Table Of Contents
Cisco BTS 10200 Softswitch
Annex E UDP Support
This document describes how the Cisco BTS 10200 Softswitch supports the User Datagram Protocol (UDP)-based Annex E feature of ITU-T Recommendation H.323 v4.
Note Annex E as used in this document refers to material that was previously in Annex E of Recommendation H.323, but is incorporated into the main body of Recommendation H.323 as of v4.
Throughout this document, Annex E refers to UDP-based Annex E functionality. (The Cisco BTS 10200 Softswitch does not implement TCP-based Annex E.) In this document, TCP refers to TCP-based signaling without Annex E functionality.
The Cisco BTS 10200 Softswitch is a class-independent network switch. In addition to performing switching functions, it can emulate up to four instances of logical H.323 gateways (GWs). The logical H.323 GW terminates H.323 signaling onto H.323 transit, or terminates the call in order to interwork with other protocols, such as MGCP, SIP, or another H.323 line. Annex E implementation allows H.323 signaling to be transported between the Cisco BTS 10200 Softswitch and the far-end H.323 endpoint using UDP (connectionless) signaling instead of TCP (connection-oriented) signaling. The choice of UDP or TCP signaling is important in a Cisco BTS 10200 Softswitch H.323 process restart or Call Agent (CA) failover scenario.
Note H.323 process restart means the H.323 GW instance in the Cisco BTS 10200 Softswitch restarts automatically.
CA failover means an automatic CA switchover, in which the standby CA side becomes active and takes over for the previously active CA side. To support the stability of active calls, the CA replicates and preserves call processing data from the active CA side to the standby CA side on a regular basis.
If an H.323 restart or CA failover occurs, a remote H.323 endpoint using TCP signaling cannot reestablish the connection with the previously active process or CA. Therefore, the stable H.323 call(s) on that connection are cleared. However, a remote H.323 endpoint using UDP to communicate with the Cisco BTS 10200 Softswitch in a connectionless session can continue to communicate with the newly active process or CA side using the same connectionless session. This allows the remote endpoint to preserve and support the active call. The Annex E feature must be enabled on the Cisco BTS 10200 Softswitch, the H.323 gatekeeper (GK), and the remote H.323 endpoint.
Note An H.323 call is considered to be stable, or in stable state, after the Connect message has been sent (or received) to (or from) the remote H.323 endpoint, and the media channels in both directions have been established. This definition differs somewhat from the typical telephony definition of a stable call.
Use of the Annex E feature is optional and configurable in the Cisco BTS 10200 Softswitch. Each H.323 trunk group (TG) in the Cisco BTS 10200 Softswitch can be independently provisioned to support either Annex E UDP-based signaling or non-Annex-E TCP-based signaling. Each logical H.323 GW instance in the Cisco BTS 10200 Softswitch can have multiple outgoing TGs, with each TG independently configured for Annex E UDP signaling or non-Annex-E TCP signaling.
Limitations on Annex E Support
Annex E must be supported and enabled on both the originating and terminating sides of a call leg for the Annex E redundancy to be supported. If the far-end H.323 device does not support Annex E UDP (or if Annex E is disabled), the following limitations apply:
•For a RAS-based call (ras parameter set to Y in the H323-TG-PROFILE table), the Cisco BTS 10200 Softswitch will automatically use TCP mode (to match the far end device), even if Annex E UDP mode is provisioned on the Cisco BTS 10200 Softswitch.
Note RAS (registration, admission, and status protocol) is defined in ITU-T Recommendation H.225. It is used to communicate between H.323 GWs, endpoints, and GKs.
•For a non-RAS call (ras parameter set to N in the H323-TG-PROFILE table), and with annexe-supp parameter set to Y in both the H323-GW table and H323-TG-PROFILE table, the Cisco BTS 10200 Softswitch will use Annex E UDP for outgoing calls. However, if the far-end does not support Annex E, these calls will fail.
Following are additional limitations on Annex E support:
•All TCP-based calls are automatically cleared if the Cisco BTS 10200 Softswitch H.323 process restarts or if the CA fails over.
•If the H.323-GW process in the Cisco BTS 10200 Softswitch restarts, or if the CA fails over to the other side, Annex E UDP-based transient calls (calls that are in the process of being set up) may be impacted as follows.
–The called party may continue to receive ringing and the calling party may continue to receive ringback, but the call will be torn down.
–After each of the parties goes on-hook, the ringing and ringback will automatically terminate.
–Each of the parties can then receive a new incoming call, or can go off-hook to receive a dial tone.
•For Annex E functionality to occur, the call-start-mode parameter in the H323-GW table must be set to fast-start (enables use of fast-connect procedure).
•If a call is established with Annex E, but reverts back to slow-start to match a far-end H.323 endpoint, the call will be dropped in event of an H.323 process restart or CA failover.
•If parameters are set in the following combination, then Annex E will be used, but stable calls will not be maintained:
–Out-of-band DTMF relay parameters, or out-of-band fax parameters, are enabled
–H.245 tunnelling is disabled
–Annex E is enabled
BTS Provisioning for Annex E Functionality
This section highlights the special parameters that must be provisioned to support Annex E functionality.
Note For basic H.323 provisioning, refer to the section on H.323-GW provisioning in the Cisco BTS 10200 Softswitch Provisioning Guide.
Caution In the following steps, the changes for Annex E support in the H323-GW table will not take effect until the H323-GW is controlled out of service (OOS), then in service (INS). Therefore, it is recommended that you perform these steps during a maintenance window.
Step 1 Login to a CLI session on the Cisco BTS 10200 Softswitch.
Step 2 To provision Annex E support for the H.323 GW instance on the Cisco BTS 10200 Softswitch, enter the following command.
change h323-gw id=city1gw; annexe-supp=Y; call-start-mode=FAST-START; annexe-udp-port=2517; annexe-retransmit-timer=500; annexe-retransmit-multiplier=2; annexe-retransmit-attempts=8;
•h323-gw id—This is the name used to identify the H.323 GW instance. This value must be a string of length 1 to 16 ASCII characters, assigned by the service provider.
•annexe-supp—This flag indicates whether this H.323 GW instance supports Annex E. The default value is Y (yes). This value must be kept as Y (yes) to support Annex E. If the remote H.323 end point does not support Annex E UDP signaling, the Cisco BTS 10200 Softswitch automatically adjusts and uses TCP signaling toward that end point.Explanation:
–If the Cisco BTS 10200 Softswitch H323-GW instance is configured as Annex E enabled (annexe-supp=Y), the Cisco BTS 10200 Softswitch indicates its Annex E capabilities while registering with the H.323 Gatekeeper (GK).
–For RAS routed calls, the GK negotiates all Annex E capabilities between the Cisco BTS 10200 Softswitch and the far-end H.323 endpoint, and notifies the originating endpoint to use Annex E.
–If the Cisco BTS 10200 Softswitch H323-GW instance is Annex E enabled (annexe-supp=Y), but the far-end H.323 end point is not, the Cisco BTS 10200 Softswitch can still complete calls using regular TCP mode.
–If the Cisco BTS 10200 Softswitch H323-GW instance is configured as Annex E disabled (annexe-supp=N), the Cisco BTS 10200 Softswitch will not receive any inbound Annex E UDP calls. It will receive inbound calls using TCP signaling only.
•call-start-mode—Specifies the preferred call start mode for outgoing H.323 calls. Possible values are FAST-START (default value) and SLOW-START. This parameter must be set to FAST-START to use Annex E functionality. If the remote H.323 end point does not support fast-start parameters, the Cisco BTS 10200 Softswitch automatically adjusts and uses slow-start mode toward that end point. The fast-start parameter is based on the fast-connect procedures described in ITU-T Standard H.323.
•annexe-udp-port—This is a number that specifies the port to be used to receive incoming Annex E messages. The default value is 2517, but a unique value must be assigned to each H.323 GW instance on the Cisco BTS 10200 Softswitch.
•annexe-retransmit-timer—This is the initial value of the retransmit timer which determines when to resend AnnexE packets if an ACK message has not been received. All subsequent retransmissions will be based on exponential back-off algorithm using annexe-retransmit-multiplier. The valid range is a number from 1 to 30,000 (in milliseconds), and the default value is 500.
•annexe-retransmit-multiplier—The multiplication factor of previous retransmit interval used for subsequent Annex E packet retransmissions. The valid range is 1 to 10, and the default value is 2.
•annexe-retransmit-attempts—Specifies how many attempts to resend a message to the remote entity before dropping the message. The valid range is 1 to 10, and the default value is 8.
Step 3 Use this step to control the H323-GW instance OOS (if necessary), and then INS:
Caution The changes to ANNEXE-SUPP parameter and other configurations in H323-GW table do not take effect until H323-GW is controlled out of service (OOS), then in service (INS). Use this step to control the H323-GW OOS (if currently INS), and then INS.
a. To display the service status of the H323-GW instance, enter the following command
status h323-gw id=city1gw;
b. View the display to see the status of the H323-GW instance.
d. Enter the following commands to place the H323-GW instance OOS:
Caution This step will clear all calls on this H323-GW instance.
control h323-gw id=city1gw; target-state=OOS;
e. Enter the following command and verify that the status of the H323-GW instance is displayed as OOS:
status h323-gw id=city1gw;
f. To place the H323-GW instance in service, enter the following command:
control h323-gw id=city1gw; target-state=INS;
g. Enter the following command and verify that the status of the H323-GW instance is displayed as INS:
status h323-gw id=city1gw;
Step 4 To provision Annex E support on the outgoing H.323 trunk groups (TGs), enter the following command.
CLI> change h323-tg-profile id=h323tg01; ras=Y; annexe-supp=Y; transport-pref-mode=UDP-MODE;
•h323-tg-profile id—This is the unique ID for this H323-TG-PROFILE. The format is 1 to 16 ASCII characters, assigned by the service provider.
•ras—Specifies whether RAS signaling to H.323 gatekeeper (GK) is supported. The allowed values are Y (yes) and N (no). Default value is Y. Set this value to Y to enable communications with the GK.
•annexe-supp—This flag indicates whether this H323-TG-PROFILE supports Annex E. The default value is Y (yes). This value must be kept as Y (yes) to support Annex E. If the remote H.323 end point does not support Annex E UDP signaling, the Cisco BTS 10200 Softswitch automatically adjusts and uses TCP signaling toward that end point.
•transport-pref-mode—Specifies what transport layer protocol to use to transmit H.323 signaling messages. Valid values are as follows:
–TCP-MODE, use TCP to transport messages.
–UDP-MODE (default value), use Annex E UDP-based message transport facility. If the remote H.323 end point does not support UDP, the Cisco BTS will adjust automatically to use TCP mode. This token must be set to UDP-MODE before annexe-supp can be set to Y.
Step 5 If not already done, enable Annex E support on the far-end H.323 end point.
Note To enable Annex E support for particular H.323 TG, it is necessary to enable Annex E for the H.323 GW instance on the Cisco BTS 10200 Softswitch, and also enable Annex E support for the far-end H.323 end point.
Events and Alarms
There are no changes to events or alarms associated with the Annex E feature.
There are no changes to traffic measurements or SNMP interfaces associated with the Annex E feature.
There are no changes in billing interfaces or billing parameters associated with the Annex E feature.
If Annex E functionality is provisioned on the Cisco BTS 10200 Softswitch, but stable calls are being dropped when the H.323 process restarts or CA switches over, check to see whether configuration is correctly registered with the GK:
Step 1 Log on to gatekeeper
Step 2 # show gatekeeper endpoints
Step 3 Examine the display of the GK, which should look similar to the example below:
CallSignalAddr Port RASSignalAddr Port Zone Name Type Flags
Step 4 Verify that both the CA and H323-GW are registered with the GK with Flags=E as shown in the above example.
The following documents have additional information about Cisco BTS 10200 Softswitch features and provisioning procedures:
•Cisco BTS 10200 Softswitch Provisioning Guide
•Cisco BTS 10200 Softswitch Operations, Maintenance, and Troubleshooting Guide
•Cisco BTS 10200 Softswitch Command Line Interface Reference Guide
•Cisco BTS 10200 Softswitch System Description