H.323 Dialing Tutorial

Introduction
This paper is a tutorial on the current state of video conferencing addressing using:

• VTEL H.323 terminals over TCP/IP networks

The purpose is to explain address format, how the address(es) are used and how the various connections complete. Address book format and user interface implications are beyond the scope of this document. In addition, the explanations here are product specific. Even though the protocols are defined to allow other connection scenarios, the products we use may restrict them.

Contents:

H.323 LAN Addresses

Circuit switched networks normally use static phone number style addresses. The address is saved away in many databases and then lookup up as needed.

In the H.323 TCP/IP world, addressing is much more dynamic. IP addresses are not static. They can change every time you boot up your system at work or connect to the internet from home. This is due to the fact that IP addresses are a limited resource. ISPs generally have a pool of IP addresses and hand them out to users as they connect to the internet. Many companies are now using Network Address Translation (NAT) schemes to reuse IP addresses behind their firewalls and then translate their address to a real internet address when the employee goes out to the internet. IP domain names usually are static, but they are not always used for all hosts. Internet e-mail names are the most static of all.

TCP/IP, in general, uses two different address formats:

Note: When calling using the VTEL system, the port number is not required. Port number 1720 is the well-known calling signaling port.

H.323 version 1 added two alias address formats:

Note: In order to use either of these formats, a H.323 version 1 (or above) gatekeeper is required on the network.

H.323 version 2 added two more alias address formats:

Note: In order to use either of these formats, an H.323 version 2 gatekeeper is required on the network. Vtel currently does not support any

H.323 terminals may have only 1 IP dotted-quad address, multiple IP domain names and multiple H.323 IDs, E.164 IDs, E-mail names and URLs.

Back to Top

Calling another H.323 terminal

To make a simple point-to-point call, the user specifies the IP dotted quad or IP domain name address of the destination.

If a gatekeeper is present on the network and both terminals have registered with the gatekeeper, then any of the alias addresses (H.323 ID, E.164 ID, e-mail name, or URL) can be used.

Example:

Address: Aliases:	192.150.109.1 user1.vtel.com user1 1000 user1@vtel.com		192.150.109.10 user10.vtel.com user10 1010 user10@vtel.com

The user would specify 192.50.109.10, "user10.vtel.com" if a DNS server is present or "user10", "1010" or user10@vtel.com if a gatekeeper is present.

Back to Top

Calling an H.320 terminal using a VideoServer Gateway

H.323 terminals must use a gateway to call H.320 terminals. Gateways may be shared by multiple H.323 terminals and there may be more than one gateway on the network.

To call an H.320 terminal through a gateway, the user must specify the address of the gateway to use along with the ISDN phone number(s) of the H.320 terminal.

Example:

192.150.109.1 user1.vtel.com user1 1000 user1@vtel.com	192.150.109.2 gw	437-2000

The H.323 terminal would specify the gateway using 192.50.109.2 (or "gw" if a gatekeeper is present) and 437-2000 to make the call.

With the Vtel H.323 terminal, the IP address of the Gateway is specified as aconfiguration item once, allowing the user to only enter the ISDN number and indicatethe call should be made using a Gateway. The VideoServer Gateway supports QuadBRI card (2x64 and 384IMUX calls only), a POTS card and a PRI card. In addition the user must specify a prefix on the outgoing call in order to select thrline rate. The prefix is also a one-time configuration item, that does not need to be entered each call by the user.

Note: the H.320 terminal may be an H.320 MCU for multipoint calls.

Back to Top

Calling from a H.320 terminal to a H.323 terminal with a VideoServer Gateway

H.320 terminals can use a gateway to call H.323 terminals. To call an H.323 terminal through a gateway, the H.320 terminal must specify the ISDN phone number(s) of the gateway to use along with one of the addresses of the H.323 terminal.

Once again, the "direct inward dialing problem" is present. There may be multiple H.323 terminals on the other side of the gateway. So the H.320 terminal has to pick which H.323 terminal it wants to connect to behind the gateway. This can be done by:

Use the new TCS-4 protocol

This is an addition to the H.320 standard that lets the H.320 specify the IP dotted-quad address or IP domain of the H.323 terminal to connect to. If a gatekeeper is present, then any alias address may also be used.

Note: if the H.320 terminal only has a numeric keypad, it must use the E.164 ID alias address of the H.323 terminal. This requires a gatekeeper and an H.323 terminal that registers E.164 ID alias addresses. This is the reason E.164 alias addresses were invented.

This option requires that the H.320 terminal and the gateway both support TCS-4.

Use DTMF tones

This method requires the H.320 terminal send touch-tones (DTMF tones) after the ISDN connection is made. This is used to specify the E.164 ID alias address of the H.323 terminal to connect to. Since only E.164 ID aliases can be used, a gatekeeper is required.

Note: The VTEL H.320 system will first attempt to connect to the Gateway using TCS-4, but if that fails, will attempt using DTMF. The VideoServer Gateway supports TCS-4.

Back to Top

Calling an H.323 MCU

The H.323 standard also defines MCUs. In H.320 MCUs, each terminal calls a different ISDN phone number. Since H.323 MCUs can use the same IP address for multiple connections, other methods must be used to specify which conference in the MCU to connect to. The method used is based upon MCU implementation.

Multipoint calls using a gatekeeper
VideoServer’s H.323 MCU registers a different name (H.323 ID) and number (e.164 ID) with the gatekeeper for each conference in the MCU. This requires that the user specify the name of the conference to use when making the connection. This method only works when both the MCU and the endpoint are registered with a gatekeeper

Multipoint calls without a gatekeeper
If a gatekeeper is not present, the VideoServer H.323 MCU allows users to connect to a single special conference named __OpenConference. This is how NetMeeting users join a conference (since they don’t use gatekeepers at this time).

Example:

192.150.109.1 user1.vtel.com user1 1000 user1@vtel.com	192.150.109.10 user10.vtel.com user10 1010 user10@abc.com		192.150.109.3 mcu conf1 conf2 conf3

To connect to the conference named "conf1", both H.323 terminal #1 and H.323 terminal #2 would specify "conf1" as the address to call.

Back to Top

Calling an H.323 terminal across a firewall

TBD.

Back to Top

Calling to/from an H.320 system connected to a Radvision VIU

TBD

Back to Top

Gatekeeper Functions

H.323 gatekeeper discovery, registration and admission

One function of H.323 gatekeepers is to translate from an alias address to an IP dotted-quad addresses. Since gatekeepers are optional items in an H.323 network, without them alias addresses cannot be used.

Gatekeeper discovery
Upon bootup - H.323 terminals, gateways and MCUs can discover their gatekeepers using multicast addresses. When more than one gatekeeper is present or if the user administrator wants a terminal/gateway/MCU to use a specific gatekeeper, the terminal/gateway/MCU may be configured for a specific gatekeeper.

Gatekeeper registration
Once discovered, the terminal/gateway/MCU register their IP dotted quad address along with all of their alias addresses, terminal type (terminal, gateway, MCU) and vendor information. The gatekeeper keeps track of all of the alias addresses. Notice that the MCU has not only registered a name for itself, but for all of it’s conferences, too (more on that later).

Each H.323 entity should be registered with only one gatekeeper. Multiple gatekeepers can be used to manage multiple sets of users. The LAN segments that a single gatekeeper manages is called a zone.

Example:

Address: Aliases:	192.150.109.1 user1 1000 user1@vtel.com		192.150.109.2 gw	192.150.109.3 mcu conf1 conf2 conf3

Gatekeeper admission:
Another function of H.323 gatekeepers is allow or disallow calls to occur. Prior to initiating a call, the H.323 terminal sends an admission request (may I make this call) to the gatekeeper. The admission request contains the destination address (or alias) and the bandwidth required for the call. The gatekeeper, based on some predetermined criteria, either allows the call with an admission confirm message, translating any alias address into a dotted-quad, or rejects the call.

When an H.323 terminal is registered with a gatekeeper, the gatekeeper can reject the call for any number of reasons including:

• the user may not have permission to call the destination (the administrator manually disallows it)
• there may not be enough bandwidth to make the call

The policies used by the gatekeeper to accept or reject the call depends on the gatekeeper implementation. Note that a terminal may override a gatekeeper reject by:

1. using a dotted-quad IP address (if known)
2. not sending an admission request to the gatekeeper, but simply addressing the destination terminal directly

The gatekeeper's enforcement power is derived from terminals adherening to the protocol and its maintainance of address information, the mapping table of alias to dotted-quad for registered endpoints

(The following section may be deleted)

H.323 <-> H.323 through Two Back-To-Back Gateways
H.323 connections can also go through two back-to-back gateways. This allows H.323 calls to go around the WAN portion of their data network. This works well for users who:

• Have limited bandwidth on their data networks (especially their WAN bandwidth)
• Have poor quality of service through their data network
• Prefer not to use up data network bandwidth for video conferences

Even though this scenario looks inviting to the customer, there are a few drawbacks:

1. If either of the two gateways doesn’t connect properly, it can be very difficult for the user to determine where the problem is occurring.
2. The user doesn’t save any money on the call since they are still using ISDN as the WAN transport.
3. Both gateways must support the same "direct inward dialing" method: TCS-4, DTMF, ISDN subaddr, or ISDN MSN (see previous section).

Example:

To make a call, the H.323 terminal on the left would specify the left gateway’s IP address (192.150.109.2 or "gw" if a gatekeeper is present), the right gateway’s ISDN number (437-2000), and one of the right’s H.323 addresses depending on:

Note: May not be supported.

References
For more information about H.323 dialing modes, see the ITU H.323 Recommendation, February 1998.

Back to Top