Understanding How Digital T1 CAS (Robbed Bit Signaling) Works in IOS Gateways

Channel Associated Signaling (CAS) is also referred to as Robbed Bit Signaling. In this type of signaling, the least significant bit of information in a T1 signal is "robbed" from the channels that carry voice and is used to transmit framing and clocking information. This is sometimes called "in-band" signaling. CAS is a method of signaling each traffic channel rather than having a dedicated signaling channel (like ISDN). In other words, the signaling for a particular traffic circuit is permanently associated with that circuit. The most common forms of CAS signaling are loopstart, groundstart, Equal Access North American (EANA), and E&M. In addition to receiving and placing calls, CAS signaling also processes the receipt of Dialed Number Identification Service (DNIS) and automatic number identification (ANI) information, which is used to support authentication and other functions.

Each T1 channel carries a sequence of frames. These frames consist of 192 bits and an additional bit designated as the framing bit, for a total of 193 bits per frame. Super Frame (SF) groups twelve of these 193 bit frames together and designates the framing bits of the even numbered frames as signaling bits. CAS looks specifically at every sixth frame for the timeslot's or channel's associated signaling information. These bits are commonly referred to as A- and B-bits. Extended super frame (ESF), due to grouping the frames in sets of twenty-four, has four signaling bits per channel or timeslot. These occur in frames 6, 12, 18, and 24 and are called the A-, B-, C-, and D-bits respectively.

The biggest disadvantage of CAS signaling is its use of user bandwidth in order to perform signaling functions.

Super Frame is an older framing standard for T1s. Also called D4 or D3/D4 framing. In the 1970s it replaced the original T1/D1 framing scheme of the 1960s in which the framing bit simply alternated.

In order to determine where each channel is located in the stream of data being received, each set of 24 channels is aligned in a frame. The frame is 192 bits long (8 * 24), and is terminated with a 193rd bit, the framing bit, which is used to find the end of the frame.

In order for the framing bit to be located by receiving equipment, a pattern is sent on this bit. Equipment will search for a bit which has the correct pattern, and will align its framing based on that bit. The pattern sent is 12 bits long, so every group of 12 frames is called a Super Frame. The pattern used in the 193rd bit is 1000 1101 1100.

Superframe remained in service in many places through the turn of the century, replaced by the improved Extended Super Frame of the 1980s in applications where its additional features were desired.

In telecommunication, an Extended Super Frame (ESF) is a T1 framing standard, sometimes called D5 framing, invented in the 1980s. It is preferred to its predecessor, Super Frame, because it includes a cyclic redundancy check (CRC) and bandwidth for a data link channel (used to pass out-of-band data between equipment.) It requires less frequent synchronization than the earlier superframe or D-4 format, and provides on-line, real-time testing of circuit capability and operating condition.

In ESF, a superframe is 24 frames long, and the 193rd bit of each frame is used in the following manner:

  • Frames 4, 8, 12, 16, 20, and 24 are used to send the framing pattern, 001011
  • Frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21 and 23 are used for the data link (totalling half of all framing bits, or 4000 bits per second)
  • Frames 2, 6, 10, 14, 18, and 22 are used to pass the CRC total for each super frame.

Note: Less-frequent synchronization frees overhead bits for use in testing and monitoring.

CAS Signaling Types

Loopstart Signaling
Loopstart signaling is one of the simplest forms of CAS signaling. When a handset is picked up (the telephone goes off-hook), this action closes the circuit that draws current from the telephone company CO and indicates a change in status, which signals the CO to provide dial tone. An incoming call is signaled from the CO to the handset by sending a signal in a standard on/off pattern, which causes the telephone to ring.

A disadvantage of loopstart signaling is the inability to be notified upon a far-end disconnect or answer. For instance, a call is placed from a Cisco router configured for Foreign Exchange Station (FXS)-loopstart. When the remote end answers the call, there is no supervisory information sent to the Cisco router to relay this information. This is also true when the remote end disconnects the call.

Note: It is possible for answer supervision to be provided with loopstart connections if the network equipment can handle line-side answer supervision. Also, loopstart provides no incoming call channel seizure. Therefore a condition known as glare can arise, where both parties (Foreign Exchange Office [FXO] and FXS ) try to simultaneously place calls. Glare can be avoided when you configure the T1-CAS gateway's port selection order in such a way that the inbound and outbound calls are in reverse order. For example, if the inbound calls are sent by the provider on the FXO ports in the order of port 1, port 2, port 3 and port 4, then configure the Cisco CallManager Route Group to route outbound calls on those same ports in the order port 4, port 3, port 2 and port 1.

With loopstart signaling, the FXS side only uses the A-bit and the FXO side only uses the B-bit to communicate call information. The AB-bits are bi-directional. This state table defines this signaling information from the CPE's perspective (FXS).

This is the FXS-loopstart timing diagram.

On an incoming call (network -> CPE) this happens:

  1. The network toggles the B-bit to indicate ringing. This is a standard ringing pattern. For instance, 2 seconds on, 4 seconds off.
  2. CPE detects the ringing and off-hook states. A-bit goes from 0 to 1.

In an outgoing call (CPE -> network) this happens:

  1. CPE goes off-hook and A-bit goes from 0 to 1.
  2. The network provides dial tone. There is no signaling change.
  3. CPE sends digits (dual tone multifrequency (DTMF) in Cisco's case).

During a disconnect from the network, this occurs:

  1. CPE detects in-band that the call has dropped (someone says good-bye or a modem drops the carrier).
  2. CPE goes on-hook and A-bit goes from 1 to 0.

During a disconnect from the CPE, only step 2 occurs.

The Answer Supervision and Disconnect Supervision States are only seen when provided by the network.

Groundstart Signaling
Groundstart signaling is very similar to loopstart signaling in many regards. It works by using ground and current detectors that allow the network to indicate off-hook or seizure of an incoming call independent of the ringing signal and allow for positive recognition of connects and disconnects. For this reason, ground start signaling is typically used on trunk lines between PBXs and in businesses where call volume on loop start lines can result in glare.

The advantage of groundstart signaling over loopstart signaling is that it provides far-end disconnect supervision. Another advantage of groundstart signaling is the ability for incoming calls (network -> CPE) to seize the outgoing channel, thereby preventing a glare situation from occurring. This is done by using the A- and B- bit on the network side instead of just the B-bit. The A-bit is also used on the CPE side. However, the B-bit can also be involved, based on the switch's implementation. Typically the B-bit is ignored by the Telco. This is a state table that defines this signaling information from the CPE's perspective (FXS).

This is the FXS-groundstart timing diagram.

On an incoming call (network-> CPE) this happens:

  1. The network goes off-hook and the A-bit goes from 1 to 0 and rings the line by toggling the B-bit between 0 and 1.
  2. CPE detects the ringing and seizure and goes off-hook and the A-bit is set to 1.
  3. The network goes off-hook and the B-bit stops toggling. B-bit is now 1.

In an outgoing call (CPE -> network) this happens:

  1. CPE goes ground on ring and A-bit and B-bit are 0.
  2. The network goes off-hook and the A-bit goes from 1 to 0. The B-bit is set to 1.
  3. The CPE goes off-hook. The A-bit and the B-bit are 1.
  4. CPE detects a dialtone and sends digits.

During a disconnect from the network, this occurs:

  1. The network goes on-hook and the A-bit goes from 0 to 1.
  2. CPE goes on-hook and the A-bit goes from 1 to 0.

During a disconnect from the CPE, the above steps are reversed.

EandM Signaling
E&M Signaling is typically used for trunk lines. The signaling paths are known as the E-lead and the M-lead. Descriptions such as Ear and Mouth were adopted to help field personnel determine the direction of a signal in a wire. E&M connections from routers to telephone switches or to PBXs are preferable to FXS/FXO connections because E&M provides better answer and disconnect supervision.

E&M signaling has many advantages over the previous CAS signaling methods discussed in this document. It provides both disconnect and answer supervision as well as glare avoidance. E&M signaling is simple to understand and is the preferred choice when you use CAS.

This is the E&M Signaling diagram.

The three types of E&M Signaling that are supported on Cisco routers are:

  • Wink-start (FGB) - Used to notify the remote side that it can send the DNIS information.
  • Wink-start with wink-acknowledge or double-wink (FGD) - A second wink that is sent to acknowledge the receipt of the DNIS information.
  • Immediate start - Does not send any winks at all.

Note: FGD is the only variant of T1 CAS that supports ANI and Cisco supports it along with the FGD-EANA variant. In addition to FGD functionality, FGD-EANA provides certain call services, such as emergency (USA-911) calls. With FGD, the gateway supports the collection of ANI inbound only. With the use of FGD-EANA, a Cisco 5300 is able to send ANI information outbound as well as collecting it inbound. This latter capability requires the user of the fgd-eana signaling type in the ds0-group command, with ani-dnis option and calling-number outbound command in the POTS dial-peer. The calling-number outbound command is supported only on the Cisco 5300 as of Cisco IOS Software Release 12.1(3)T.

Therefore, on an incoming call (network-> CPE) this process happens:

  1. The network goes off-hook. The A-bit and B-bit equal 1.
  2. CPE sends wink. The A-bit and B-bit equal 1 for 200 ms. This only occurs when you use wink-start or wink-start with wink acknowledgement. Ignore this step for immediate start.
  3. The network sends DNIS information. This is done by sending inband tones which are decoded by the modem.
  4. CPE sends a wink acknowledgement. A-bit and B-bit equal 1 for 200 ms. This only occurs for wink-start with wink acknowledgement. Ignore this step for immediate start or wink-start.
  5. CPE goes off-hook when a call is answered. A-bit and B-bit equal 1.

In an outgoing call (CPE -> network) the same procedure occurs. However, the network just described is the CPE and vice-versa. This is because the signaling is symmetric.

During a disconnect from the network, this process occurs:

  1. The network goes on-hook. A-bit and B-bit equal 0.
  2. CPE goes on-hook. A-bit and B-bit equal 0.

During a disconnect from the CPE, these two steps are reversed.

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