Voice-Enabling the Data Network

  • Dynamically change from voice to fax demodulation

  • AAL2 Type 3 cells for reliable dual tone multifrequency (DTMF) relay

  • Dynamically change the compression rate to G.711 for fax calls in midcall

  • Indicate end of speech burst for background noise generation during

    silence periods at the egress ATM switch Transport up to 248 voice calls with different compression schemes within one or more ATM permanent virtual circuit (PVC).

This architecture provides a Class 4 interconnect replacement, which enables an enterprise to bypass the local Tandem Switch.

A Tandem Switch is a switch that incumbent local exchange carrier (ILEC) networks use to route calls between COs in the same local access and transport area (LATA). These calls are referred to as intraLATA calls. Trunks at each CO are typically interconnected by a Synchronous Optical Network (SONET) ring. The Tandem Switch also connects to an IXC Tandem Office, which is called a point of presence (POP) A POP houses a Class 4 switch that connects into the ILEC ’s Tandem Switch. The Tandem Switch aggregates interLATA traffic from multiple COs and a trunk facility. An IXC Tandem Office can have dedicated trunks to an ILEC ’s CO in cases where a high concentration of traffic exists between the CO and the IXC. An IXC handles interLATA traffic.

A Class 5 switch is located in an end office and a Class 4/5 switch is located in a Tandem Office. A Class 5 switch provides local services in the PSTN to the end user. A Class 5 switch provides enhanced calling features, such as call waiting and three-way calling to end users.

An end-to-end trunking architecture does not require a call agent. This architecture can reduce the complexity of a mesh of narrowband circuits by having only a single integrated voice and data network. IADs can support the transport of data and voice by using AAL2 and AAL5 from the customer premises to the service provider. The architecture includes IAD 2400 at the business customer site, which terminates into a MGX8000 Voice Gateway at the service provider’s network edge. The MGX8000 Voice Gateway adds packet voice capabilities to the MGX 8850 that includes VoIP, VoAAL, and VoAAL2.The voice signaling from the enterprise customer is tunneled through the MGX8000 Voice Gateway to a Class 4 switch by using AAL2 point-to-point trunking. The data is tunneled through to an ATM switch, such as a Cisco BPX 8600.

Multiple T1s or E1s with Primary Rate Interface (PRI) or channel-associated signaling (CAS) terminate from the PSTN to the MGX8000 Voice Gateway. The PSTN cloud represents another service provider offloading its voice traffic to another carrier. The integrated access service uses AAL2 PVCs between two MGX 8850s within the service provider’s network. In this application, the MGX8000 Voice Gateway uses an ATM User Service Module (AUSM) card. The IAD 2400 aggregates both voice and data traffic over a T1 access line to the service provider.

CAS and PRI signaling can be supported in this architecture. The CAS information is carried in the AAL2 PVC across the network. CAS is a signaling technique that uses robbed bits within a multiframe, such as a D4 Super Frame (SF) or Extended Superframe. These robbed bits, referred to as ABCD, represent various states and transitions of a voice call. These ABCD bits are transported over the same AAL2 channel as the one used for voice because CAS does not use a separate signaling channel, such as in-band signaling; the CAS bits use AAL2 Type 3 packets as they provide CRC checks for reliability whereas voice traffic uses AAL2 Type 1 packets that are without CRC checks. An important feature that this architecture provides is idle channel suppression idle channel suppression stops sending idle channel bits that are generated from the CAS source (for example, a PBX). This mechanism results in significant amounts of bandwidth savings in the service provider ’s ATM network; this mechanism has no benefit in common channel signaling (CCS) configurations.

The PRI signaling channels, for example, in T1 the 24th time slot and in E1 the 16th time slot, are carried across the ATM network in the AAL5 PVC while the voice traffic, that is the bearer traffic, is carried by the AAL2 PVC. Thus, two different PVCs traverse the end-to-end network. One carries all the data traffic, and the other carries all the signaling traffic. D-channel information is transported across an AAL5 PVC because the signaling is in High-Level Data Link Control (HDLC) format. AAL2 does not support HDLC but AAL5 does.

Fundamentals of AAL2

The AAL2 protocol has two layers:

  • Service specific convergence sublayer (SSCS)
  • Common part sublayer (CPS)

The SSCS encodes different information streams for the transport by AAL2 over a single ATM connection. The information streams might be active voice encodings, silence insertion descriptors, dialed digits, or fax. SSCS can provide error control on critical information (CAS signaling and dialed digits) by using a 10-bit CRC. This is called an AAL2 Type 3 cell. The SSCS segments the information that is being passed from a higher layer application, such as samples of voice from a digital phone into a number of units of data, and submits these units of data to the CPS for transmission. The length of the segmented data can be between one and the maximum length supported by the CPS connection, which is either 45 or 64 bytes. At the SSCS receiver, the units of data are reassembled back into the information before being passed to the higher layer application.

The second layer, the CPS, is specifically responsible for transporting end-to-end connections across the network.

Part three will focus on CPS layer and how it can enable multiplexing of variable length voice packets.

James F. Durkin is the author of Voice-Enabling the Data Network from Cisco Press. The following article was an excerpt from the book.

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