5.2 ISDN Protocol Layers
5.2.1 ISDN layer 1
Similar to other communication standards, ISDN uses many protocols. This section describes the ISDN protocols.
Layer 1, the physical layer, is responsible for the circuit switching of the connection. It supports the attachments of a terminal adapter/Network Termination 1 (TA/NT1), or even the connection of multiple devices. Both the B and the D channels share this physical layer. The standards involved at Layer 1 are as follows:
  • I.430 for BRI, which defines the communication across the S/T reference point
  • I.431 for PRI, which is a full-duplex, point-to-point, serial, synchronous connection
  • ANSI T1.601 for BRI, which defines the communication across the U interface for North America

As shown in Figure , Layers 2 and 3 operate for the D channel. However, the B channel operates in either a High-Level Data Link Control (HDLC) or Point-to-Point Protocol (PPP) encapsulation mode to encapsulate the upper-layer protocols, instead of using Layers 2 and 3 directly.

Link Access Procedure on the D channel (LAPD) is the framing protocol used for D-channel data. The Digital Subscriber Signaling System No. 1 (DSS1) is the Layer 3 protocol for the D channel. Specifically, only Q.931 is
used here, not the entire DSS1 protocol suite.

The D channel is governed by dial-on-demand routing (DDR), which is the mechanism for building connections over either an analog or ISDN connection. The B channel is governed by IP or Internetwork Packet Exchange (IPX) protocols for data transmission.

Figure shows what the I.430 BRI frames between the TE and the NT look like.

An interesting note is that the format of the frames is different, depending on which direction they are going. Also, note that time-division multiplexing is occurring between the D channel and the two B channels. The frame can have 16 bits from each of the B channels and four bits from the D channel. The framing always occurs, even if no data is being transmitted across the B channels. The NT1 constantly provides clocking and framing out to the local exchange.

B channels have the following characteristics:

  • The ISDN BRI bit rate is 192 Kbps.
  • The ISDN user bandwidth is 144 Kbps, 2 x B channels plus 1 D channel (2 x 64) + 16).
  • The ISDN signaling requirement is 48 Kbps.

The TE and the ISDN switch are constantly communicating. That is, there is a constant stream of ones and zeros exchanged between the router and the ISDN switch located at the central office. The stream of data exchanged must follow a specific sequence. Such a sequence, in which each bit has a specific function, is referred to as framing. The I.430 standard defines the BRI framing bit field key as follows:

  • B1 bit—Bit within the B channel 1
  • B2 bit—Bit within the B channel 2
  • D bit—D-channel bit
  • F bit—Framing bit used for synchronization
  • L bit—DC balancing bit adjust average bit value
  • E bit—Echo of previous D bit
  • A bit—Activation bit
  • S bit—Spare bit