Supervision Signaling on Digital Trunks

R2 signaling operates over E1 digital facilities (described in the "Physical Layer—MTP L1" section of Chapter 4). E1 has 32 time-slots, numbered TS0 to TS31, whereby TS1-TS15 and TS17-TS31 are used to carry voice encoded with pulse code modulation (PCM), or to carry 64 kbps data.

Sixteen consecutive frames are in the SF format, and they are numbered 0 to 15. TS16 in frame 0 is used for SF alignment, and TS16 in the remaining frames (1-15) is used for trunk signaling. Four status bits are used from TS16 for signaling. They are called a, b, c, and d.

In the case of CCITT-R2 signaling, only the a and b bits are used. The c and d bits are set to 0 and 1, respectively. An idle state is denoted when a and b are equal to 1 and 0. Signaling is continuous. For two-way trunks, the supervision roles for forward and backward signaling vary on a call-by-call basis. Table 3-9 illustrates the R2 supervision signal, transition, and direction used on digital trunks.

Table 3-9. R2 Supervision Signaling on Digital Trunks

Direction

Signal Type

Transition

Forward

Seizure

a,b: 1,0 to 0,0

Forward

Clear-forward

a,b: 0,0 to 1,0

Backward

Seizure acknowledgment

a,b: 1,0 to 1,1

Backward

Answer

a,b: 1,1 to 0,1

Backward

Clear-back

a,b: 0,1 to 1,1

Backward

Release-guard

a,b: 0,1 to 1,0

0 0

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