|  | 5.2 | How ISDN Relates to the OSI Reference Model
| 5.2.2 | The ISDN physical layer
ISDN physical-layer (Layer 1) frame formats differ depending on whether the frame is outbound (from terminal to network-the TE frame format) or inbound (from network to terminal-the NT frame format). Both of the frames are 48 bits long, of which 36 bits represent data. Actually, the frames are two 24 bit frames in succession consisting of 2 8-bit B channels, a 2-bit D channel, and 6 bits of framing information (2*(2*8B+2D+6F) = 32B+4D+12F = 36BD+12F = 48BDF). Both physical-layer frame formats are shown in the figure. The 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. B1 channel bits. B2 channel bits. 8 Added channel bit counts bits. D channel bits - Used for user data. Note that each of the ISDN BRI frames are sent at a rate of 8000 per second. There are 24 bits in each frame (2*8B+2D+6F = 24) for a bit rate of 8000*24 = 192Kbps. The effective rate is 8000*(2*8B+2D) = 8000*18 = 144Kbps. Multiple ISDN user devices can be physically attached to one circuit. In this configuration, collisions can result if two terminals transmit simultaneously. ISDN therefore provides features to determine link contention. These features are part of the ISDN D channel, which is described in more detail later in this chapter.
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