Table of Contents

Voice Circuit Types
Voice Circuit Signal Flow

Voice Signal Flow with CDP Card
Voice Channel Signaling
Signaling Bit Flow with CDP

Voice Processing Features

ADPCM Voice Compression
Speech Detection
Modem Detection
Echo Cancelling
Voice and Signaling Conditioning
Level Adjustment

Voice Connections

This chapter is provided for users who wish to have an in-depth knowledge of the Cisco IPX narrowband/Cisco IGX 8400 series multiband voice connections and related functions. This chapter describes voice and signaling flow as well as digital processing topics such as voice compression, speech detection, modem detection, and echo cancelling.

The chapter contains the following:

• Voice Circuit Types

• Voice Circuit Signal Flow

• Voice Processing Features

Voice Circuit Types

An Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch voice circuit is transmitted using one or more DS0 time slots in the T1 bit stream. The circuit is defined within the node by one of nine voice frequency (VF) connection types depending on the compression characteristics applied as indicated in Table 15-1. These characteristics are with respect to the CDP card (Cisco IPX narrowband switch) and the CVM card (Cisco IGX 8400 series multiband switch) and are set in software for each connection. See the "Voice Processing Features" section for more information.

Voice Circuit Signal Flow

Figure 15-1 illustrates the general voice circuit signal flow through a Cisco IPX narrowband switch or a Cisco IGX 8400 series multiband switch. Encoded voice circuits, originating with PABX's and D4 channel banks, are applied via a T1 or E1 circuit line to an interface card on the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch that converts the input data to a standardized DS1 or E1 format. The encoded VF data is then packetized by a CDP (Cisco IPX narrowband switch) or CVM (Cisco IGX 8400 series multiband switch) card.

Voice Signal Flow with CDP Card

Note   This description is with respect to the CDP card in the Cisco IPX narrowband switch, but the CVM card in the Cisco IGX 8400 series multiband switch functions similarly.

Voice traffic originates from a PABX or channel bank and is transmitted over a circuit line to the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch where it is terminated on an E1 or T1 back card, depending on the type of transmission facility used for the circuit line (Figure 15-2). The back card is primarily an interface card that monitors the received signal for alarm conditions or codes, converts the bipolar signal to unipolar logic level data, and extracts clock from the input signal. The back card makes this clock available to the node for frame synchronization.

The back card receives this input data and stores it in a frame buffer to reduce the effects of jitter on the circuit line. The BC-E1 can be programmed to operate with either of the two E1 line formats, AMI or HDB3. The BC-T1 can be programmed for either of the three T1 line formats, AMI, B8ZS, or ZCS.

For all voice circuit types except the "t" type, the data stream from the user will be assumed to be carrying voice circuit signaling information in specific bit positions (interpretive mode). With the transparent mode for "t" type connections, all bits are assumed to be data bits and no signaling processing is performed on these circuits. For E1 circuits, the signaling bits may be associated with each frame, Channel Associated Signaling (CAS), or in a separate frame by itself, Common Channel Signaling (CCS) and is specified in the system software.

For circuits defined as one of the interpretive modes, the signaling bits are stripped off by the back card and forwarded separately to the CDP for further signaling bit processing (see next section). The voice data bits are then passed to the CDP card at 125 µs. intervals via the Local Bus.

The CDP (Figure 15-2) performs specified signal processing functions on standard PCM voice data on a per-channel basis. The circuit data, temporarily stored in the CDP's buffers, is analyzed for silent intervals for VAD as well as for modem detection. Gain or loss may be added to the circuit data, any A-law to µ-law conversion may be provided, and voice compression in the form of VAD and/or ADPCM may be performed. If a modem tone is detected on a channel, any compression or gain control is disabled for the duration of the modem activity.

Each CDP may be equipped with optional Integral Echo Cancellers (IECs). The IECs come in two models, a 24-channel IEC for use with T1 circuit lines and a 32-channel for E1 circuit lines. This provides quality voice connections while eliminating the need for any external echo cancellers.

Voice Channel Signaling

Note   This description is with respect to the CDP card in the Cisco IPX narrowband switch, but the CVM card in the Cisco IGX 8400 series multiband switch functions similarly.

Signaling on digitally encoded voice channels consists of one or two bits that accompany each voice channel to represent the status of the channel (on-hook, off-hook, dialing, etc.). Changes in the status of a particular voice channel are represented by changes in the signaling bit state for the particular channel. Timing of signaling bit transitions with respect to voice information on the same connection is not precisely maintained through the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch network as the transmission paths are not the same.

On a T1 circuit line unless "no signaling" is selected with the cnfvchtyp command, signaling is carried using up to four bits for each channel, the A, B, C, and D bits. These bits are inserted in the least significant bit position of the PCM sample in frames 6 and 12 of every superframe by the PABX or D4 Channel Bank. On an E1 circuit line, signaling is performed using these same four bits for each channel. The A, B, C, and D bits for a particular channel are carried in timeslot 16 in every sixteenth frame (each frame carries the four signaling bits for two channels in channel 16).

Regardless of the type of circuit line (T1 or E1), the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch recognizes two types of dialing formats: inband (for DTMF) and dial pulsing. These are software configured channel-by-channel. The Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch supports the transmission of both of these signaling types with appropriate time delays.

Signaling Bit Flow with CDP

Note   This description is with respect to the CDP card in the Cisco IPX narrowband switch, but the CVM card in the Cisco IGX 8400 series multiband switch functions similarly.

Signaling bits are extracted by the BC-T1 or BC-E1 back card. The CDP demultiplexes the T1 or E1 signal and separates the voice data bits from the signaling bits. Both voice bits and signaling bits are temporarily stored in RAM buffers in the back card. An Interdigit Signaling timer debounces the signaling bit transitions to eliminate noise. Upon demand, the back card sends the signaling bits to the CDP over a local bus. A separate local bus is used for the voice bits for maximum data transfer. Signaling bits, occur at a slower rate and are transferred using a second local bus.

The CDP detects signaling bit transitions and integrates the changes of state to minimize false signaling. The CDP data processing modules perform any protocol conversions that may be specified for a channel and packetizes the signaling transition. Each change of state of a signaling bit is packetized in a timestamp packet. In addition to the packet timestamp, the CDP adds a delay field in the packet payload that is used to ensure the signaling packets are received in the proper sequence and with acceptable end-to-end delay to support a maximum of 11 pulses/second dial pulsing.

The signaling packets are sent over the MUXBUS where they are picked off by the NTC and forwarded to their destination. If there are no signaling bit transitions detected, heartbeat packets are sent every 2 to 30 seconds to maintain the far end signaling state.

At the far end, the signaling packets are received by the NTC, which forwards them directly to the appropriate CDP over the MUXBUS (Figure 15-3). The signaling state is detected and the signaling data is forwarded to the associated back card where it is inserted in the circuit's receive data frame and sent to the user device.

Voice Processing Features

Digital signal processing provides many of the features available for voice connections through the Cisco IPX narrowband switch. The following paragraphs describe in detail these features.

ADPCM Voice Compression

The Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch uses a standard algorithm (ITU-T G.721, G.723, and G.726) to implement Adaptive Differential Pulse Code Modulation and encodes only the difference between one encoded voice sample and the following sample, not the actual value of the sample amplitude. Since the range of values for the difference is much smaller than the actual value of the sample, it requires fewer bits for encoding this difference.

With ADPCM, a formula is applied to the previous outgoing sample that predicts the range of values for the next sample. The transmitter then encodes the difference in level between the previous sample and the present sample, on a scale set by the prediction, and transmits this information to the far end receiver. Since the receiver has the same data to work with as the transmitter, it is able to reconstruct the PCM data input to the transmitter.

Conversion of 8-bit PCM samples to ADPCM, as well as packetizing the ADPCM, is performed together in the CDP (Cisco IPX narrowband switch) or CVM (Cisco IGX 8400 series multiband switch) card. Compressed voice code words from the CDP/CVM can be selected by the user to be either 4-bit (using 32 Kbps of bandwidth), 3-bit (24 Kbps) or 2-bit (16 Kbps) rather than 8-bit PCM samples. The resulting bandwidth savings with ADPCM generally compromises voice quality.

ADPCM compression relies on the signal being close to its model of speech to make such bandwidth reduction. High-speed modem and FAX signals are distorted by ADPCM to the extent that operation above 4800 baud generally will not work reliably on ADPCM connections. When PCM channels are used for either of these two functions, all compression should be disabled.

Speech Detection

If the Voice Activity Detector (VAD) option is active, a speech detector is used to identify periods of silence or idle VF channels to provide further compression in addition to ADPCM. This function is performed by the CDP card.

Since the speech detection test takes some time to make, speech was actually present some time before the speech detector was switched on. To accommodate this, a buffer is used and the first few packets of speech are sent out using high priority packets to minimize additional delay factors. This practice prevents front-end clipping of talkspurts but this increases overall connection delay.

The Adaptive Voice feature automatically disables VAD during periods when there is unused network bandwidth available. This allows transmission of the compressed voice without the effects of VAD. As the extra bandwidth is allocated to other connections, VAD is dynamically enabled on all voice circuits to free up the needed bandwidth.

Modem Detection

The V.25 modem detector recognizes the steady 2100 Hz tone output by V.25 compliant modems
and FAX machines to disable echo cancellers at the beginning of transmission.

When a channel is declared V.25 modem, a message is sent to the controller card, and software coordinates upgrade the connection at both ends from ADPCM to PCM (unless already PCM, or otherwise configured in the "cnfvchparm V.25 Detect" field). Therefore, the connection effectively becomes "p" until the modem removal criteria (bidirectional power below "MDM Low Pwr Thrsh" for a duration greater than "MDM Detect Silence Max") are met, at both ends, when it returns to the configured state. Software attempts to synchronize changes at both ends to within 100 msec, but there is a short silence on the channel while reconfiguration takes place.

Echo Cancelling

It is practically impossible to obtain good echo performance from an Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch network without using echo cancellers. The echo cancellers are provided internally as an option with the CDP/CVM card. The performance of echo cancellers depends on a number of factors that can be optimized in the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch by the advanced user for particular conditions.

Echo is a function of how much energy is reflected back to the talker or listener, and the time delay between the original signal and the reflection. The graph in Figure 15-4 illustrates the generally accepted boundary between acceptable and unacceptable echo.

The reflections of speech are produced by impedance mismatches in 2-wire circuits that are found to some extent in all 2-wire networks. The reflected power is generally in the order of 10 to 20 dB less than the forward power level as indicated in the figure. User perception of echo is aggravated by circuit delay. In circuit-switched environments, with a typical delay less than 5 msec, the perceived echo is below the threshold of annoyance.

However, when Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch equipment is introduced into the network, the delay increases by 30-100 msec. The increased delay is primarily due to voice activity detection, packetizing, and queuing processes. Figure 15-4 shows that for this delay, the reflected signal must be 20 to 30 dB less than the original signal. If nothing were done, the echo would be intolerable.

Echo cancellers have a tone detection feature for FAX/fast modem detection. If a 2100 Hz tone is detected, echo cancelling is disabled until the signal power in the connection has fallen, indicating the end of the call. This feature should normally be enabled.

Voice and Signaling Conditioning

With almost all digital multiplexers and PABXs, the signaling bits are forced to a predetermined state when the transmission link has failed. This is used to drop all calls in progress and block any new access to the voice circuits. Usually, the predetermined state is a "busy" but other conditioning sequences may be used (for example idle for a short interval to drop all calls in progress followed by a permanent busy until the fault clears).

With a point-to-point transmission link, where all 24 channels end up in the same destination, the channel bank or PABX conditioning detects the fault on the digital trunk line and provides the conditioning. However, with transmission through the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch, each DS0 circuit may have a different destination from all others from the same source. Therefore, it is up to the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch to condition the signaling when it detects a network trunk failure (such as packet out of frame).

The signaling conditioning for each individual voice circuit is specified by a conditioning template that the user selects using the Configure Voice Channel Interface Type (cnfvchtyp) command. The command specifies the channel on-hook (idle) state and the signaling state forced by the CDP (Cisco IPX narrowband switch) or CVM (Cisco IGX 8400 series multiband switch) when a connection fails. After detecting a connection failure, the channel voice and signaling conditioning is instantaneously applied.

Level Adjustment

Voice connections in a network must concern themselves with providing sufficient talk volume at the receive end with a low level of circuit noise and distortion for acceptable voice conversations to be held. In worldwide telephone networks this is accomplished with the use of loss plans. Loss plans assign acceptable circuit loss for each element of the network that a telephone call may traverse.

In general, subscriber loops (from the telephone to the central office) are allowed to have anywhere from 0 dB to 6 or 8 dB of voice frequency (VF) loss. This loss is primarily determined by the size and length of the cable pairs that connect the local phone or to the office switching equipment. Rarely are there any gain devices employed in the local loop. Since this local loop is found at both ends of the talk path, the total loss may be as high as 15 or 16 dB, which would be quite noticeable to both parties.

Most local calls and all long distance calls travel over one or more connections (hops) between central switching offices on circuits called VF trunks. These differ from subscriber lines in that trunks are not dedicated to a particular caller but are used as needed to route calls between central offices. It is in these trunks that VF level control can be accomplished.

Many businesses employ Private Branch Exchanges (PBXs) so the local loop may be short (between the phones in an office and the PBX) but the distance from the PBX to the central office may be some distance. PBX's usually connect to telephone offices using trunks.

Bell System loss objectives for various types of trunks are listed in Table 15-2. These trunks may or may not have gain associated with them. Most trunks in the telephone networks of industrialized nations use some form of digital transmission (T1 or E1 or higher bit rates) where circuit gain or loss can be tightly controlled.

Via Net Loss (VNL) ranges from 0.5 dB for short trunks up to 165 miles to 2.9 dB for extremely long trunks up to 1850 miles. For any trunk that employs echo suppressors, VNL should be 0 dB. In addition to insuring adequate talk volume at each end of the circuit, the loss plan is used to reduce the effects of echo on the voice path. A certain amount of loss is required to prevent echo levels from becoming objectionable. If the loss plan for each trunk type as stated in Table 15-2 is followed, there should be sufficient loss to keep echo from being a problem.

It is strongly recommended that the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch voice paths be equipped with echo cancellers because of the delay that voice packets experience over an Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch network. Note that it is not desirable to have more than one echo suppressor device on a talk path in attempt to further reduce echo as they tend to interfere with each other sometimes resulting in worse echo performance than with no echo cancellers at all.

Digital interoffice trunks do not have the problem of loss so much as analog trunks as once the voice signal is encoded, the level remains constant and the primary concern is to keep the bit error rate low enough to avoid noticeable clicks on the talk path. Digital circuits are inherently 4-wire trunks until terminated on a 2-wire channel unit thereby avoiding the repeated 2-wire to 4-wire conversions that contribute to echo problems.

In the Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch, the per-channel voice frequency gain can be digitally adjusted over a range of -8 to +6 dB in the CDP/CVM. The default for all voice channels is 0 dB. It is recommended that any gain adjustment be done at the circuit ends on an analog basis (at the channel bank or PABX channel) rather than within the network to minimize the distortion introduced.

Most VF encoders in channel banks, digital PABX's, digital switching offices, and even the Cisco IPX narrowband switch have a limited range over which they will encode linearly. In addition, too high a signal level input to the encoder will overload the encoder resulting in clipping of the VF signal. The overload point is approximately +3 dBm. And any signal level below approximately -45 dBm0 will not be encoded accurately.

Table 15-3 lists the typical quantization distortion added for a single-hop Cisco IPX narrowband switch/Cisco IGX 8400 series multiband switch trunk with 32 Kbps ADPCM employed. The impairment indicated is approximately the same for all digital PAD values in the range of 1 to 8 dB. One exception is the 6 dB (or multiple of 6 dB) A-Law PAD. It introduces negligible impairment for signals ranging to approximately -30 dBm0, and therefore attracts zero units of quantizing distortion.

Posted: Tue Jan 16 11:11:39 PST 2001
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