Signaling Systems

Configuring Cisco Systems voice equipment to interface with other equipment requires an understanding of the signaling that conveys supervision between the systems. Proper troubleshooting also requires an understanding of these signaling systems.

This section describes the various signaling systems used between telephony systems, such as common channel signaling and channel associated signaling. It also explores signaling between PBXs, signaling between PBXs and COs, and specialized signaling, such as ISDN.


Channel Associated Signaling

Channel associated signaling (CAS) is a signaling method commonly used between PBXs. Although this can manifest itself in many forms, some methods are more common than others. Signaling systems can also be implemented between a PBX and a Cisco voice device.


T1 Channel Associated Signaling

PBXs and Cisco devices use T1 and E1 interfaces to convey voice. Originally, this was the main purpose of T1, which carries signaling information using two methodologies: CAS and common channel signaling (CCS). Figure 2-31 illustrates the format of the T1 digital signal.


The characteristics of the T1 digital signal format are as follows:

• A T1 frame is 193 bits long, 8 bits from each of the 24 time slots (digital service zeros [DS0s]), plus 1 bit for framing. A T1 repeats every 125 microseconds, resulting in 8000 samples per second (8 bits * 24 time slots + 1 framing bit * 8000 samples per second = 1.544 Mbps).
  

• T1 has two major framing and format standards:

- Super Frame (SF), or D4, specifies 12 frames in sequence. The D4 framing pattern used in the F position in Figure 2-31 is 100011011100 (a 1 goes with the first frame, a 0 goes with the second frame, a 0 goes with the third frame, and so on, all the way through 12 frames). This unique framing pattern allows the receiving T1 equipment to synchronize within four frames, since any four consecutive frame bits are unique within the 12-bit pattern. Because there are 8000 T1 frames transmitted per second, 8000 F bits are produced and used for framing.

- Extended Superframe (ESF) format was developed as an upgrade to SF and is now dominant in public and private networks. Both types of formatting retain the basic frame structure of one framing bit followed by 192 data bits. However, ESF repurposes the use of the F bit. In ESF, of the total 8000 F bits used in T1, 2000 are used for framing, 2000 are used for cyclic redundancy check (CRC) (for error checking only), and 4000 are used as an intelligent supervisory channel to control functions end to end (such as loopback and error reporting).

Because each DS0 channel carries 64 kbps, and G.711 is 64 kbps, there is no room to carry signaling. Implemented for voice, the T1 uses every sixth frame to convey signaling information. In every sixth frame, the least significant bit (LSB) for each of the voice channels is used to convey the signaling, as shown in Figure 2-32. Although this implementation detracts from the overall voice quality (because only 7 bits represent a sample for that frame), the impact is not significant. This method is called robbed-bit signaling (RBS). When SF employs this method, the signaling bits are conveyed in both the 6th (called the "A" bit) and 12th (called the "B" bit) frames. For control signaling, A and B bits provide both near- and far-end off-hook indication.


The A and B bits can represent different signaling states or control features (on hook or off hook, idle, busy, ringing, and addressing). The robbed bit is the least significant bit from an 8-bit word.

ESF also uses RBS in frames 6, 12, 18, and 24 to yield four signaling bits, providing additional control and signaling information. These four bits are known as the A, B, C, and D bits.

Because the signaling occurs within each DS0, it is referred to as in band. Also, because the use of these bits is exclusively reserved for signaling each respective voice channel, it is referred to as CAS.

The robbed bits, depicted in Figure 2-33, are used to convey E&M status or FXS/FXO status and provide call supervision for both on hook and off hook.


T1 CAS has the following characteristics:

• SF has a 12-frame structure and provides AB bits for signaling.

• ESF has a 24-frame structure and provides ABCD bits for signaling.

• DTMF, or tone, can be carried in band in the audio path. However, other supervisory signals must still be carried via CAS.

E1 Channel Associated Signaling

In E1 framing and signaling, 30 of the 32 available channels, or time slots, are used for voice and data. Framing information uses time slot 1, while time slot 17 (E0 16) is used for signaling by all the other time slots. This signaling format, illustrated in Figure 2-34, is also known as CAS because the use of the bits in the 17th time slot is exclusively reserved for the purpose of signaling each respective channel. However, this implementation of CAS is considered out of band, because the signaling bits are not carried within the context of each respective voice channel, as is the case with T1. E1 CAS is directly compatible with T1 CAS, because both methods use AB or ABCD bit signaling. Although the signaling for E1 CAS is carried in a single common time slot, it is still referred to as CAS because each individual signaling time slot represents a specific pair of voice channels.


In the E1 frame format, 32 time slots make up a frame. A multiframe consists of 16 E1 frames, as depicted in Figure 2-35.