Information About V.92 and V.44 Support for Digital Modems
The following sections provide information about the V.92 and V.44 Support for Digital Modems feature.
ITU-T V.92 Modem Standard
The ITU-T V.90 modem standard recommendations have been enhanced with V.92, a new standard that meets the need for a digital modem and analog modem pair on the Public Switched Telephone Network (PSTN). V.92 improves the upstream data signaling rate and adds new features that enhance modem usability.
This feature supports the following parts of the V.92 modem standard:
- V.92 Modem on Hold
- V.92 Quick Connect
V.92 is implemented at the modem level as new modem protocols and standards. The new V.92 features co-reside with existing portware features and have no impact on the hardware configuration of either the hex MICA technologies module (HMM) or dual MICA technologies module (DMM), including on memory requirements.
V.92 Modem on Hold
V.92 Modem on Hold allows a dial-in customer to suspend a modem session to answer an incoming voice call or to place an outgoing call while engaged in a modem session.
When the dial-in customer uses V.92 Modem on Hold to suspend an active modem session to engage in an incoming voice call, the Internet service provider (ISP) modem listens to the original modem connection and waits for the dial-in modem to resume the connection. When the voice call ends, the modem signals the telephone system to end the second call and return to the original modem connection, then the modem signals the ISP modem that it is ready to resume the modem call. Both modems renegotiate the connection, and the original exchange of data continues.
Note This feature is designed for use on telephone lines that are configured for the call-waiting service; call-waiting signals trigger the suspension of the modem session. If call-waiting service is not present on the subscriber’s line, callers receive a busy signal, and the modem session is not interrupted.
Use of the V.92 Modem on Hold feature for Cisco MICA portware can be controlled globally using AT commands (modemcaps) or can be controlled on a per-caller basis using the RADIUS distributed client/server system. (You are not required to have a RADIUS server to use the V.92 Modem on Hold feature.)
V.92 Modem on Hold AT Commands and S-Registers
V.92 Modem on Hold is disabled by default and is controlled with standard AT commands and S-registers. V.92 is enabled and disabled with the S29 S-register (S29 = 12), and MOH is controlled with the S62 S-register (S62 must be set to enable MOH). AT commands download the configuration to the modem at the end of every call. The ATSn=v and ATSn? AT commands are used to configure the V.92 Modem on Hold feature on Cisco MICA platforms. Table 1 lists additional S-register parameters used to enable and disable the feature.
Table 1 V.92 Modem on Hold S-Registers
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MOH Timeout |
S62 |
63 |
0 MOH Disabled |
0: MOH Disabled 1: 10 seconds 2: 20 seconds 3: 30 seconds 4: 40 seconds 5: 1 minute 6: 2 minutes 7: 3 minutes 8: 4 minutes 9: 6 minutes 10: 8 minutes 11: 12 minutes 12: 16 minutes 13: No limit |
To disable V.92 Modem on Hold, you can use a modemcap (for example, s62=0s63=3s21=15s29=12) or set the S29 register to any number other than 12. You can also use the RADIUS VSA to disable MOH if the feature was initially enabled by the default value (modemcap).
Note If the feature is enabled using S29=12 and V.92 Modem on Hold is disabled using S62=0, statistics for the number of times a dial-in customer requests an on-hold function are tracked in the MOH link information parameters. However, completely disabling the feature by setting S29 to a value other than 12 disables the reporting of all MOH statistics.
For detailed information about the AT commands and S-registers used to control V.92 on Cisco MICA platforms, refer to the AT Command Set and Register Summary for MICA 6-Port Modules.
RADIUS Authorization
Per-user control of V.92 Modem on Hold can be configured for caller authorization using a RADIUS server. RADIUS servers use the VSA capability to configure MOH for individual users. The current attribute=value protocol syntax has been extended with the new Modem on Hold attribute. You can enter an unsigned integer in the range from 0 through 65,535 that represents the maximum number of seconds that a modem may remain on hold, and which can extend from 10 seconds to an unlimited number of minutes.
Note Although the integer attribute values can be specified as any value in a contiguous range, V.92 specifications limit configuration to a limited set of values. Attribute values are rounded down to the next permitted value when they are used to configure a modem.
Alternatively, you can enter the inf keyword, which allows the modem to remain on hold indefinitely.
If the V.92 Modem on Hold configuration is not present, or if it is not syntactically correct, the modem uses its default configuration for MOH. The default operation can be modified by using a modemcap string. Following authentication of a dial-in user, an additional control command is sent to the modem if the dial-in user does not have a default MOH configuration.
Note Code space requirements for RADIUS support is less than 2 KBs. There are no additional data space requirements.
V.92 Modem on Hold running on systems using RADIUS authorization increases the length of RADIUS response packets by approximately 20 bytes. RADIUS databases increase in size by approximately the same amount for each dial-in user with a nondefault MOH attribute value.
With RADIUS, authentication and authorization occur as part of the same process. When a caller connects to the access server, the caller enters a user ID and password in response to prompts from the access server. This information is formatted as a RADIUS request packet and is sent to the appropriate RADIUS server. If the user ID is valid and the password matches, the RADIUS server responds with a packet containing authorization data for the connection. This authorization data contains the MOH configuration if it is present in the RADIUS database.
The access server interprets the response from the RADIUS server and performs any actions associated with the authorization data. For V.92 Modem on Hold, a command is sent from the Cisco IOS host to the Cisco MICA channel to set the MOH parameters. No confirmation from the channel is required.
When the client modem requests a V.92 Modem on Hold operation, the modem switches to an on-hold state, which prevents further data from being queued to the connection. When the Cisco IOS software receives the request to go on hold, any transmit packets queued to local Cisco IOS queues (packets not already posted to the queues shared between the Cisco IOS software and the modem) are discarded. This mechanism reduces the possibility that stale data will be transmitted to the modem when the connection is reactivated. It also reduces the number of buffer resources that are unavailable while a modem is on hold.
Note When a RADIUS server is used, placement of commas is important. The asterisk in the modem-on-hold attribute indicates that the attribute is optional. If a modem does not support V.92 Modem on Hold, then the call might continue anyway. If the asterisk is replaced by an equal sign (=), the attribute is required, and modems that do not support MOH terminate the calls following authentication. The value of the attribute is the number of seconds allowed for the on-hold state. This value is rounded down by the Cisco IOS software to one of the permissible values. The number can be replaced with the inf command, which allows unlimited on-hold time. Case is significant for both attribute names and values.
Note If your router is configured for RADIUS, the RADIUS server must be accessible to the router. The server must also be capable of responding to authentication requests with VSA attributes.
V.92 Quick Connect
V.92 Quick Connect speeds up the client-to-server startup negotiation, reducing the overall connect time by up to 30 percent. The client modem retains line condition information and characteristics of the connection to the ISP, which reduces connect time by avoiding some of the initial signal handshaking.
V.92 Quick Connect AT Commands and S-Registers
V.92 Quick Connect is enabled by default and is controlled with standard AT commands and S-registers. S29 is used to enable V.92 sequence detection and S63 is used to enable V.92 Quick Connect, both of which are enabled by default. The S63 S-register controls the feature through the ANSpcm signal. You can choose the power level of the ANSpcm signal, which defaults to –12 dBm (the transmit power level for the United States). The ATSn=v and ATSn? AT commands are used to configure V.92 Quick Connect on Cisco MICA platforms.
To disable V.92 Quick Connect or to set a different ANSpcm value, you can use a modemcap (for example, v92_v44:MSC=s62=8s63=0s21=15s29=12), or you can set the S29 register to any number other than 12. Table 2 lists the S-registers used to enable and disable the V.92 Quick Connect feature.
Table 2 Quick Connect (QC) Config S-Registers
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V.92 QC Config |
S63 |
64 |
0x3 QC Enabled ANSpcm Level –12 dBm |
Bit 0: QC Enable 0: Disabled 1: Enabled Bits 1–2 ANSpcm Level 00: –9 dBm 01: –12 dBm 10: –15 dBm 11: –18 dBm |
Note If the V.92 Modem on Hold feature is enabled using S29=12, and QC is disabled using S63=0, then the QC Exchange Link Information Parameter is updated to show if QC was requested. However, completely disabling the feature by setting S29 to a value other than 12 disables the reporting of QC requests.
For detailed information about the AT commands and S-registers that are used to configure the V.92 Quick Connect feature on Cisco MICA platforms, refer to the AT Command Set and Register Summary for MICA 6-Port Module s.
V.44 LZJH Compression
V.44 LZJH is a new compression standard based on Lempel-Ziv that uses a new string-matching algorithm that increases upload and download speeds to make Internet access and Web browsing faster. The V.44 call success rate (CSR) is similar to V.42 bis with significant compression improvement for most file types, including HTML files. V.44 applies more millions of instructions per second than V.42 bis toward the same application data stream and yields better compression rates in almost any data stream in which V.42 bis shows positive results.
V.44 supports automatic switching between compressed and transparent modes on supported platforms. Automatic switching allows overall performance gain without loss in throughput for file streams that are not compressible.
V.44 is globally controlled through dialed number identification service (DNIS), calling line identification (CLID), and resource pool manager server (RPMS) virtual groups, and performance improvement is determined by the LZJH algorithms.
To support V.44 LZJH compression, the control switch module (CSM) has been modified. MIBs that show the status of V.42 bis have been extended to show V.44 configuration status. New disconnect reasons help manage V.44 session status and debugging.
V.44 AT Commands and S-Registers
V.44 compressor/decompressor optimizes throughput and the CSR using the standard AT command interface. New AT commands are provided so that V.44 can be enabled or disabled at the modem level. Modemcaps (dial in) or chat scripts (dial out) are required to enable or disable V.44 globally on the access server.
The AT command interface supports new S-registers for tuning V.44 negotiation parameters and lets you monitor V.44 configuration and session status. The ATSn=v AT command is used to configure V.44, and the ATSn?, AT%R, and AT\S commands are used for obtaining V.44 status. These commands modify the operation of S21 to enable and disable the algorithm.
Table 3 shows the S21 register values, and Table 4 shows additional S-register values used with the V.44 feature. V.44 is enabled by default.
Table 3 V.44 S-Registers (Existing)
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Data Compression |
S21 |
NextPort 0x800b MICA 11 |
0xf |
Bit 2: V.44 Tx Bit 3: V.44 Rx |
Table 4 V.44 S-Registers (Redefined)
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V.44 Max Tx Codewords |
S65 |
NextPort 0x8042 MICA 66 |
2048 |
256–2048 |
V.44 Max Rx Codewords |
S66 |
NextPort 0x8043 MICA 67 |
2048 |
256–2048 |
V.44 Max Tx String Length |
S67 |
NextPort 0x8044 MICA 68 |
255 |
0–255 characters |
V.44 Max Rx String Length |
S68 |
NextPort 0x8045 MICA 69 |
255 |
0–255 characters |
V.44 Max Tx History Size |
S69 |
NextPort 0x8046 MICA 70 |
10240 |
256–10240 bytes |
V.44 Max Rx History Size |
S70 |
NextPort 0x8047 MICA 71 |
10240 |
256–10240 bytes |