A
Basic Rate Interface (BRI)
uses a single pair of copper wires to provide up to 192Kbps of
bandwidth for both voice and data calls A BRI uses two 64Kbps B channels and one 16Kbps
D channel. An additional 48Kbps are used for framing and synchronization.
To review the math, each B channel is 64Kbps, so that totals 128Kbps. Add the 16Kbps
D channel, and the usable bandwidth for ISDN BRI is now at 144Kbps. Finally, add the 48Kbps
for framing and synchronization to get a total circuit speed of 192Kbps. Figure 26.1 shows the
ISDN protocol layers.
Both the B and D channels share layer 1. Layers 2 and 3 operate over the D channel, but the B
channel operates in either an HDLC or PPP encapsulation mode. This architecture is used to encapsulate
the upper layer protocols instead of using layer 2 and layer 3 directly. LAPD is the framing
protocol used for the D channel data. DSS1 (digital subscriber signaling system number 1) is the
layer 3 protocol for the D channel where Q.931 is used. B channels are used by the IP or IPX protocols
for data transfer, and the D channel is used by dial-on-demand routing (DDR), which builds
the connection over ISDN.