Internet Telephony Gateway Concepts
The ITG enables the transmission of voice and fax traffic over any IP network by digitizing voice and fax signals, encapsulating the information within IP packets, and then sending the packets across the IP network
1. The TIM inside the ITG digitizes analog voice signals at 8 Kbps.
2. ITG system software handles the:
Ÿ Capture of telephone number presented as DTMF tones.
Ÿ Mapping the telephone number to the IP address of remote ITG.
Ÿ Setting up calls with remote ITGs utilizing H.323 call control protocol.
Ÿ Digitizing, compressing and encapsulating the voice into IP packets and transmission of the IP packets onto the Ethernet LAN.
3. A router attached to the LAN forwards the IP packets across the WAN, where they will be received by another ITG at the remote.
4. The process is reversed at the remote ITG.
To allow you to easily dial a telephone or fax on the network, the ITG maps a series of dialed digits to the IP address of the remote ITG whose phone or fax you are calling. This mapping information is contained in a database inside each ITG called the dial plan.
Based on the dial plan the
Address Translation and Parsing Manager (ATPM)
inside the ITG
translates telephony numbers to IP addresses of remote ITGs. The ATPM collects
telephone number dialed by users, decides
whether the dial string is part of the dial plan and, if it is, maps it a remote
ITG. When the call is set up to the destination, a substring of the original
dial string will be sent along to the remote ITG.
The destination is where a call is terminated. Typically, for inbound calls from IP network, the ITG terminals the call at one of the telephony ports. The destination for the call is the telephony port where the call terminated. For calls initiated from telephony ports, the ITG forward the call to a remote ITG via IP network, and the remote ITG terminal the call. The destination of the call is the remote ITG.
Instead of directly mapping a phone number to a destination, the ATPM first maps the phone number to a group of destinations known as a Hunt Group. A hunt group is a group of destinations that are equivalent. For example, the customer support group of a company might have 20 people who can handle support calls. Access to customer support is through a single phone number but the next available support person is actually connected upon each incoming call. These 20 phones would be configured as a hunt group. A hunt group consists of a phone number and a list of destinations (members of the group). When an incoming phone number matches the phone number of the hunt group, the ITG attempts to terminate the call at each of the destinations in the hunt group, one at a time until a call is successfully completed.
Every destination that can be reached by dialing a phone number is a member of at least one hunt group. When an address is presented to ATPM for lookup, the output is a hunt group ID number. As a second step, the hunt group ID is presented to ATPM to get the list of members. To effectively bypass the hunt group feature, simply make a unique hunt group for each destination and one member in each hunt group.
The dial plan is a database inside the ITG for the ATPM to map telephony numbers users dialed to the IP address of remote ITGs. The dial plan consists of the destination table, hunt group table and the address table. Users need to setup these tables, so that the ITG knows how to setup calls with remote ITGs.
The address table maps a phone number to a hunt group. The table contains entries that specify the following information:
· Telephone number
· The hunt group the phone number maps to.
· The minimum number of digits to collect before the ATPM starting address lookup.
· The maximum number of digits the ATPM collects before it considers the dial string is complete.
· Number of digits forward to the destination.
Address table sample:
|
Address Entry |
Hunt Grp_Id |
Min. Digits |
Max. Digits |
Prefix strip |
Prefix Address |
|
200 |
3 |
3 |
0 |
None |
|
|
201 |
3 |
3 |
3 |
0 |
None |
|
899 |
11 |
3 |
3 |
0 |
None |
|
8 |
12 |
3 |
3 |
0 |
None |
|
0 |
5 |
1 |
1 |
0 |
None |
|
03 |
5 |
10 |
10 |
2 |
“0” |
The hunt group table maps a hunt group to a list of destinations. Hunt group sample
Group id
Type #Members
Member ids
----------------------------------------------------------------------------
1 2
1
1
3 2
1
2
5 2
1 4
11
2
1
11
12
2
1 12
The destination table maps a destination to a telephony port or the IP address of a remote ITG.
Destination table sample
Dest
id Mode
Destination
-------------------------------------------------------
1
Local PORT
= 0
3
Local
PORT = 2
5 Local
PORT = 4
11 H.323
Dest = 192.168.0.55/1720 TCP
12 DNS
Dest = itg0021.dyndns.org /1720 TCP
Voice from PSTN is compressed by the ITG before sending across the IP network and then decompressed by the destination ITG. The voice coders supported by the ITG are designed for ideally compressing and decompressing human voice. If the compression / decompression process is performed on DTMF tone which needs to be conveyed across IP network, distortion might be too significant to be not cognizable in the receiving end. To overcome the shortcoming that the voice coders can not perfectly encode DTMF tone, the ITG encodes DTMF tone into special packets. The packets are then sent to the destination ITG via a separate IP connection. The destination ITG decodes the packets, generates the DTMF tone, and then sends the tone to the PSTN. The way the ITG handles DTMF tone is so called DTMF relay .
The ITG handles DTMF relay per H.323 specifications. Certain third party VoIP devices may handle DTMF relay per IMTC standard. For the ITG to interoperate with those VoIP devices, users need to specify which remote VoIP devices uses IMTC conforming DTMF relay technique. Refer to CLI command set h323 imtc_dtmf {add|del} ip_addr on user's manual for detailed information on how to select DTMF relay mode.
Voice codecs supported by the ITG include G.711, G.723.1 5.3kbps, G.723.1 6.3kbps and G.729 AB. When setting up a call, two ITG automatically negotiate with each other until an agreed upon codec is determined.
Copyright EUSSO Technologies, Inc. 2000,2001,2002