the ISDN transport

Work on developing ISDN began in the  1970s but  would  not  be sold  to the  bandwidth-hungry customer until the  early  to mid 1990s. Many said it was too  little too  late, and  the  consumer market for ISDN never took  off. After the  news  of the  first VoIP telephony call over  the Internet spread in 1995, a renewed interest in ISDN emerged for a short while. But by this  time our attention was turned to the  emerging DSL technology first deployed in 1998.

The eventual ISDN standard provided for two flavors of ISDN: Basic Rate Interface (BRI) and  Primary Rate Interface (PRI). The ISDN standard defines the  basic unit of bandwidth as a B channel, which provides 64 Kbps of band-  width. B stands for bearer channel, which is another name for the  channel that carries POTS calls over  the  PSTN.

In the  digital  world, all transport lines  provide one or more  channels, just like your  cable television provides different channels to carry  various programs. Unlike POTS calls,  ISDN calls originate in digital  form and  travel over  the  switched network to the  destination being  called.

BRI
By the  time ISDN rolled out  to the  public, other transports and  services had  evolved that provided more  bandwidth without the  complexities and cost factors associated with the  BRI flavor of ISDN. Some BRI customers are still out  there, but  they  are usually in the  process of converting to DSL, cable modem, or some variation of wireless technology. The monthly recurring charges for BRI transport services are considered exorbitant — and  are even  higher than POTS recurring charges. As a result, BRI has  generally dropped out  of the  VoIP picture.

PRI
The PRI implementation of ISDN has  proven to be a cost-effective transport option for companies with a single  location seeking to run IP telephony on their LAN. PRI supports local POTS calls into the  PSTN.

PRI is the  one ISDN transport that has  remained useful  for supporting VoIP. It provides a customer with 23 B channels of digital  bandwidth. In addition, the  carrier configures the  PRI to have  one 64 Kbps D channel, which is used to manage the  line. The industry summarizes the  PRI aggregate digital  band-  width as “23 B + 1 D.”

In general, a VoIP call is made over  a PRI transport as follows:
1.  A caller  uses  their IP-enabled phone to dial a number.
2.  The number initiates a packet  on the LAN. For inside calls,  the packet  stays on the LAN. For outside calls,  the packet  is switched to the PSTN gateway. A PSTN gateway  is a hardware device whose basic function is to sit between two dissimilar networks and  translate the  packets that pass through into the  format required by the  destination side.  Many different levels  of gateways include other functions, such as routing and  network management.
3.  The PSTN gateway may use a PRI transport  to connect directly to the PSTN. When the gateway gets the call request, it allocates a B channel on the PRI transport,  initiates the call, and passes the call to the PRI transport.
4.  After the call is on the PRI, it is translated for operation over  the PSTN as a circuit-switched call.
5.  When the call reaches the destination telephone, a circuit is estab- lished between the caller  and the receiver for the life of the call.

6.  When the call is complete and either  party hangs  up, the PRI B chan- nel is returned to the channel pool  controlled by the PSTN gateway.

Because PRI is a switched transport, it easily  connects to the  PSTN, which is also  switched. PRIs are compatible with the  call control used on the  PSTN to manage POTS-related calls.  (Call control is discussed at some length later in this  chapter.)

In effect, the PRI is capable of handling twenty-three individual telephone calls simultaneously, delivering an aggregate bandwidth capacity of 1.472 Mbps over  the PSTN. In addition, the PRI transport can be used for computer data as well as videoconferencing. The PRI transport continues to be employed because it
is effective, compatible with the PSTN, and cheap, averaging about $275 to
$425 per month. Local recurring telephony charges still apply, as they do with
POTS or any off-net VoIP call.

The PRI is not  a good  fit for every  VoIP network environment. But for a single- location LAN running IP telephony (VoIP on the  LAN) with separate Internet access, the  PRI is an effective solution for gaining  access to the PSTN for your  network.