring a CAC Policy to Use a Codec List: Example” section on page SBC-367.)

Configuring a CAC Policy to Use a Codec List: Example
The following example shows how to configure a CAC policy to use a codec list:
configure
sbc mysbc
sbe
cac-policy-set 1
first-cac-table table1
first-cac-scope call
cac-table table1
match-type adjacency
entry 1
match AdjA
codec-restrict-to-list allowable_codecs
action cac-complete
end

OL-15854-01 SBC-367

Additional References

Note The codec list CLI commands are entered only at the per-call level in the CAC policy tables. If you
configure a codec list at any other level, the CAC policy set does not activate. However, a log is displayed
after you commit the configuration and the policy set is marked as “complete”.

Where to Go Next
After configuring SBC transcoding, you can configure the quality-of-service (QoS) marking. To
configure QoS, see the Implementing SBC Quality of Service (Marking) module.

The following sections provide references related to implementing SBC transcoding and codec
restriction.

Related Documents

Related Topic Document Title
Cisco IOS XR master command reference Cisco IOS XR Master Commands List
SBC Transcoding Commands on Cisco IOS XR Software Cisco IOS XR Session Border Controller Command
Reference
SBC Media Gateway Controller Commands on Cisco IOS XR
Software
SBC Policy Commands on Cisco IOS XR Software
Initial system bootup and configuration information for a Cisco IOS XR Getting Started Guide
router using the Cisco IOS XR Software
Cisco MGX 8880 documentation http://www.cisco.com/en/US/products/hw/gatecont/ps3869/i
ndex.html

Standards

Standards Title
No new or modified standards are supported by this feature, —
and support from existing standards has not been modified by
this feature.

SBC-368 OL-15854-01


MIBs

MIBs MIBs Link
— To locate and download MIBs using Cisco IOS XR software,
use the Cisco MIB Locator found at the following URL and
choose a platform under the Cisco Access Products menu:
http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

RFCs

RFCs Title
RFC 2833 RTP Payload for DTMF Digits, Telephony Tones and
Telephony Signals
RFC 3261 SIP: Session Initiation Protocol

Technical Assistance

Description Link
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pages of searchable technical content, including links to
products, technologies, solutions, technical tips, and tools.
Registered Cisco.com users can log in from this page to
access even more content.

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Implementing SBC Transcoding and Codec Restriction

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