Call Setup

The V.34 initial handshaking procedure between an originating modem and a terminating modem can be broken down into the four primary phases shown in Figure 1-18.

Figure 1-18 V.34 Modem Call Setup Procedure

V.8 Messaging - Disable Echo Cancellers, Exchange Capabilities, Determine Call Modulation

Line Characterization, Determination of Carrier Frequency, Symbol Rate, and Round Trip Delay

Half Duplex Equalizer and Echo Canceller Training, Digital Impairment Learning (V.90)

Full Duplex Training and Fine Tuning, Selection of the Final Data Rate

Each of these phases has its own separate message exchange to fulfill its precise role in setting up a successful modem call. This section discusses each of these phases in detail and elaborates on the state of the call as it progresses through its handshake sequence.

Phase I: Network Interaction

The V.8 recommendation defines the first messages that are communicated between V.34 modems. This specification is used by a number of ITU-T V series modulations to establish communication and preliminary negotiations before the actual modulation begins. The V.8 negotiation identifies the capabilities of each device and determines the best modulation to use and some other parameters. Figure 1-19 illustrates the V.8 message exchange that occurs at the beginning of a V.34 modem call.

When the call is first answered by the terminating modem, it plays an answer tone called ANSam. Consisting of a 2100 Hz tone that is phase reversed approximately every 450 ms and amplitude modulated by a sine wave at 15 Hz, the ANSam lasts for a duration of three to four seconds and is used to disable network echo cancellers in the call path.

Phase I

Network Interaction

Phase II

Probing/Ranging

Phase III

Equalizer and Echo Canceller Training

Phase IV

Final Training

Figure 1-19 V.34 Phase I Startup Procedure (V.8) Call Connect

Figure 1-19 V.34 Phase I Startup Procedure (V.8) Call Connect

>= 0.2 s 75 +-5 ms

When the calling modem detects the ANSam, it sends a sequence of continuous CM (Calling Menu) messages. The CM message contains a detailed capabilities list (including modulation) of the calling modem. After detecting at least two identical CM messages, the answering modem responds with a continuous sequence of JM (Joint Menu) messages that contain the capabilities common to both calling and answering modems. If the answering modem for some reason does not support one of the capabilities advertised by the calling modem in the CM message, it may offer an alternative in its JM message sequence.

After detecting at least two identical JM messages, the calling device then transmits a CM terminator message (known as CJ) to acknowledge the JM message. After sending the CJ message, the calling modem pauses for about 75 ms and then begins operating in the selected mode. The CJ message just indicates the termination of the V.8 session, and it does not contain any additional information. When the answering modem receives the CJ message, it stops transmitting the JM sequence and pauses for approximately 75 ms before beginning to operate with the capabilities selected by the CM/JM exchange.

NOTE

V.8 messages are transferred by V.21 at 300 bps.

V.8 was released along with V.34 as a means of speeding up the initial answering sequence of modems. Older modulations used a large range of answer tones exchanged between the calling and called modems until they found one that matched as a means to determine an initial modulation. The V.8 CM/JM automatic modulation determination procedure (automode) did away with this and allowed for faster initial training.

It is worth noting that V.90 optionally uses a revised version of V.8 known as V.8bis. Both are procedures for exchanging capabilities and setting the mode of operation, but there are some differences between the two. One difference is that V.8bis has a more detailed list of modes that it can negotiate. It doesn't support some of the older modulations that V.8 does, but it does add newer ones. Another key difference is that V.8bis can negotiate a change in operating mode in the middle of a call, whereas V.8 can only do it at the beginning.

TIP V.8bis was designed to facilitate switching a voice call into data mode, but this was never widely used, so the use of V.8bis is optional (unless using K56Flex), and in fact it can be useful to turn it off, because disabling it saves two or so seconds at the start of the training.

Phase II: Probing/Ranging

After a modulation has been selected via the V.8 procedure, the call progresses to Phase II. In this phase, the modems collect and exchange information about the characteristics of the line. The two primary methods of obtaining this detailed view of the channel are line probing and ranging. Line probing determines the SNR and the bandwidth of the channel, whereas ranging determines the network round-trip delay. This line analysis allows V.34/ V.90 modem to choose the optimum operating parameters for the fastest and most robust connection for a particular line condition.

An elementary way of describing the message exchanges in Phase II is that it is an initial information exchange (INFO_0), followed by the ranging sequence, then the line probing sequence, and ending with a final information exchange (INFO_1). Figure 1-20 illustrates this simplified view of Phase II.

NOTE Both INF0_0 and INFO_1 signals are designated by an a if sent by the answering modem or c if sent by the calling modem.

The initial information exchange in Phase II is via the INFO_0 message to convey the available capabilities and modulation parameters of both the calling and answering modems. An example of a parameter exchanged via INFO_0 is the supported symbol rates for the call.

Figure 1-20 V.34 Phase IIStartup Procedure

Calling Modem

Answering Modem

TONE B/B

L1

L2

INF0_0a

TONE A/A

TONE

L1

L2

TONE A/A

_M

A

• • • • • w

( Information Ranging

( Exchange

Line Probing

( Information Ranging

( Exchange

Line Probing m

V.34 Phase 3,4

TONE A

INFO_1a m

Information Exchange #2

The next step of Phase II is ranging, which is the determination of the round-trip delay of the connection. This round-trip delay is used to set the modem's far-end echo canceller. The modems obtain this information by sending tones and transitioning to a phase reversal of that tone.

TONE B/B

NOTE Tone A is 2400 Hz, and Tone A is the tone obtained by a 180 degree phase reversal of Tone A. Likewise, Tone B is 1200 Hz and Tone B is its phase reversed counterpart.

When ranging has completed, the line probing process begins. This process yields the characteristics of the line, such as the bandwidth and SNR of the channel. The modems analyze the channel by sending the L1 and L2 signals, which are known as line-probing signals. L1 and L2 are made up of a list of tones with frequencies from 150 Hz to 3750 Hz in increments of 150 Hz (with 900 Hz, 1200 Hz, 1800 Hz, and 2400 Hz missing). The modems transmit these multitone signals to sweep the band and thus analyze the resultant distortion, SNR, and bandwidth of the channel.

The last thing to occur in Phase II is the information exchange between the two modems via the INFO_1 messages. The principal information carried by the INFO_1 messages is the results of the line probing that were collected. These results are provided as projected maximum data rates at the various symbol rates.

NOTE

Phase II in V.90 uses virtually an identical structure as V.34.

Phase III: Equalizer and Echo Canceller Training

The next step in setting up the physical connection is the equalizer and echo canceller training phase. An equalizer is an adaptive device in the modem that needs to be trained to compensate for distortion introduced by the PSTN transmission facilities. Likewise, the echo canceller in a modem requires training to eliminate signal reflections caused by hybrids in the call path. A hybrid is a tunable, impedance-matching device that converts between two-wire and four-wire circuits in telephony networks.

Phase III trains both these devices with a half-duplex training procedure as Figure 1-21 illustrates. The half-duplex nature of this phase is evident in Figure 1-21, which shows the originating modem is quiet while the terminating modem sends a sequence of signals to train its echo canceller. Correspondingly, the answering modem is quiet while the calling modem transmits a similar sequence to train its echo canceller.

Figure 1-21 V.34 Phase IIIStartup Procedure

Calling Modem

S/S

MD

S/S

PP

TRN

V.34 Phase 4

S/S

MD

S/S

PP

TRN

AAAAr

This training sequence is composed of an optional manufacturer-specific echo canceller training signal (MD) whose duration is determined by the round-trip delay that was obtained in Phase II. This proprietary MD signal is present in case the mandatory train signal (TRN) is unable to train the modem echo canceller. In addition to training the echo canceller of the transmitting modem, the TRN signal assists in training the remote modem's equalizer. The PP signal is a sliding frequency signal used for initially training the equalizer of the remote modem. Lastly, the J sequence is a repetition of a 16-bit sequence that determines the constellation size to be used in phase IV.

One big difference between V.34 and V.90 occurs in Phase III. V.90 adds a digital impairment learning (DIL) capability, which is used to identify digital impairments, such as robbed bit signaling (RBS) trunks, digital padding, and so on. This process occurs by the analog modem (client side) sending manufacturer-specific parameters, known as the DIL descriptor, to the digital modem (server side). The digital modem uses this description of the DIL sequence and then generates the DIL tone. This proprietary DIL tone is specific to each modem manufacturer. The analog modem receives this DIL sequence and analyzes it for possible digital impairments. Although it is theoretically possible to negotiate V.90 without the DIL sequence, it is extremely unlikely.

Phase IV: Final Training

As a result of the work in Phase III, the echo characteristics for the connection have now been learned, and the echo cancellers have been trained. Therefore, in Phase IV, both modems can transmit simultaneously and use the full bandwidth of the channel for the first time. Phase IV is a short phase that includes a fine-tuning while determining the final data rate to be used. Figure 1-22 shows this full-duplex training and its associated messaging.

Figure 1-22 V.34 Phase IVStartup Procedure

Calling Modem

V.34 Phase 4

DATA

S/S

TRN

MP

MP

MP'

MP'

E

B1

The TRN signal is a training sequence used to fine-tune the echo canceller and equalizer. This is followed by a sequence of exchanged modulation parameters, known as the MP signals. The modulation parameters sent between the modems are to be used for data mode transmission and include such things as maximum call modem-to-answer modem data signaling rate, maximum answer modem-to-call modem data signaling rate, and the trellis coder selection. The MP' signals are merely MP signals with the acknowledge bit set.

The E signal is a 20-bit sequence that marks the end of modulation parameter exchange. The last signal sent is B1, which is one data frame made up of scrambled 1s as its payload. Because this is the first time the modem is sending information using all the selected modulation parameters that will be used in data mode, this serves as a final test of the connection. After this, regular user data transmission begins.

V.90 has a similar Phase IV negotiation as V.34. One main difference is that in V.90 there are constellation parameters (CP) sent from the analog modem to the digital modem rather than MP signals. These constellation parameters contain information such as the downstream data signaling rate and the set of constellations.

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