Layer 1 - Physical Layer
The ISDN Physical Layer is specified by the ITU
G-series documents. The U interface provided by the telco for BRI is a 2-wire, 160 kb/s digital connection. Echo cancellation is used to reduce noise, and data encoding schemes (2B1Q in North America, 4B3T in Europe) permit this relatively high data rate over ordinary single-pair local loops.
2B1Q (2 Binary 1 Quaternary) is the most common signaling method on U interfaces. This protocol is defined in detail in 1988 ANSI spec T1.601. In summary, 2B1Q provides:
- Two bits per baud
- 80 kilobaud (baud = 1 modulation per second)
- Transfer rate of 160 kb/s
This means that the input voltage level can be one of 4 distinct levels (note: 0 volts is not a valid voltage under this scheme). These levels are called Quaternaries. Each quaternary represents 2 data bits, since there are 4 possible ways to represent 2 bits, as in the table above.
Each U interface frame is 240 bits long. At the prescribed data rate of 160 kb/s, each frame is therefore 1.5 ms long. Each frame consists of:
- Frame overhead - 16 kb/s
- D channel - 16 kb/s
- 2 B channels at 64 kb/s - 128 kb/s
|12 * (B1 + B2 + D)
- The Sync field consists of 9 Quaternaries (2 bits each) in the pattern +3 +3 -3 -3 -3 +3 -3 +3 -3.
- (B1 + B2 + D) is 18 bits of data consisting of 8 bits from the first B channel, 8 bits from the second B channel, and 2 bits of D channel data.
- The Maintenance field contains CRC information, block error detection flags, and "embedded operator commands" used for loopback testing without disrupting user data.
Data is transmitted in a superframe consisting of 8 240-bit frames for a total of 1920 bits (240 octets). The sync field of the first frame in the superframe is inverted (i.e. -3 -3 +3 +3 +3 -3 +3 -3 +3).
December 5, 2006
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