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Protocol Structure - ISDN: Integrated Services Digital Network
General structure of the ISDN frame
| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
|---|---|---|---|---|---|---|---|
| Protocol discriminator | |||||||
| 0 | 0 | 0 | 0 | Length of reference call value | |||
| Flag | Call reference value | ||||||
| 0 | Message type | ||||||
| Other information elements as required | |||||||
Protocol discriminator
The protocol used to encode the remainder of the Layer.
Length of call reference value
Defines the length of the next field. The Call reference may be one or two octets long depending on the size of the value being encoded.
Flag
Set to zero for messages sent by the party that allocated the call reference value; otherwise set to one.
Call reference value
An arbitrary value that is allocated for the duration of the specific session, which identifies the call between the device maintaining the call and the ISDN switch.
Message type
Defines the primary purpose of the frame. The message type may be one octet or two octets (for network specific messages). When there is more than one octet, the first octet is coded as eight zeros. A complete list of message types is given in ISDN Message Types below.
ISDN Information Elements
there are two types of information elements: single octet and variable length. Single octet information elements- the single octet information element appears as follows:
| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
|---|---|---|---|---|---|---|---|
| 1 | Information element identifier | Information element | |||||
Variable length information elements - The following is format and the variable length information element:
| 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
|---|---|---|---|---|---|---|---|
| 0 | Information element identifier | ||||||
| Length of information elements | |||||||
| Information elements (multiple bytes) | |||||||
The information element identifier identifies the chosen element and is unique only within the given Codeset. The length of the information element informs the receiver as to the amount of the following octets belonging to each information element.
ISDN Message Types - The possible ISDN message types: Call Establishment, Call Information Phase, Call Clearing, and Miscellaneous.
Codeset - Three main Codesets are defined. In each Codeset, a section of the information elements are defined by the associated variant of the protocol:
| Codeset 0 | The default code, referring to the CCITT set of information elements. |
| Codeset 5 | The national specific Codeset. |
| Codeset 6 | The network specific Codeset. |
CPE - Customer Premises Equipment - refers to all ISDN compatible equipment connected at the user sight. Examples of devices are telephone, PC, Telex, Facsimile, etc. The exception is the FCC definition of NT1. The FCC views the NT1 as a CPE because it is on the customer sight, but the CCITT views NT1 as part of the network. Consequently the network reference point of the network boundary is dependent on the variant in use.
ISDN Channels B, D and H - The three logical digital communication channels of ISDN perform the following functions:
