Introduction
This presentation describes addressing standards that are used at Northwestern University to place calls between H.323 endpoints located at Northwestern as well as between Northwestern endpoints and endpoints located at other universities. While the emphasis is on H.323 addressing, some attention is also paid to H.320 addressing using the Northwestern gateway. Users who do not have a basic understanding of the interoperability of H.323 components (gatekeepers; endpoints such as client terminals, MCUs, and gateways; and terms such as the H.323 extension), should first read the information in H.323 Basics.

Point-to-Point calling
First consider H.323 calls between two client terminals (endpoints) that are both registered with the Northwestern gatekeeper. The IP address of the Northwestern gatekeeper is 129.105.253.250, and this IP address must be configured into the user's client terminal in order for it to register with the Northwestern gatekeeper.

Once registered, one terminal can call the other using the called terminal's H.323 extension. Since both are registered with the same gatekeeper, the gatekeeper sets up the call between the two terminals, and once set up, the audio/video streams flow between the terminals over the Internet.

At Northwestern University, we have decided to use a locally defined convention for the format of the H.323 extension (video address). H.323 extensions will be six characters in length. The first character will be a '1'. The next five digits will be the last five digits of the user's telephone number. Assuming that a user's telephone number is 847 467 2345, the number the user would configure in their terminal as their H.323 extension would be 172345. Others users could then call this user by entering (dialing) 172345 using their client terminal interface. This provides an easy way to remember a user's video address.

Not all universities follow the same conventions for assigning H.323 extensions; in fact, there is a wide range of H.323 addressing conventions in use. In the sections "H.323 Zones" and "Inter-zone Calling" below, methods for calling users registered with gatekeepers administered at other universities is described. Frequently, it is convenient to have non-Northwestern users register with the Northwestern gatekeeper. They need not follow the Northwestern convention for H.323 extension assignment. The gatekeeper will allow terminals with any H.323 extension to register. The gatekeeper does not limit the size of the H.323 extension to six characters ("1" followed by the last five digits of your telephone number) as used at Northwestern University. The important fact to remember is that each user must have a unique H.323 extension configured when registering their client terminal with the gatekeeper. We cannot have two users with the same H.323 extension registered at the same time.

Some times H.323 terminals are not located in an office, but they are located in a conference room. We suggest that the number of the telephone in the conference room be used to construct the H.323 extension. Also, there are times when a user has more than one H.323 terminal but only one telephone number assigned. We suggest that in these cases, a seven-digit H.323 extension be used. The first six digits are constructed as before starting with a "1" and the last five digits of the user's telephone number, but then a "1" is appended to the end of the string. The resultant string is assigned to the first terminal (e.g. 1723451). Append a "2" for the second terminal (e.g. 1723452), and so forth for any another terminals.

Multipoint calling
So far we have been discussing point-to-point calls. If more than two sites are involved in the videoconference, a multipoint videoconference is necessary using a Multipoint Control Unit (MCU). The system administrator will have defined services on the MCU. When the MCU is powered on, it registers any services that have been defined with the gatekeeper.

At Northwestern University we have defined a service (service 70) that supports multiple videoconferences, each running at 384 Kbps. A given videoconference is defined by the service number and by a password. A system administrator assigns a password when the videoconference is scheduled and gives the password to the conference chairman who made the request for the videoconference. The conference chairman then sends the password to all the people that are to attend the videoconference. Using the interfaces on their client terminals (e.g. a terminal such as a Polycom Viewstation), a user at each site calls (dials) the MCU session in a manner similar to the way a user calls another user in a point-to-point call. Assuming that you want to enter a particular videoconference with a password of 1234 on service 70, then you would dial 701234. Note, however, that this only works if the user's client terminal and the MCU are both registered with the same gatekeeper. If they were not, then you would have to follow the procedures outlined in the following section.

H.323 Zones and Inter-Zone Dialing
So far we have discussed H.323 point-to-point and multipoint calls when user terminals and MCUs are all registered with the same gatekeeper. An H.323 zone is defined as a single gatekeeper and all of the client terminals, MCUs, and gateways that are registered with that gatekeeper.

A zone is identified by the IP address of the zone's gatekeeper and by a "zone number". Most universities in the United States and indeed internationally have adopted the architecture defined by ViDeNet for zone numbering. The convention for zone numbering has been in a state of transition for some time now and is currently on version 3. Zone managers, including the Northwestern University zone manager, register their gatekeepers with ViDeNet and are assigned a zone number. The current zone number for the Northwestern gatekeeper is 0011169.

As defined in the ViDeNet Version 3 architecture, a Global Dialing Scheme (GDS) provides a structure for peering between gatekeepers, such that any endpoint associated with a ViDeNet zone may dial an endpoint in any other ViDeNet zone. The new GDS gatekeeper hierarchy features redundant global gatekeepers in the US, Ireland, Wales and Australia for fully resilient operation.

Lets consider two H.323 zones, one being the Northwestern zone (0011169) and the other being the Ohio State University zone (0011109). Assume that you are calling a user at Ohio State University (OSU) that has a H.323 extension of 2000. Then you would dial 0011109 2000. Note that you do not dial the spaces; they are only included here for readability. Also note that OSU does not follow the Northwestern convention for H.323 extensions. They do not start their H.323 extensions with a "1".

Assume that you want to join a videoconferencing session on the OSU MCU that has the address 70*1234. Note that OSU separates the service number (70) and the password (1234) with an "*". We do not use this convention at Northwestern. To join the session you would type 0011109 70*1234. Note again that you do not dial the spaces, but you must dial the "*".

If your client terminal had a H.323 extension of, say, 172345 and you wanted to tell someone registered in another zone what address they should use to call you, it would be: 0011169 172345. If you wanted to have someone registered in another zone join a session, say 701234, on the Northwestern MCU, then you would tell them to dial: 0011169 701234. Note that in some cases, zone administrators may have defined an "out of zone prefix" that their users would have to dial before dialing these numbers. We do not use an out of zone prefix at Northwestern in order to initiate out of zone calls.

Gateways
Gateways provide the means for H.320 clients to be able to videoconference with users that have H.323 client terminals (point-to-point videoconferences) or for H.320 clients to join videoconference session hosted on an H.323-based MCU (multipoint videoconferences). H.320 clients connect to the Northwestern gatekeeper by dialing it over ISDN lines. The ISDN number that users must call to connect to the Northwestern gateway is 847 467 0001. When the gateway number is dialed, the gateway will respond with a voice requesting the user to dial an extension. The extension must be an H.323 extension of a H.323 client terminal (point-to-point call) or a service/password address of a session on the Northwestern MCU. Not all H.320 clients are capable of dialing the second dial sequence that represents the extension number although most can. Users need to consult the user documentation for their particular H.320 client to determine the appropriate escape sequence that allows them to dial the second set of numbers. If the user does not enter the extension, the gateway will time out and connect the H.323 client to a default extension of 701234 on the MCU.