The difference between Zone Prefix and Tech Prefix

An H323 gateway uses a gatekeeper for telephone (E164) number to destination gateway ip address resolution and/or bandwidth authorization. An Admission Request (ARQ) message is sent to the gatekeeper by the originating gateway and assuming the gatekeeper knows the whereabouts of the telephone number contained within the ARQ and there is ample bandwidth provisioned, an Admission Confirm (ACF) message containing the IP Address of the destination gateway is returned to the originating gateway.

What is often overlooked is exactly how gatekeeper isolates a particular telephone number as belonging to a specific gateway. Gatekeeper uses a decision tree when determining whether to send an ACF and indeed what destination gateway IP Address to use within the ACF message or whether to reply with an Admission Reject (ARJ) message.

The most simplified method of routing a call though gatekeeper is used when the gatekeeper contains an E164 number to Call Signaling IP Address/Port mapping.  This can occur when analog devices are attached to FXS ports on a voice gateway (IOS or ATA for example). In this situation it is possible for the gateway to inform the gatekeeper of all E164 numbers known to the gateway. When a call arrives with a matching number then the gatekeeper will inform the originating gateway of the Call Signaling IP/Port using an ACF message. The underlying technology SCCP Phones use when registered to a UCME is the same as analog devices connected to FXS ports and hence it is possible to register SCCP phone extension numbers to the gatekeeper. The same is not true for SIP Phones registered to UCME.

Assuming the called number in the ARQ message did not match a registered E164 number, the next branch of the decision tree used by gatekeeper is Technology Prefix analysis. The “Tech Prefix” is designed to give the Called Number an additional layer of granularity that the E164 number does not provide- gateway type. If we take a look at an example E164 number such as +1 408 7773434 we can see that we do not have any information on whether to route this call to a video or voice endpoint. The purpose of the Tech Prefix is to allow gatekeeper to route the call to the appropriate gateway once the destination zone has been determined. You can think of all gateways in a particular area code or country code as being in a separate zone- just a logical grouping of gateways. Video gateways sending a call to gatekeeper might use a Tech Prefix of 1# and Voice gateways sending a call to gatekeeper might use a Tech Prefix of 2#. Users do not dial the actual Tech Prefix- the originating gateway inserts this into the Called Number. All of our video and voice gateways will have registered with the Tech Prefix of 1# and 2# respectively. Gatekeeper will inspect the Called Number contained within the ARQ message (e.g. 1#14087773434) and determine that the call should be routed to a video gateway (2#) in the San Jose (1408).

The list of all known Tech Prefixes is contained with the “gw-type-prefix” table as shown below.

SJC-RTR#sh gatekeeper gw-type-prefix
GATEWAY TYPE PREFIX TABLE
=========================
Prefix: 1#*
Zone US master gateway list:
10.10.210.10:1720 US-VIDEO-GW

Prefix: 2#*
Zone US master gateway list:
10.10.210.11:1720 US-VOICE-GW

Prefix: 1#*
Zone Spain master gateway list:
10.10.110.3:1720 SPAIN-VIDEO-GW

Prefix: 2#*
Zone Spain master gateway list:
10.10.110.4:1720 SPAIN-VOICE-GW 

The result of the Tech Prefix analysis is not conclusive- we need to find a destination zone before the gatekeeper can generate the appropriate response. The remainder of the Called Number (digits after Tech Prefix) are inspected and compared against the zone prefixes which are contained in the “zone prefix” table as shown below.

SJC-RTR#sh gatek zone prefix
ZONE PREFIX TABLE
=================
GK-NAME               E164-PREFIX
-------               -----------
Spain               34*
US                    1*

Sticking with the same example the Called Number of 1#14087773434 would be sent to “US-VIDEO-GW” because the Tech-Prefix match is 1# and the zone prefix match is “1″. The originating gateway would send the entire Called Number string in a SETUP message after the gatekeeper has resolved the number- the destination gateway would most likely strip the Tech Prefix of 1# and route the call onward based on the remaining digits (E164 number). Incidentally if there is no zone prefix match then gatekeeper defaults the destination zone to be the source zone which could consequently result in the call being sent to the correct gateway type in the wrong zone. The only time you are able to get away from using a zone prefix is when there is only a single zone configured on the gatekeeper in which case the source and destination zone will always be identical.

Other than the first branch of the decision tree that was mentioned earlier (E164 numbers registered to gatekeeper), there is one instance when the zone prefix is not used and the tech prefix alone is used to route the call and this is known as hopoff tech prefix. The hopoff Tech Prefix can be used to bypass the zone prefix and is configured statically on the gatekeeper using the “gw-type-prefix hopoff” command within IOS CLI. So sticking with the same example used:

SJC-RTR(config-gk)#gw-type-prefix 1#34 hopoff Spain gw ip 10.10.110.3 1720

With the above command the gatekeeper on receiving a Called Number of 1#3412345 would route the call to “SPAIN-VIDEO-GW” and the “zone prefix Spain 34*” would not be used.

Aside from when E164 numbers are registered to gatekeeper, there is one instance when the tech prefix is not used and this is when there is a matching zone prefix resolving a telephone number to a remote zone – a zone defined on another gatekeeper.

SJC-RTR#sh gatek zone prefix
ZONE PREFIX TABLE
=================
GK-NAME               E164-PREFIX
-------               -----------
Spain               34*
US                    1*
India              91*

If the zone India (country code 91) was defined as a remote zone, then the Tech Prefix is not required – the zone prefix alone would be able to route the call and it is therefore possible for the originating gatekeeper to not include any Tech Prefix in the Called Number.

Finally, there is final branch of the decision tree and this can be considered a catch all “gateway of last resort” option . This is known as the Default Technology Prefix and is used when gatekeeper is unable to route the call based on E164 number registration, there is no matching Tech Prefix and there is no zone prefix sending the call to a remote zone. The benefit of using this option is that originating gateways do not need to include the Tech Prefix and destination gateways do not need to strip the Tech Prefix in order to successfully route the call and therefore configuration and design is less complex. This would be the most sensible option to use when there is no mix of gateway types registered to the gatekeeper (i.e. voice only gateways or video only gateways). In order to successfully use this option the zone prefix is still required otherwise the destination gateway will be the “gateway of last resort” in the incorrect (source) zone.

Vik Malhi, IPexpert Inc

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Tags: ccie voice, ccie voice blog, tech prefix, zone prefix