40-400-00001, Rev. A
13
TEI Assignment
Most ISDN Terminating Equipment support a feature called automatic TEI assign-
ment. After a device is connected and synchronized, packets are exchanged with
the switch to establish the TEI. The switch knows which TEI’s are assigned to that
line and can query each device if in doubt. A unique TEI between 64 and 126 is then
formulated and returned to the requesting device. TEI 127, the default broadcast
TEI, is used for this operation. Some switches or devices do not support automatic
assignment, however. The user is then responsible for configuring the device with a
unique
fixed TEI between 1 and 63.
Layer 3 - Network Operation
Once packet exchange is established, the Network Layers of the switch and Termi-
nating Equipment can begin the Call Control dialog that replaces conventional
telephony signaling. The dialog is based on a set of commands that operate on one
or more parameters, similar to a computer Operating System language. Commands
are called
Messages and parameters are called Information Elements (IE). There
are over 45 Messages and 65 Information Elements currently used by North Ameri-
can ISDN telephone networks. Information elements may contain between one and
over 100 bytes of constant or variable data.
For example, the SETUP Message initiates an incoming or outgoing call on de-
mand. Several Information Elements are contained in a SETUP Message, and each
are expected to be a particular value or variable type. The SETUP Message usually
contains the
Channel ID, Bearer Capability, and Calling Party Number Information
Elements, among others. The Channel ID determines which B or D Channel is used
for the connection. The Bearer Capability describes the call as voice, high-speed
data, or packet mode data and sets the data rate and other parameters. The Calling
Party Number is the caller’s Directory Number in ASCII code.
Other Messages and Information Elements enable tones, transport keypad num-
bers, alert the called party, connect the call across the network, and disconnect the
call and release its resources when either party hangs up. Standardized Message
types and Information Elements are listed in Chapter 5. Call Control is extensible,
however, and the number of Messages and Information Elements continues to
increase.
Multiple versions of ISDN Call Control currently exist. Two common ISDN switches
use proprietary Call Control: Northern Telecom’s DMS-100 series and AT&T’s 5E
series. The Call Control standard to which these switches are being upgraded and
to which current switches are designed is called
National ISDN-1. The European
Economic Community, (ECC) is currently consolidating various national versions to
comply with the European Telecommunications Standards Institute (ETSI) protocol.
Consequently, Messages and Information Elements may differ in function and
coding, depending on the switch and the vintage. A special Message, NETwork
ESCape, is implemented to alert the switch when a proprietary Message coding is
used. Information Element codings that are not yet adopted by the ITU-T are
preceded by a
Locking Shift Information Element.
Unfortunately, these practices do not ensure that all switches recognize all devices.
Any Message that is not recognized by the switch is ignored. For this reason, not all
ISDN equipment is compatible at all locations.