Primary Rate ISDN

If you have ever tried or have taken the CCNA test one annoying thing you have to learn are ISDN device and reference terms for one question on the test. Since 1997 I have been able to implement and turn up both Primary Rate and Basic rate ISDN without having to know these terms. The reference points Idea are confusing to me; basically if it is PRI then it is standard T-1 connections to the line card in your PBX or media gateway. And the BRI in the United States is a 2 wire circuit that is polarity sensitive, so you may have to transpose the tip and ring to get your adapter to work. All BRI adapters on the market now Ethernet and USB capable so there is no need to mess with RS-232 cables to PC serial ports.

Here is the official specification (Legacy as it is)
ISDN Devices connecting a CPE and a network. In addition to facsimile, telex, PC, telephone, ISDN devices may include the following:

• TA Terminal Adapters – devices that are used to portray non-ISDN equipment as ISDN compatible.
• LE Local Exchange – ISDN central office (CO). The LE implements the ISDN protocol and is part of the network.
• LT Local Termination – used to express the LE responsible for the functions associated with the end of the Local Loop.
• ET Exchange Termination – used to express the LE responsible for the switching functions.
NT Network Termination equipment exists in two forms and is referred to accordingly. The two forms are each responsible for different operations and functions.
• NT1 – Is the termination of the connection between the user sight and the LE. NT1 is responsible for performance, monitoring, power transfer, and multiplexing of the channels.
• NT2 – May be any device that is responsible for providing user sight switching, multiplexing, and concentration: LANs, mainframe computers, terminal controllers, etc. In ISDN residential environments there is no NT2

TE Terminal Equipment – any user device e.g.: telephone or facsimile. There are two forms of terminal equipment:
• TE1 – Equipment is ISDN compatible.
• TE2 – Equipment is not ISDN compatible

ISDN Reference Points
Reference points define the communication points between different devices and suggest that different protocols may be used at each side of the point. The main points are as follows:

• R A communication reference point between a non-ISDN compatible TE and a TA.
• S A communication reference link between the TE or TA and the NT equipment.
• T A communication reference point between user switching equipment and a Local Loop Terminator.
• U A communication reference point between the NT equipment and the LE. This reference point may be referred to as the network boundary when the FCC definition of the Network terminal is used.



The Common ISDN standards in the ITU Q series are:

Q.921—ISDN Data Link Layer Specification

Q.931—Call-Control and Signaling Specification
The ITU Q.921 data link layer function and format uses the Link Access Procedure on the D channel (LAPD) encapsulated in the high-level data link control (HDLC) protocol.

Q.921 provides reliable transport for Layer 3 signaling messages (Q.931), by establishing a Data (D) channel which provides identification of frames, flow control mechanisms for data transmission and reception. The Layer 2 functionality of ISDN and SS7 is similar to Frame Relay in the Open System Interconnection (OSI) model as they all use a variation of HDLC framing. Also in Frame Relay a DLCI is only locally significant between the Frame Relay interface on a router and the Frame Relay switch. Routers that connected via point or multipoint in the Frame cloud to each other do not know each others DLCI with Network provider Frame Relay switch. Likewise Q.921 communication takes place between two immediate points like a PBX to PSTN switch with the PBX is user side and the PSTN switch is the network side of a point to point connection. This does not represent a call end-to-end as it would involve other switching elements and possibly another PSTN Switch to PBX at the other end. I will go into more detail about the DLCI a little later in this chapter.


There are  two common Q.921 frame formats which for discussion purposes are defined here as Type A and Type B. Both are the same except Type B has an information field that is of variable length from 0 to 260 octets.
The Q.921 is a HDLC Frame with some variations there is an additional 8 bit address field, also FCS or Frame Check Sequence can 16 to 32 bits the control byte can be 8 or 16 bits

On both ends of the Q.921 frame are flag bytes to identify the beginning and the end of the frame. The flag bit sequence for both flag types is 01111110, which is hex (0x7E) if there is more than one frame then the closing flag of one frame can count as the opening flag of the next frame.


2 bytes of address space divided into a high-order octet address and a low-order octet address. The contents of the high-order octet starting from right to left:
1. Extended Address (EA) bit
2. Command/response (C/R) bit
3. Service Access Point Identifier (SAPI).

The second Low order octet contains starting from right to left:
1. EA bit
2. The 7-bit field Terminal Endpoint Identifier (TEI).

End of Address or EA bits are either a 1 or 0 to identify the end of the address octet. Either address octet that that has a 1 serves as the last address octet. In normal Q.921 operation both address octets are used and the bits are set to (read right to left) 0 and 1 respectively.

Command/Response C/R bit is either a “0: or a “1” for a command or a response. The direction of the message and the message type determine the bit value. For example a PBX would be on the user side of the ISDN connection sending a message; it uses a 0 value for a command and a 1 value for a response. The network side sends commands with a bit value of 1 and a response with a bit value of 0.

Command / Response Direction
Command             C/R value

Network to User       1
User to Network       0


Network to User      0
User to Network      1

Service Access Points Identifier Q.921 uses Service Access Points (SAPs) to provide service to Q.931. These access points have an identifier called a SAPI a SAPI determines the function of the data link and it identifies a Layer 3 user of LAP-D, and thus corresponds to a Layer 3 protocol entity within a user device, which contains a value identifying different types of traffic. The different SAPI values are listed in Table 5 and their defined data representations.







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