eadquarters-Tokyo or Nx56 or Nx64 Phone per port Nx56 or Nx64 Nx56 Nx64 T1/PR 1 to 8 Router Access Device Global Nx56 or Nx64 or dual 56=112K Switched Digital Video System Services Video System Router BRI Desktop Video Domestic Regional Office-San Francisco SW384 or Nx64 or Nx56 or dual 56ml 12K T1/PRI SW384 or Video System Nx54 or Nx56 Router Figure 11-3. Global Switched Video Network (courtesy of Ascend). Circuit-switched Networks The ISDN Terminal Adapter The ISDN specification also requires a terminal adapter or TA. ISDN TAs place calls, make connections, and transfer digital data across the ISDN circuit. They consist of a call-control module and a B channel and D channel transport module. The call control module manages the resources of the ISDN line, using the Q.931 protocol to perform signaling functions. Q.931 is discussed below. TAs can be internal (operate from the computer's local bus) or external (in which they are limited to the speed of the computer serial port-typically 115 Kbps. In videoconferencing applications, the TA is usually built into software. Configuring a TA can be tricky. For instance, the person doing the set-up must know the type of CO with which the TA is communicating. Between North America and Europe, there are many different versions of the D-channel protocol. Some of these are based on standards (see National ISDN, below). Others are unique to a type of central office (e.g., AT&T 5ESS Custom, Northern Telecom DMS-100, European 1TR6, NET3, etc.). One of the biggest configuration challenges in setting up the TA had to do with entering the proper Service Profile Identifier (SPID). The customer must enter the SPID into the ISDN TA before a device can, using the D channel, initialize at layer three of the OSI Reference Model. A TA sends out its SPID each time it communicates with the CO-and each TA has a unique SPID. The SPID is associated with a set of ISDN features and capabilities. SPIDs can be assigned only after a customer has notified the LEC about what ISDN service and features