VISM Voice Features
This section provides an overview of the features supported in VISM Releases 1.5, 2.0, and 2.1. VISM firmware releases are not coupled with PXM1 firmware releases, unlike other service modules.
VISM Release 1.5 Features
VISM Release 1.5 is supported on MGX 8250/8850-PXM1 switches. For VISM services on MGX 8230 switches, you must use VISM Release 2.0.
VISM Release 1.5 supports the following voice features:
Voice over IP (VoIP) using real-time transport protocol (RTP)—VISM Release 1.5 supports standards-based VoIP using RTP (see RFC 1889) and real-time transport control protocol (RTCP) protocols. This allows VISM to interwork with other VoIP gateways.
Multiservice Access with Call Control—This application also uses VoIP mode. In this application, the VISM/MGX 8850 combination is used to concentrate voice and user data services onto a single broadband circuit for transmission over the packet network using VoIP and ATM AAL5 techniques. VISM can also function with a call agent to perform call control.
Voice over AAL2 (VoAAL2) with subcell multiplexing PVC—The VISM supports standards-compliant AAL2 adaptation for the transport of voice over an ATM infrastructure. The AAL2 trunking mode is supported.
Codec support—G.711 Pulse Code Modulation (PCM) A-law and _-law, G.726 Adaptive Differential PCM (ADPCM), and G.729a/b Conjugate Structure Algebraic Code Excited Linear Predictive (CS-CELP) coding.
Eight T1/E1 lines—The VISM supports eight T1 or eight E1 interfaces when G.711 PCM coding is used. For higher-complexity coders such as G.726-32K and G.729a-8K, the density drops to six T1 or five E1 interfaces and a maximum of 145 connections.
Echo cancellation—The VISM provides on-board echo cancellation on a per-connection basis. Up to 128 milliseconds of user-configurable near-end delay can be canceled. Echo cancellation is compliant with ITU G.165 and G.168 specifications.
Voice Activity Detection (VAD)—VISM uses VAD to distinguish between silence and voice on an active connection. VAD reduces a voice connection's bandwidth requirements by not generating traffic during periods of silence in an active voice connection. At the far end, comfort noise is generated.
Fax/modem detection for echo cancellation and VAD control—The VISM continually monitors and detects fax and modem carrier tones. When carrier tone from a fax or modem is detected, the connection is upgraded to full PCM to ensure transparent connectivity. Fax and modem tone detection ensures compatibility with all voice-grade data connections.
Channel Associated Signaling (CAS) tunneling via AAL2 (for AAL2 trunking mode)—The VISM in AAL2 mode facilitates transport of CAS signaling information. CAS signaling information is carried transparently across the AAL2 connection using Type 3 packets. In this mode, VISM does not interpret any of the signaling information.
Primary Rate Interface (PRI) tunneling via AAL5 (for AAL2 trunking mode)—VISM supports transport of D-channel signaling information over an AAL5 VC. The signaling channel is transparently carried over the AAL5 VC and is delivered to the far end. In this mode, VISM does not interpret any of the signaling messages.
Voice connection admission control (CAC)—VISM can be configured to administer CAC so that the bandwidth distribution between voice and data can be controlled in AAL2 mode.
Type 3 Packet for dual tone multifrequency (DTMF)—The VISM in AAL2 mode facilitates transport of DTMF signaling information. DTMF information is carried transparently across the AAL2 connection using Type 3 packets.
Dual (redundant) PVCs for bearer/control—The VISM can configure two PVCs for bearer/signaling traffic terminating on two external routers (dual-homing). VISM continually monitors the status of the active PVC by using OAM loopback cells. Upon detection of failure, the traffic is automatically switched over to the backup PVC.
64 kbps clear channel transport—The VISM supports 64 kbps clear-channel support. In this mode, all codecs are disabled, and the data is transparently transported through the VISM.
DTMF relay for G.729—In VoIP mode, DTMF signaling information is transported across the connection using RTP named signaling event (NSE) packets.
Media Gateway Control Protocol (MGCP) Version 0.1 for VoIP with Softswitch control—VISM supports MGCP Version 0.1. This open protocol allows any Softswitch to interwork with the VISM module.
Support for call agent Simple Gateway Control Protocol (SGCP) version 1.0, SGCP 1.1+
Resource Coordination via Simple Resource Control Protocol (SRCP)—SRCP provides a heartbeat mechanism between the VISM and the Softswitch. In addition, SRCP also provides the Softswitch with gateway auditing capabilities.
Courtesy downing of ongoing voice calls when the VISM is taken out of service for maintenance or other reasons
Support for CCS (Common Channel Signaling) transport across an AAL5 trunk
Support for full continuity testing (COT)—Supports origination and terminating loopback and transponder COT toward the packet bearer and the TDM sides.
1:N cold redundancy using SRM-3T3 capabilities (bulk mode support for T1 lines only)—Calls do not persist during switchover.
VISM Release 2.0 Features
VISM Release 2.0 supports the VISM Release 1.5 features just listed, in addition to the features listed in this section. VISM Release 2.0 is supported on MGX 8230, 8250, and 8850-PXM1 switches.
Here are the key features of the VISM Release 2.0 card set:
PRI backhaul to the Softswitch using Reliable User Data Protocol (RUDP)—The PRI backhaul capability provides PRI termination on the VISM with the Softswitch providing call control. ISDN Layer 2 is terminated on the VISM, and Layer 3 messages are transported to the Softswitch using RUDP.
Codecs preference—VISM provides the capability to have the codecs negotiated between a call's two endpoints. The VISM can be configured, for a given endpoint, to have a prioritized list of codecs. Codec negotiation can be directly between the endpoints or can be controlled by a Softswitch.
31 timeslots for E1 with 240 channels only—Although all 31 timeslots on an E1 port can be used, there is a limitation of 240 connections per card.
VISM Release 2.1 Features
VISM Release 2.1 supports the VISM Release 1.5 and 2.0 features listed previously. It also includes voice compression to G.711, G.726-16k, 24k, 32k, 40k, G.729a, and G.729ab standards.
VISM offers different solutions for carrying voice traffic, including Voice Over AAL2 network, VoIP network, and Voice Over ATM services, as described in the following sections. An introductory discussion of voice connections is presented first.