Procedure on the D Channel (LAPD) protocol. LAPD is a data link layer protocol based on HDLC. In North America and Japan, PRI offers twenty-three 64 kbps B channels and one 64 kbps D channel. A PRI offers the same service as a T1 or DS1 connection. In Europe and much of the rest of the world, PRI offers 30 B channels and one D channel in order to offer the same level of service as an E1 circuit. PRI uses a Data Service Unit/Channel Service Unit (DSU/CSU) for T1/E1 connections. Web Links Integrated Services Digital Network (ISDN) http://www.cisco.com/univercd/cc/ td/doc/ cisintwk/ito_doc/ isdn.htms

Content 4.1 ISDN Concepts 4.1.3 ISDN 3-layer model and protocols ISDN utilizes a suite of ITU-T standards spanning the physical, data link, and network layers of the OSI reference model:

The ISDN BRI and PRI physical layer specifications are defined in ITU-T I.430 and I.431, respectively.
The ISDN data link specification is based on LAPD and is formally specified in the following:
- ITU-T Q.920
- ITU-T Q.921
- ITU-T Q.922
- ITU-T Q.923

The ISDN network layer is defined in ITU-T Q.930, also known as I.450 and ITU-T Q.931, also known as I.451. These standards specify user-to-user, circuit-switched, and packet-switched connections. BRI service is provided over a local copper loop that traditionally carries analog phone service. While there is only one physical path for a BRI, there are three separate information paths, 2B+D. Information from the three channels is multiplexed into the one physical path. ISDN physical layer, or Layer 1, frame formats differ depending on whether the frame is outbound or inbound. If the frame is outbound, it is sent from the terminal to the network. Outbound frames use the TE frame format. If the frame is inbound, it is sent from the network to the terminal. Inbound frames use the NT frame format. Each frame contains two sample frames each containing the following:

8 bits from the B1 channel
8 bits from the B2 channel
4 bits from the D channel
6 bits of overhead

ISDN BRI frames contain 48 bits. Four thousand of these frames are transmitted every second. Each B channel, B1and B2, have a capacity of 8*4000 = 64 kbps, while channel D has a capacity of 4*4000 = 16 kbps. This accounts for 144 kbps of the total ISDN BRI physical interface bit rate of 192 kbps. The remainder of the data rate are the overhead bits that are required for transmission. The overhead bits of an ISDN physical layer frame are used as follows:

Framing bit – Provides synchronization
Load balancing bit – Adjusts the average bit value
Echo of previous D channel bits – Used for contention resolution when several terminals on a passive bus contend for a channel
Activation bit – Activates devices
Spare bit – Unassigned

Note that the physical bit rate for the BRI interface is 48*4000 = 192 kbps. The effective rate is 144 kbps = 64 kbps + 64 kbps + 16 kbps (2B+D). Layer 2 of the ISDN signaling channel is LAPD. LAPD is similar to HDLC. LAPD is used across the D channel to ensure that control and signaling information is received and flows properly. The LAPD flag and control fields are identical to those of HDLC. The LAPD address field is 2 bytes long. The first address field byte contains the service access point identifier (SAPI), which identifies the portal at which LAPD services are provided to Layer 3. The command/response bit (C/R), indicates whether the frame contains a command or a response. The second byte contains the terminal endpoint identifier (TEI). Each piece of terminal equipment on the customer premises needs a unique identifier. The TEI may be statically assigned at installation, or the switch may dynamically assign it when the equipment is started up. If the TEI is statically assigned during installation, the TEI is a number ranging from 0 to 63. Dynamically assigned TEIs range from 64 to 126. A TEI of 127, or all 1s, indicates a broadcast. Web Links Integrated Services Digital Netowrk (ISDN) http://www.cisco.com/univercd/cc/ td/doc/ cisintwk/ito_doc/ isdn.htm

Content 4.1 ISDN Concepts 4.1.4 ISDN functions Several exchanges must occur for one router to connect to another using ISDN. To establish an ISDN call, the D channel is used between the router and the ISDN switch. Signal System 7 (SS7) signaling is used between the switches within the service provider network. The D channel between the router and the ISDN switch is always up. Q.921 describes the ISDN data-link processes of LAPD, which functions like Layer 2 processes in the OSI reference model. The D channel is used for call control functions such as call setup, signaling, and termination. These functions are implemented in the Q.931 protocol. Q.931 specifies OSI reference model Layer 3 functions. The Q.931 standard recommends a network layer connection between the terminal endpoint and the local ISDN switch, but it does not impose an end-to-end recommendation. Because some ISDN switches were developed before Q.931 was standardized, the various ISDN providers and switch types can and do use various implementations of Q.931. Because switch types are not standard, routers must have commands in their configuration specifying the ISDN switch to which they are connecting. The following sequence of events occurs during the establishment of a BRI or PRI call:

The D channel is used to send the called number to the local ISDN switch.
The local switch uses the SS7 signaling protocol to set up a path and pass the called number to the remote ISDN switch.
The remote ISDN switch signals the destination over the D channel.
The destination ISDN NT-1 device sends the remote ISDN switch a call-connect message.
The remote ISDN switch uses SS7 to send a call-connect message to the local switch.
The local ISDN switch connects one B channel end-to-end, leaving the other B channel available for a new conversation or data transfer. Both B channels can be used simultaneously.

Interactive Media Activity Drag and Drop: ISDN Functions When the student has completed this activity, the student will be able to correctly identify the ISDN establishment cycle. Web Links Integrated Services Digital Network (ISDN) http://www.cisco.com/univercd/cc/ td/doc/cisintwk/ito_doc/isdn.htm

Content 4.1 ISDN Concepts 4.1.5 ISDN reference points ISDN standards define functional groups as devices or pieces of hardware that enable the user to access the services of the BRI or PRI. Vendors can create hardware that supports one or more functions. ISDN specifications define four reference points that connect one ISDN device to another. Each device in an ISDN network performs a specific task to facilitate end-to-end connectivity. To connect devices that perform specific functions, the interface between the two devices needs to be well defined. These interfaces are called reference points. The reference points that affect the customer side of the ISDN connection are as follows:

R – References the connection between a non-ISDN compatible device Terminal Equipment type 2 (TE2) and a Terminal Adapter (TA), for example an RS-232 serial interface.
S – References the points that connect into the customer switching device Network Termination type 2 (NT2) and enables calls between the various types of customer premises equipment.
T – Electrically identical to the S interface, it references the outbound connection from the NT2 to the ISDN network or Network Termination type 1 (NT1).
U – References the connection between the NT1 and the ISDN network owned by the telephone company.

Because the S and T references are electrically similar, some interfaces are labeled S/T interfaces. Although they perform different functions, the port is electrically the same and can