ISDN Specifications
ISDN has been standardized in a host of different specifications from groups such as the International Telecommunication Union (ITU), Telcordia (formerly Bellcore), and the American National Standards Institute (ANSI). These groups, along with the ISDN forum, are responsible for modeling the ISDN industry. Standards that are in the I group consist of standards set forth for ISDN, both narrow and broadband. The Q group of standards has to do with switching and signaling. The Q standards also include topics such as SS7 and Intelligent Networks (INs). Several standards are associated with ISDN, but the most common standards are as follows:
• Q.921—ISDN Data Link Layer Specification
• Q.931—Call-Control and Signaling Specification
The physical layer specifications for ISDN are I.430 and I.431. They identify the electrical, mechanical, and functional specifications of the circuits. I.430 specifies the basis for a BRI frame as 48 bits. The BRI frame is cycled at 4000 times per second, which gives a total bandwidth of 192,000 bps (48 * 4000). Although 192 kbps is available on the BRI circuit, the subscriber typically only uses a maximum of 128 kbps. 144 kbps is possible if the D channel is also used for data, but D channel use depends on the provider.
I.431 describes the use of either a T1 or E1 circuit for the electrical basis of a PRI circuit. There are specifications for a Japanese PRI (INS-1500), but it is not included in the ITU specification.
ITU Q.921
The ISDN protocol stack consists of three layers: physical, data link, and network. The data link layer function and format is described in ITU Q.921. Q.921 is also referred to as Link Access Procedure on the D channel (LAPD). This part of the protocol stack is largely based on the high-level data link control (HDLC) protocol.
The purpose of Q.921 is to provide for a reliable transport for Layer 3 signaling messages (Q.931), provide identification of frames, and provide flow control mechanisms for data transmission and reception. If you're thinking that these functions look familiar by now, they should. The Layer 2 functionality that you find in ISDN, SS7, Frame Relay, and other technologies are basically the same thing. This is due to the description of what the function of a Layer 2 service should be in functional models such as the Open System Interconnection (OSI) model.
NOTE Not all technologies implement layer 2 functions in the same way.
Because Q.921 deals with Layer 2 functions, it is important to remember that transmission of frames is involved between ISDN nodes. Q.921 communication takes place only between two immediate points and is not end-to-end. Before you move on, you should understand the Q.921 frame structures and how they are used. Figure 8-6 shows Frame Types A and B for ISDN Q.921.
Figure 8-6 Q.921 Frame Structure
Transmitted First
|
Flag |
FCS |
FCS |
Control |
Control |
Address Low |
Address High |
64 1 Bit Number Frame Type B 8
Bit Number V = Variable Length Field There are two frame formats that are commonly associated with Q.921, and they are denoted as Type A and Type B. The main difference between the two frame types is the fact the Type B has an information field that is of variable length. The Flag fields that are on either end of the Q.921 frame identify the beginning and the end of the frame. The flag sequence for both flag types is 01111110, and the closing flag of one frame can count as the opening flag of the next frame. Otherwise, there are back-to-back flag octets in every frame after the first flag octet in the signal stream. The address octets are set up as a high-order octet and a low-order octet. The high-order octet contains information such as the extended address (EA) bit, command/response (C/R) bit, and the service access point identifier (SAPI). The second octet contains an EA bit and a 7-bit field that is referred to as the terminal endpoint identifier (TEI). Figure 8-7 shows the placement of each of these fields. Figure 8-7 Q.921 Address Octets Transmitted First Figure 8-7 Q.921 Address Octets 
The EA bits identify where the address octets end. Whichever of the two octets are identified by a 1 serves as the last address octet. In normal Q.921 operation you use both address octets, and the bits are set to (read right to left) 0 and 1 respectively. The C/R bit identifies whether a frame is intended to be either a command or a response. The bit value is determined by the direction of the message and the message type. If the user side of the ISDN connection is sending the 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. Service access points (SAPs) are the points that Q.921 uses to provide service to Q.931. The access points are denoted with an identifier called a SAPI. In other words, the SAPI is a value that identifies different types of traffic. Table 8-1 lists the different SAPI values and their defined data representations.
The last field in the address low octet is the TEI. The TEI identifies the terminal equipment on the data link connection. The values range from 0 to 127, and they are split into two different sub-categories: static and dynamic TEI assignment: • TEI values 0 to 63 are for static assignment, and TEI 0 is most commonly associated with ISDN PRI circuits. • TEI values 64 to 126 are for dynamic TEI assignment and are commonly found on ISDN BRI circuits. • TEI 127 is reserved as the broadcast TEI. For instance, when an ISDN device on a BRI circuit comes online and requests a TEI from the network, it uses TEI 127 as the vehicle for the request. The next several octets in the frame are called control octets, and they can be 16 or 24 bits total (depending on whether or not there is an info octet). The control octets, in conjunction with the C/R bit, identify which type of frame is being sent. Three types of frame groups are used, numbered information (I), unnumbered information (U), and supervisory (S). The main difference between I and U frame types is that U frame types do not guarantee delivery, and they do not require acknowledgments. Table 8-2 lists some of the more common frame types and their functions. Table 8-2 Q.921 Control Octet Frame Types
continues continues
Figure 8-8 shows the three main frame categories and their associated bit locations. Figure 8-8 Q.921 Control Octets Transmitted First Frame Type A 88 888888 Frame Type A 88 888888 
M = Modifier Function P/F = Poll Bit if Used as the First Bit, N(R) = Receive Sequence Number and Final Bit if Used as the Last Bit N(S) = Transmit Sequence Number x = Reserved S = Supervisory M = Modifier Function P/F = Poll Bit if Used as the First Bit, N(R) = Receive Sequence Number and Final Bit if Used as the Last Bit N(S) = Transmit Sequence Number x = Reserved S = Supervisory NOTE Something to note about Figure 8-8 is the state of the first two bits. The following applies to the Q.921 control octets: • If the first bit is equal to 0, it is an I frame. • If the first bit is equal to 1 and the second bit is equal to 0, it is an S frame. • If the first bit is equal to 1 and the second bit is equal to 1, it is a U frame. During the data link negotiation process within Q.921, the purpose is to get the link to a Multiple Frame Established (MFE) state. After the link is in MFE, the devices are ready to transmit upper-layer data between them. The negotiation process contains several steps that you need to comprehend to effectively understand and troubleshoot Q.921 operation. Continue reading here: Q921 Timers Was this article helpful? |