Table of Contents

Circuit Emulation Services Overview

The T1 and E1 CES Interfaces
Features and Functionality

CES-IWF
Unstructured CES
Structured CES
Channel-Associated Signaling and On-Hook Detection for Structured CES
Advantages
Limitations

Network Clocking for CES and CBR Traffic

Synchronous Clocking
SRTS Clocking
Adaptive Clocking

CES Configurations

Before you Begin
About Cell Delay Variation
General Procedure for Creating Soft PVCs for CES
T1/E1 Unstructured CES

Hard PVCs for Unstructured Services
Soft PVCs for Unstructured Services

T1/E1 Structured CES Services

Hard PVCs for Structured Services without CAS
Soft PVCs for Structured Services without CAS
Soft PVCs for Structured Services with CAS
Multiple Soft PVCs on the Same CES Port
Soft PVCs for Structured Services with CAS and On-Hook Detection Enabled

Simple Gateway Control Protocol

How it Works

Circuit Emulation Services and Voice over ATM

This chapter provides an overview of using circuit emulation services (CES) for connecting the ATM switch router and traditional time-division multiplexing (TDM) devices. This chapter also includes a description of the Simple Gateway Control Protocol (SGCP), a software feature that allows your ATM switch router to function as a call gateway in a voice over ATM environment.

This chapter includes the following sections:

• "Circuit Emulation Services Overview" section

• "Network Clocking for CES and CBR Traffic" section

• "CES Configurations" section

• "Simple Gateway Control Protocol" section

Circuit Emulation Services Overview

Real-time data, such as voice and video, have low tolerance for transfer delays. For voice, it is generally <50 ms; for video, it is <150 ms; delays >50 ms usually require echo cancellation. The constant bit rate (CBR) traffic category provides the low delay characteristics required for such traffic. Using CBR PVCs or soft PVCs, circuit emulation extends T1/E1 services carrying real-time data directly from traditional TDM devices across the ATM cloud.

Figure 8-1 provides an example of CES applications in an ATM network. TDM devices that do not have ATM interfaces---multiplexers, PBXs, and video codecs---are directly attached to the ATM network, which transports the data over its infrastructure as CBR traffic.

The T1 and E1 CES Interfaces

There are two types of T1 and E1 port adapters available for the ATM switch router with different framing characteristics at the physical level. The T1 and E1 ATM port adapters send and receive ATM cells and must be connected to an external host that can accept native ATM. The T1 and E1 CES port adapters, however, connect TDM equipment across an ATM network using CBR ATM virtual connections.

Features and Functionality

The CES modules provide three key functions:

Figure 8-2 shows an example of using the CES modules in an ATM network for both unstructured and structured services.

CES-IWF

CES-IWF is based on an ATM Forum standard that allows communication between CBR and ATM UNI interfaces, that is, between non-ATM telephony devices (such as classic PBXs or TDMs) and ATM devices (such as an ATM switch or router with an ATM interface). CES-IWF works by packaging incoming native T1 or E1 frames into AAL1 cells at the ingress, and performing the opposite function at the egress.

Figure 8-3 illustrates the use of CES-IWF between non-ATM devices (traditional PBXs) and other end devices. In the case of communication between two non-ATM end devices (traditional PBXs), CBR data in native T1 format received from an edge device on one side of the network is segmented into ATM cells and propagated through the ATM network. After traversing the network, the ATM cells are reassembled into a CBR bit stream that matches the original user data. This native T1 CBR data is then passed out of the network to the edge device at the destination endpoint. In the case of communication between the traditional PBX and the ATM DSU, it is the DSU that performs the reassembly into the CBR bit stream for its attached PBX. Both cases illustrate the use of CES-IWF.

Unstructured CES

Unstructured CES in an ATM network emulates point-to-point connections over T1 or E1 leased lines. This service maps the entire bandwidth necessary for a T1 or E1 leased line connection across the ATM network. Unstructured CES operations do not decode or alter the CBR data in any way.

• CES T1 port adapter---Supports DSX-1 physical interfaces to provide T1 unstructured (clear channel) CBR data transmission services at 1.544 Mbps.

• CES E1 port adapter (either 120-ohm or 75-ohm version)---Supports G.703 physical interfaces to provide E1 unstructured (clear channel) CBR data transmission services at 2.048 Mbps.

Figure 8-4 shows how T1/E1 unstructured CES might be used to connect PBXs with an ATM switch router equipped with a CES T1 or E1 port adapter.

Structured CES

Structured CES is designed to emulate point-to-point fractional T1 (N x 64 Kbps) connections. N x 64 refers to a circuit bandwidth (data transmission speed) provided by the aggregation of N x 64-Kbps channels. The 64-Kbps data rate, or the DS0 channel, is the basic building block of the T carrier systems (T1, T2, and T3).

With T1/E1 structured CES services, networks can be simplified by eliminating TDM devices and allocating T1/E1 bandwidth to PBXs and teleconferencing equipment. In addition, the Simple Gateway Control Protocol (SGCP) can be used to control structured CES circuits for voice over ATM. See the "Simple Gateway Control Protocol" section.

The CES modules provide the following structured services:

• CES T1 port adapter---Supports DSX-1 physical interfaces to provide T1 channelized data transmission services at a rate of 1.544 Mbps.

• CES E1 port adapter---Supports G.703 physical interfaces to provide E1 channelized data transmission services at a rate of 2.048 Mbps.

By supporting T1/E1 structured CES, the CES module can function in the same way as a classic digital access and crossconnect system (DACS) switch. Figure 8-5 illustrates the digital crossconnect and channelized mapping functions supported by an ATM switch router equipped with a CES module.

Figure 8-5: DACS Functions of CES Modules

Either single or multiple DS0 time slots can be mapped across the ATM network. Each time slot (or DS0 channel) can transmit CBR data at a rate of 64 Kbps. Note also that multiple N x 64 circuits can be connected to a single port, using separate time slots.

In Figure 8-6, for example, structured CES allows DS0 timeslots to be combined into circuits and transported using ATM PVCs. The PVCs can be routed to many different destination CES interfaces. Similarly, circuits from many different CES interfaces can be interconnected to a single CES interface, where the various circuit DS0 timeslots are interleaved to form an outgoing T1 bit stream. Thus, you can combine structured CBR data in a highly flexible way for transport across an ATM network.

Figure 8-6 illustrates how 24 available N x 64 DS0 time slots in a CES T1 port adapter can be combined in a number of ways to accomplish structured CBR data transport in an ATM network.

In Figure 8-6, for example, time slots 7, 8, and 24 are bundled to form a single 192-Kbps circuit. At the other end of the connection, you can bundle any of three (available and different) DS0 time slots (such as 18, 19, and 20) to complete the CES circuit.

Figure 8-7 illustrates how 31 available N x 64 DS0 time slots can be provided for structured CES services in a CES E1 port adapter. The rule for DS0 time slot allocation with a CES T1 port adapter also applies to the CES E1 port adapter: the specific DS0 time slot numbers assigned at one end of the circuit in a CES E1 port adapter do not need to map identically to the DS0 time slot numbers at the other end of the CES circuit. Only the aggregate number of DS0 time slots at each end of the circuit must agree.

Channel-Associated Signaling and On-Hook Detection for Structured CES

Since the CES T1 and E1 port adapters emulate CBR services over ATM networks, they must be able to support channel-associated signaling (CAS) information introduced into structured CES circuits by PBXs and TDMs. The optional CAS feature for the CES T1 and E1 port adapters meets this requirement. CAS, also called robbed bit signaling, uses 8k out of each 64k channel to carry signaling information, leaving 56k for the voice channel.

The alternative to CAS, Common Channel Signaling (CCS), in which one entire 64k channel is used for signaling, is not directly supported on the CES T1 and E1 port adapters. However, the Cisco VSC 2700 signaling controller, in conjunction with SGCP can provide similar functionality. See the "Simple Gateway Control Protocol" section.

A second feature, on-hook detection, allows the bandwidth of a quiet circuit to be used by other virtual connections, based upon the CAS. This feature frees unused CBR bandwidth for other preexisting ABR or UBR circuits.

These features can be configured for structured CES in the following ways:

• CAS not enabled (the default state)

In this case, the CES module does not sense the CAS information (carried as ABCD bits in the CBR bit stream) and does not provide support for CAS functions.

• CAS enabled without on-hook detection

In addition to packaging incoming CBR data into ATM adaptation layer 1 (AAL1) cells in the usual manner for transport through the network, the CES module in the ingress ATM switch router (see Figure 8-8) senses the ABCD bit patterns in the incoming data, incorporates these patterns in the ATM cell stream, and propagates the cells to the next node in the network. The ATM cells flow across the network from link to link until reaching the egress ATM switch router node.

At the egress node, the CES module strips off the ABCD bit patterns carried by the ATM cells, reassembles the CAS ABCD bits and the user's CBR data into their original form, and passes the frames out of the ATM network on the proper DS0 time slot.

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Note All these processes occur transparently.

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• CAS and on-hook detection enabled

The CAS and on-hook detection features work together to allow an ingress node in an ATM network to monitor on-hook and off-hook conditions for a specified 1 x 64 structured CES circuit. As implied by 1 x 64, the on-hook detection (or bandwidth-release) feature is supported only in a structured CES circuit with a single DS0 time slot at each end of the connection, as shown in Figure 8-8.

For structured CES services, you can invoke CAS with the ability to detect on-hook or off-hook conditions for any given structured CES circuit. The hook state indicates the following:

• On-hook---Circuit is idle or unconnected.

• Off-hook---Circuit is in use and connected.

The CAS mechanism allows dynamically allocated T1/E1 bandwidth and is released by hard PVCs or soft PVCs configured for structured CES services.

When you configure CAS, the ingress CES module monitors the ABCD bits in the incoming CBR bit stream to detect on-hook and off-hook conditions in the circuit. In an off-hook condition, all the bandwidth provided for the specified CES circuit is used to transport ATM AAL1 cells across the network from the ingress node to the egress node.

Conversely, in an on-hook condition, the network periodically sends dummy ATM cells from the ingress node to the egress node to maintain the connection. However, these dummy cells consume only a fraction of the circuit's reserved bandwidth, leaving the rest of the bandwidth available for other network traffic. This bandwidth-release feature enables the network to make more efficient use of its resources by making unused bandwidth available to bursting or oversubscribed traffic. However, the released bandwidth cannot be reserved by other virtual connections.

Advantages

Potential advantages of using CES in your ATM network include the following:

Limitations

Potential limitations of CES include the following:

Network Clocking for CES and CBR Traffic

For your CES environment to function properly, clocking must be carefully set up. Clock sources and their priorities, along with a distribution mode, must be properly configured, as described in the chapter "Network Clock Synchronization."

The CES port adapters are capable of using three locking modes to meet the timing requirements of CBR data:

• Synchronous---the default clocking mode for CES. If you have a single PRS, such as a clock signal from a telephone company, you should use synchronous mode. If you are using structured CES, you must use synchronous mode.

• Synchronous residual time stamp (SRTS)---allows equipment at the edges of a network to use a clocking signal that is different (and completely independent) from the clocking signal being used in the ATM network. SRTS mode also allows two CPEs to have different clocks.

• Adaptive---typically used when it is not possible to implement either synchronous or SRTS mode. This is the least precise and least recommended method.

Table 8-1 summarizes, in order of preference, the characteristics of the three clocking modes you can configure on a CES module.

Synchronous Clocking

Synchronous clocking mode is the only one that supports full CES functionality. SRTS and adaptive clocking do not support structured CES services. In addition, synchronous clocking is typically used in public telephony systems, making a precision reference signal readily and widely available for synchronizing CBR data transport. With synchronous clocking mode, every device must get its clocking from a single PRS, such as a PBX connecting to a public telephone network.

Figure 8-9, for example, shows how a PRS for synchronous clocking can be provided to an edge node of an ATM network and propagated through the network to synchronize the flow of CBR data between the communicating ATM end nodes.

In this network scenario, a PRS is available to the network by the PBX at the edge of the network. The PRS is present at the port of a CES module in edge node A (the ingress node). From there, the PRS is propagated into the first ATM network through an ATM port and conveyed across an OC-3 trunk to an adjacent ATM network. This same clocking signal is then used to synchronize the handling of CBR data in edge node B (the egress node).

Since synchronous clocking is the default for CES, you do not need to perform any per-interface configuration when using this mode. This assumes that you have one PRS and have properly configured your network clocking, as described in the chapter "Network Clock Synchronization."

SRTS Clocking

Synchronous residual time stamp (SRTS) clocking mode is typically used when there are multiple different clock sources; for example, a when you already have your ATM network synchronized to a reference source, then add a PBX that receives its clock from another service provider source. If you want to avoid resynchronizing your network to their source, SRTS provides a solution by reconciling the different timing signals in the course of CBR data transport through the ATM network.

A common scenario for using SRTS clocking is when your edge equipment is driven by a different clocking signal than that being used in the ATM network. For example, user equipment at the edges of the network can be driven by clock A, while the devices within the ATM network are being driven by clock B. Figure 8-10 shows such an operating scenario, in which a timing signal is provided to edge nodes independently from the ATM network.

Using Figure 8-10, assume that the user of edge node 1 wants to send CBR data to a user at edge
node 3. In this scenario, SRTS clocking works as follows:

1. Clock A is driving the devices within the ATM network.

2. At edge node 1, the user introduces CBR traffic into the ATM network according to clock B.

3. As edge node 1 segments the CBR bit stream into ATM cells, it measures the difference between user clock B, which drives it, and network clock A.

4. As edge node 1 generates the ATM cell stream, it incorporates this delta value into every eighth cell.

5. The cells then propagate through the network in the usual manner.

6. As destination edge node 3 receives the cells, this node not only reassembles the ATM cells into the original CBR bit stream, but also reconciles, or reconstructs, the user clock B timing signal from the delta value carried within every eighth ATM cell.

Thus, during SRTS clocking, CBR traffic is synchronized between the ingress (segmentation) side of the CES circuit and the egress (reassembly) side of the circuit according to user clock signal B, while the ATM network continues to function according to clock A.

Configuration Overview

Configuring SRTS clocking for CES requires the following steps:

Step 1 Configure the global clocking as described in the chapter "Network Clock Synchronization."

Step 2 From interface configuration mode, enable SRTS clocking mode on the CES interfaces.

Adaptive Clocking

The name adaptive clocking mode reflects the fact that the rate at which CBR data is propagated through an ATM network is driven essentially by the rate at which CBR data is introduced into the network by the user's edge equipment. The actual rate of CBR data flow through the network might vary from time to time during adaptive clocking, depending on how rapidly (or how slowly) CBR data is being introduced into the network. Nevertheless, CBR data transport through the network occurs in a "pseudo synchronous" manner that ensures the integrity of the data.

CES Configurations

This section provides some general guidelines and considerations when configuring CES connections. This section also includes examples of various types of unstructured and structured service connections you can configure for CES.

Before you Begin

Before you begin configuring physical interfaces and virtual circuits for CES operation, you should be aware of the information you need and the associated tasks:

You must have clocking properly configured for CES operations to be successful. Configuring synchronous clocking is described in the chapter "Network Clock Synchronization." Special clocking cases for CES are described in the "Network Clocking for CES and CBR Traffic" section in this chapter.

• Service type

You must decide whether to use unstructured or structured services. See the "Unstructured CES" section and the "Structured CES" section for descriptions.

• Physical level characteristics:

  • Framing type

  • Line buildout

  • Line code type

  • Loopback test method

Refer to your ATM switch router software documentation for details on configuring these and other physical level parameters.

• Cell Delay Variation (CDV)

CDV can be critical due to the delay-sensitive nature of CBR data; refer to the "About Cell Delay Variation" section.

• PVC (hard, soft)

You must decide whether to use hard or soft PVCs. Manual configuration is required for hard PVCs, while soft PVCs are set up through signaling. Also, soft PVCs can reroute in the event of failure, while hard PVCs cannot. For more information, see the "General Procedure for Creating Soft PVCs for CES" section.

About Cell Delay Variation

Cell delay variation (CDV) refers to the distortion caused by change in interarrival times between cells, also known as jitter, measured in microseconds.

Each end-to-end CES circuit exhibits delay characteristics, based on the following factors:

• The delay characteristics of the individual devices participating in the CES circuit.

Each network device contributes some increment of delay, reflecting the unique electrical characteristics of that device.

• The number of intermediate hops through which the CBR data must pass in traversing the network from source to destination.

• The type and speed of the trunk lines interconnecting the ATM networks.

• The volume of traffic being handled by the trunk lines at any given time; that is, the degree to which the network is experiencing congestion conditions.

The network designer or administrator calculates a CDV value for each hop in the data path as a means of establishing a maximum allowable CDV value for the network as a whole. To some degree, the network's maximum allowable CDV value is a measure of the network's expected performance. By establishing this CDV threshold for the network, appropriate buffer sizing can be derived for the network devices involved in any given CES circuit, ensuring that the network operates as expected.

In a CES module, for example, the maximum allowable CDV value for the network is used to determine an appropriate size (depth) for the SAR buffer built into the board. This sizing of the SAR buffer is done to prevent buffer overflow or underflow conditions. An overflow condition can cause a loss of frames, while an underflow condition can cause frames to be repeated.

The actual CDV value for a circuit varies according to the particular data path used for the circuit. Consequently, the depth of the SAR buffer increases or decreases in proportion to the CDV value for the CES circuit being set up.

Tips You can issue the CLI show ces circuit interface command in an unstructured circuit to measure the current CDV value. In many cases the configured default value is satisfactory.

For an unstructured hard permanent virtual circuit (PVC), the CDV value for the circuit (including all hops) must not exceed a maximum allowable CDV value.

For an unstructured soft PVC, the network automatically determines the best data path through the network and handles the routing of CBR traffic. The network accomplishes this task dynamically through the ATM connection admission control (CAC) mechanism. The CAC mechanism determines the best path through the network by executing a routing algorithm that consults local routing tables in network devices.

If the requested data path is equal to or less than the maximum allowable CDV value established by the network administrator, the connection request is granted. If the requested CES circuit exceeds the maximum allowable CDV value, the connection request is denied.

For example, when a user requests a connection from source node A at one edge of the network to destination node B at the opposite edge of the network, the CAC mechanism accounts for the CDV value for each hop in the requested connection to determine a suitable path through the network that does not exceed the network's maximum allowable CDV value.

General Procedure for Creating Soft PVCs for CES

You can create either hard private virtual circuits (PVCs) or soft PVCs, depending on your particular CES application requirements, for use in your CES operations. This section provides a general procedure for configuring soft PVCs for CES.

Step 3 Configure the destination (passive) side of the soft PVC.

Step 4 Retrieve the CES-IWF ATM address of the soft PVC's destination end. You can use the show ces address command to display the CES-IWF ATM addresses.

You must determine this address, as well as the VPI/VCI values for the circuit (see Step 5), and use these elements as part of the command string when you configure the source (active) end of the soft PVC (see Step 6).

Step 5 Retrieve the VPI/VCI values for the circuit. You can use the show ces circuit command to display the VPI/VCI values.

Step 6 Configure the source (active) end of the soft PVC last, using the information derived from Step 4 and Step 5.

You must configure the source end of the soft PVC last, because that end not only defines the configuration information for the source port, but also requires you to enter the CES-IWF ATM address and VPI/VCI values for the destination port.

T1/E1 Unstructured CES

This section provides an overview of the procedures for configuring CES modules for unstructured CES services.

The circuit you set up on a CBR port for unstructured service is always identified as circuit 0, since you can establish only one unstructured circuit on any given CBR port. An unstructured circuit uses the entire bandwidth of a T1 or E1 port, as follows:

Hard PVCs for Unstructured Services

A CES module converts CBR traffic into ATM cells for propagation through an ATM network. CBR traffic arriving on a given CES module port must first be segmented into ATM cells. This cell stream is then directed to an outgoing ATM port or CBR port using a PVC.

Figure 8-11 displays unstructured CES services configured on an ATM switch router switch using ATM and CES interface modules to create a hard PVC.

Configuring a hard PVC, such as the one shown in Figure 8-11, requires the following steps:

Step 1 Select the CES interface to configure and enter interface configuration mode.

Step 2 Configure the clock source for the CES interface.

If you are using synchronous or SRTS clocking mode, you must configure the global clocking distribution mode for the chassis and the clock source on the interface; you do not need to perform these steps if you are using adaptive clocking. For more information on configuring clock sources, see the chapter "Network Clock Synchronization."

Step 3 Configure the CES AAL1 service as unstructured.

Step 4 Configure the CES AAL1 clock mode.

See the "Network Clocking for CES and CBR Traffic" section.

Step 5 Configure the CES interface circuit identifier and specify a circuit name.

Step 6 Configure the hard PVC cross-connect to the ATM interface with VPI/VCI values.

Soft PVCs for Unstructured Services

In a soft PVC, as well as a hard PVC, you configure both ends of the CES circuit. However, a soft PVC typically involves CES modules at opposite edges of an ATM network, so a soft PVC can be set up between any two CES modules anywhere in your network.

For detailed soft PVC requirements, see the section "General Procedure for Creating Soft PVCs for CES."

The destination address of a soft PVC can point to either of the following:

• A port in the same ATM switch router chassis

• A port in any other CES module in the network

For simplicity, the procedure in this section assumes that you are creating a soft PVC between interface modules in the same ATM switch router chassis.

Figure 8-12 shows a logical representation of the soft PVC used in the following example procedure.

Configuring a soft PVC for unstructured CES services, such as the one shown in Figure 8-12, is a two-phase process:

• Phase 1---Configure the destination (passive) side of the soft PVC.

• Phase 2---Configure the source (active) side of the soft PVC.

Configuring the destination side of the soft PVC requires the following steps:

Step 1 Select the CES interface to configure and enter interface configuration mode.

Step 2 Configure the clock source for the CES interface.

If you are using synchronous or SRTS clocking mode, you must configure the global clocking distribution mode for the chassis and the clock source on the interface; you do not need to do these steps if you are using adaptive clocking. For more information on configuring clock sources, see the chapter "Network Clock Synchronization."

Step 3 Configure the CES AAL1 service as unstructured.

Step 4 Configure the CES AAL1 clock mode.

See the "Network Clocking for CES and CBR Traffic" section.

Step 5 Configure the CES interface circuit identifier and specify a circuit name.

Configuring the source side of the soft PVC requires the following steps:

Step 1 Display the CES-IWF ATM address for the destination end of the circuit.

Step 2 Enter global configuration mode, then select the CES interface to configure and enter interface configuration mode.

Step 3 Disable the CES circuit.

Step 4 Configure the soft PVC to the destination CES-IWF ATM address and VPI/VCI of the circuit.

T1/E1 Structured CES Services

This section provides an overview of the procedures you use when configuring CES modules for structured (N x 64 Kbps) CES services.

An important distinction between structured and unstructured CES services is that structured CES services allow you to allocate the entire T1/E1 bandwidth. Structured CES services only use the T1/E1 bandwidth actually required to support the active structured circuit(s) you configure. For example, configuring a CES module for structured services allows you to define multiple hard PVCs or soft PVCs for any CES T1 or E1 port.

Hard PVCs for Structured Services without CAS

This section describes configuring a hard PVC for structured CES services without channel-associated signaling (CAS).

Figure 8-13 illustrates a hard PVC for a structured CES connection configured with the following parameters:

• Four time slots (DS0 channels 1 to 3, and 7) are configured for a circuit named CBR-PVC-A.

• CAS is not used.

• ATM port 0/0/0 in the ATM switch router chassis is designated as the destination port of the hard PVC.

For more information on configuring clock sources, see the chapter "Network Clock Synchronization."

Step 5 Configure the CES AAL1 service as structured.

Configuring the hard PVC requires the following steps:

Step 1 Select the CES interface to configure and enter interface configuration mode.

Step 2 Configure the CES interface circuit identifier and list of T1 time slots that comprise the circuit.

Step 3 Configure the CES interface circuit identifier and specify a circuit name.

Step 4 Configure the hard PVC to the ATM interface with VPI/VCI values.

Soft PVCs for Structured Services without CAS

This section describes the procedures used to configure a soft PVC for structured service based on the following assumptions, as illustrated in Figure 8-14:

• Four time slots (DS0 channels) are configured for the soft PVC, as follows:

  • For circuit CBR-PVC-A: DS0 channels 1 to 3 and 7 are used on port CBR0/1/0

  • For circuit CBR-PVC-B: DS0 channels 10 to 13 are used on port CBR0/1/1

• Channel-associated signaling (CAS) is not used.

• The source (active) side of the soft PVC is named CBR-PVC-A.

• The destination (passive) side of the soft PVC is named CBR-PVC-B.

Soft PVCs for Structured Services with CAS

The procedures in this section build on the configuration information in the "Soft PVCs for Structured Services without CAS" section. However, this procedure enables channel associated signaling (CAS) for the soft PVC.

The following procedure is based on the following assumptions, as illustrated in Figure 8-15:

• One time slot is configured for the soft PVC:

  • For circuit CBR-PVC-A: DS0 channel 1 on port CBR0/1/0

  • For circuit CBR-PVC-B: DS0 channel 10 on port CBR0/1/1

• CAS is enabled for the circuit.

• The signaling mode for the CBR ports is set to robbedbit.

Multiple Soft PVCs on the Same CES Port

The procedures in this section describe creating more than one structured service PVC on the same T1 or E1 port. Figure 8-16 illustrates how you can configure multiple CES circuits on a single T1 or E1 port.

The following assumptions apply to creating multiple soft PVCs on the same T1 or E1 port, as illustrated in Figure 8-16:

• The source (active) side of a soft PVC named CBR-PVC-A is already created on port CBR0/1/0.

• The destination (passive) side of a soft PVC named CBR-PVC-B is already created on port CBR0/1/1.

• A new source (active) side of a soft PVC named CBR-PVC-AC is created on port CBR0/1/0 of the CES module, thereby creating a multiple CES circuit on this particular port.

• A new destination (passive) side of a soft PVC named CBR-PVC-CA is created on port CBR0/1/2 of the CES module.

For more information on configuring clock sources, see the chapter "Network Clock Synchronization."

Step 5 Disable the CES circuit.

Step 6 Configure the CES AAL1 service as structured.

Step 7 Configure the CES circuit identifier and list of time slot numbers that comprise the
CES circuit.

Step 8 Configure the CES circuit identifier and specify a circuit name.

Configuring the source side of multiple soft PVCs requires the following steps:

Step 1 Select the CES interface to configure and enter interface configuration mode.

Step 2 Configure the CES circuit identifier and list of time slot numbers that comprise the
CES circuit.

Step 3 Configure the CES circuit identifier and specify a circuit name.

Step 4 Exit interface configuration mode and display the following information:

• CES address for the destination end of the circuit

• Detailed soft PVC interface configuration information

Step 5 Enter global configuration mode, select the interface to configure, and enter interface configuration mode.

Step 6 Disable the CES circuit.

Step 7 Configure the soft PVC to the ATM interface with VPI/VCI values.

Soft PVCs for Structured Services with CAS and On-Hook Detection Enabled

This section outlines the additional steps that you must take to activate the on-hook detection (bandwidth-release) feature in a 1 x 64 structured soft PVC CES circuit.

Step 1 Configure the soft PVC with CAS enabled, as described in the "Soft PVCs for Structured Services with CAS" section.

Step 2 Select the CES interface to configure and enter interface configuration mode.

Step 3 Disable the CES circuit.

Step 4 Enable on-hook detection for the CES circuit.

Step 5 Reenable the CES circuit.

Simple Gateway Control Protocol

The Simple Gateway Control Protocol (SGCP) running on the ATM switch router, in combination with the Cisco VSC 2700, a virtual switch controller, provides switched voice on demand within a campus or metropolitan-area network. Designed to support common channel signaling protocols, this solution can eliminate the need for expensive tandem switches when interconnecting multiple PBXs.

Networks that include multiple PBXs require a tandem switch, which inter-PBX calls must traverse. To accommodate multiple PBXs, they might all be connected to the tandem switch. Or, as in Figure 8-17, the PBXs might be connected through the ATM network using CES interfaces and clear-channel T1 or E1 links. All inter-PBX calls are required to traverse the tandem switch. This scenario works well where the number of PBXs to be connected is low. As the size of the campus or MAN grow, however, the size of the required tandem PBX also grows, as does the number of primary rate interfaces (PRIs) required.

In Figure 8-18 the virtual switch controller (VSC) is connected to an ATM switch router. SGCP, running on both the VSC and the ATM switch router, carries VSC instructions to set up and tear down connections based on the signaling between the VSC and the PBXs. When a call must be established between any two PBXs, the VSC instructs the ATM switch router to provision a soft PVC (64 Kb CBR) between the appropriate two endpoints, providing bandwidth on demand. This reduces the total number of interfaces required and eliminates the need for a tandem PBX.

Additional Advantages

How it Works

SGCP controls Voice over IP gateways by an external call control element (called a call-agent). This feature has been adapted to provide control of ATM switch router CES circuits (called endpoints in SGCP). The resulting system (call-agents and gateways) allows for the call-agent to engage in common channel signaling (CCS) over a 64-Kbps CES circuit, governing the interconnection of bearer channels on the CES interface. In this system, the ATM switch router acts as a Voice over ATM gateway.

Figure 8-19 illustrates how 64-Kbps CCS channels on the CES T1 and E1 ports are backhauled or carried to the VSC 2700 units.

Posted: Mon Aug 16 14:05:51 PDT 1999
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