Understanding the network.pdf - Back to Home

its for control data), and operates at a rate of 666 frames per second, which
maintains the 64bps B channel rate and the 16bps D channel rate.
The PRI essentially uses the TDM data path model. The PRI is a two-pair full-duplex
transmission path. It uses the same signaling and framing scheme employed by the
standard T1/E1 carrier specification, which is 24 or 30 64-bit time-slots. Each frame
contains 193 bits with a sampling rate of 8,000 frames per second, for a total
operating rate of 1.544Mbps. When H channels are configured, the remaining 64-bit
channels are used for B and D channels. PRI also provides the means for using a
single D channel to transport control data for multiple PRIs.
ISDN Layer 2
To provide reliable communications for ISDN signaling and control messages, the
ITU-T has developed the Link Access Procedure on the D channel (LAPD). LAPD is
defined in ITU-T Q.920 AND 1.921. The LAPD frame format is illustrated in Figure
5.13.
NOTE
Figure 5.13. LAPD frame format.
It is not uncommon to get T1 and PRI circuits confused, as they both support the
same data transmission rate (1.544Kbps). PRIs are used almost exclusively for
providing voice and digital modem services. They are not used for point-to-point
data transport. What makes a PRI distinct from a T1 is the ISDN signal provisioning
used between the LEC and CPE device, which is used to allocate the transport
circuit's bandwidth. On a standard T1, this additional provisioning does not exist.
The circuit is clear, and no additional management information is exchanged
between the CPE and the LEC. The datastream is just sent across the transport, with
the actual bandwidth allocation being left to the CPE devices attached on either end
of the circuit.
The LAPD frame contains the following fields: