|
|
This document provides an introduction to Cisco's Open Packet Telephony architecture and its key element, the virtual switch network.
This document contains the following sections:
Use this document with the Cisco Configuration Tool Guide, Cisco Dial Plan Provisioning Guide, Cisco Software Operations and Maintenance Guide, and Cisco VSC2700 Network Solution Integration Guidelines.
In today's telecommunications arena, data is growing faster than voice. As a result, customers are looking for networking solutions which include both voice and data. These customers also want to be in a position to support emerging technologies. Cisco is a decision-making member of the forum developing new infrastructures for supporting multiservice networks. To address our customer's needs, Cisco is implementing the Cisco Open Packet Telephony (OPT) architecture as the structure within which a multiservice network can be implemented.
The most important characteristic of the OPT architecture is the clean separation of call control from the switching fabric. This split is achieved by the definition of an open interface between the two functional layers. Historically, the call control and switching function have been tightly coupled in traditional TDM systems. This coupling left service providers with little control and almost complete dependence on the switching manufacturers for enhancements.
This document discusses Cisco's effort to provide a multiservice network in which switched voice services transport over data networks. This solution uses a Cisco virtual switch controller (VSC) together with Cisco gateways to create a virtual switch network performing the functions of an asynchronous transfer mode (ATM) transmission network and a tandem switch. Cisco's OPT provides a framework for the integration of traditional telephony over data infrastructures. This integration is handled efficiently with minimal overhead and allows for the seamless integration of traditional voice networks.
Packet telephony routes information in bundles called packets. Packet routing eliminates the need to physically switch circuits. These packets are assembled, transmitted, and routed through packet switches which manage the packet's routing.
Packet technology allows you to add transmission of Voice Over Packet Network (VoPN) to your existing transmission of data, video, and audio. Packet telephony takes advantage of the statistical nature of the call process in provisioning network resources (that not all of the channels on an DS1 or a higher capacity E1 or T1 trunk would have calls in-progress at one time.) This capability reduces required transmission bandwidth and allows dynamic resource allocation inside the packet network. This ability is ideal for supporting the bursty traffic associated with data transmission.
Packet telephony helps lower your operating costs because it is easier to operate and maintain. Revenues are higher because new value added services can be incorporated at lower costs.
Cisco's Open Packet Telephony architecture provides you with the tools necessary to effectively build state of the art telecommunication networks with the ability to effectively accommodate voice, data, video, and audio.
Open refers to the use of established industry standards regarding network interconnections rather than using proprietary interfaces. Cisco's OPT can coexist with your existing technology while helping you migrate to the next level of network interoperability.
Cisco's OPT uses communication and control protocols which provide open interfaces to your operations support systems and service creation environments. It also consists of universal building blocks that allow you to build reliable networks of any scale. Cisco uses the inherent strengths of packet technology and augments them by the using virtual switches.
Cisco's OPT can be deployed with any packet switching fabric in the information delivery layer. Cisco's patented Any to Any protocol conversion engine can talk to existing telecom switching equipment around the world. This ability preserves the investment in existing technology and provides an easy migration path from time-division multiplexing to virtual switching technology.
The concept of a virtual switched network relies on the premise that existing voice networks are based on TDM technology and that these technologies can coexist with, migrate to, or be replaced by packet technologies. Another key feature of packet networks is that their configurations contain telecommunication distributed network elements. Components of a virtual switched network are:
This collection of network components constitutes a virtual switched network.
Cisco's virtual switch network uses a virtual switch controller (VSC) to provide the call control functions for the virtual switch. To provide a frame of reference for readers who are more familiar with TDM networks, the VSC manages the software elements which would be associated with the service switching point (SSP).
Within the various protocols being incorporated into the VSC, many different titles are applied to the functions that the VSC performs. Following are the most common references:
The Cisco VSC architecture provides you with a large number of open interfaces and the ability to program user specific functions and services into it.
Cisco's virtual switch architecture is has three logic layers as does Cisco's virtual switch architecture. The three logic layers are:
The three layers are organized hierarchically. The transport layer is the lowest while the service layer is the highest. Each layer represents a different function of voice service and interacts with the other layers through specific interfaces.
The VSC controls the voice gateways located at the network's edge. These gateways use voice cards to terminate either E1 or T1 trunks which is why they are called voice gateways. In Figure 3, the voice gateways shown are:
All gateways are controlled by the VSC using the Simple Gateway Control Protocol (SGCP) interface. Any of the Cisco units listed above can serve as a voice gateway within an ATM packet network.
SGCP is designed to control Voice over IP (VoIP) gateways by an external call control element (called a call-agent in SGCP). Cisco has adapted SGCP to control ATM Circuit Emulation Services (CES) circuits (called endpoints in SGCP).
The resulting system allows the call-agent, in this case the Cisco VSC2700, to engage in common channel signaling (CCS) over a 64kbps CES circuit. This CCS engagement governs the interconnection of bearer channels on the CES interface, in this network, the voice gateways. The following SGCP messages are exchanged between the Cisco VSC2700 and the voice gateways. Communication between the gateways and the VSCs uses the ATM network as its transport layer. Table 1, displays what SGCP messages are sent to the VSC.
| Message | Usage |
|---|---|
Create Connection | Used by Cisco VSC2700 to set up a reserve Soft-PVC ATM CES connection on Cisco voice gateways. |
Modify Connection | Used by the Cisco VSC2700 to nail down the Soft-PVC ATM CES connection on Cisco voice gateways. |
DeleteConnection | Used by the Cisco VSC2700 to tear down Soft-PVC at call completion. |
The transport layer executes commands received from the VSC to establish virtual connections, see Figure 3. The transport layer may be an IP or ATM network. The example in Figure 3, is an ATM network. Customer equipment, private branch exchange (PBX) is connected at this level. A very important role is performed by the gateways within the transport layer. Gateways provide connections between networks using different communication protocols. Cisco gateways provide translations between these diverse networks. Cisco voice gateways manage:
The call control layer is where the call logic is controlled, see Figure 4. Call logic functions are:
This layer is responsible for processing call requests. It also directs the connection control layer to establish the appropriate bearer connections. End-to-end call context is maintained across the network at this level. Depending on the type of user request, the call control logic may initiate a service request to the service plane.
The call control logic also directs the connection control layer to establish appropriate bearer connections. In Figure 4, the gateways are controlled using SGCP or Media Gateway Control Protocol (MGCP). MGCP is the next evolution of SGCP. This protocol is still being defined.
In a network with more than one VSC, communications between the VSCs is conducted using Cisco-Integrated Services Digital Network User Part (C-ISUP) protocol.
The Integrated Services Digital Network (ISDN) User Part (ISUP) defines the protocol used to set-up, manage, and release trunk circuits that carry voice and data between terminating line exchanges (e.g., between a calling party and a called party). ISUP is used for both ISDN and non-ISDN calls. However, calls that originate and terminate at the same switch do not use ISUP signaling.
Communications between VSCs is performed using Cisco-Integrated Services Digital Network User Part (C-ISUP) protocol. C-ISUP is an ISUP based protocol that has been extended to transport information related to the IP and ATM networks. C-ISUP uses a subset of ISUP messages. C-ISUP communication between VSCs uses a reliable IP as its transport layer.
The C-ISUP message formatting contains many of the same characteristics of the existing ISUP message format. The primary difference between existing ISUP and C-ISUP messages is in the area of the routing label and a new information element (IE) to carry circuit unit (CU)-related data between VSCs. The own point code (OPC) and the destination point code (DPC) in the routing label is replaced with the originating virtual switch controller address and the destination virtual switch controller addresses. The CIC is replaced with a Global Call Reference (GCR) generated by the originating virtual switch controller. Table 2 lists the ISUP messages supported in C-ISUP.
| Message | Usage |
|---|---|
Initial Address Message (IAM) | Similar to the ISUP IAM. In addition to ISUP IEs for A-number, B-number, and bearer capabilities, the IAM carries a Connection Descriptor end-to-end. This field is necessary for virtual switch controllers to communicate connection information to CUs. An ISUP Generic Digits IE is used to deliver this information. |
Address Complete Message (ACM) | Similar to the ISUP ACM. In addition to ISUP IEs the ACM carries a Connection Descriptor end-to-end. This field is necessary for virtual switch controllers to communicate connection information to CUs. C-ISUP supports the ISUP Generic Digits IE as an extension to the standard IEs defined in ISUP. |
Subsequent Address Message (SAM) | Used similar to the ISUP ANM. |
Answer Message (ANM) | Similar to the ISUP SAM. In addition to ISUP IEs, the ANM carries a connection descriptor end-to-end. This field is necessary for virtual switch controllers to communicate connection information to CUs. C-ISUP supports the ISUP generic digits IE as an extension to the standard IEs defined in ISUP. |
Release Message (REL) | Similar to the ISUP REL. In addition to ISUP IEs the REL message will carry a Connection Descriptor end-to-end. This field is necessary for virtual switch controllers to communicate connection information to CUs. An ISUP generic digits IE is used to deliver this information. |
Release Complete Message (RLC) | Similar to the ISUP RLC. Presently no additional IEs are required for C-ISUP. |
The call control layer processes the signaling data within the VSC or between multiple VSCs in large networks. The VSC also performs the following functions:
Service logic is managed from the service layer. Service logic is administered through an existing intelligent network or third party services application where service logic is applied.
In Figure 5, the VSC recognizes a request for additional service at a specific stage of a call and triggers service execution logic from the service layer using standard telecommunications interfaces like Transaction Capabilities Application Part (TCAP).
TCAP supports the exchange of non-circuit related data between applications across an SS7 network using the signaling connection control part (SCCP) connectionless service. Queries and responses sent between service switching points (SSPs) and service control points (SCPs) are carried in TCAP messages. For example, an SSP sends a TCAP query to determine the routing number associated with a dialed 800/888/877 number and to check the personal identification number (PIN) of a calling card user. In mobile networks (IS-41 and GSM), TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming.
The virtual switch controller (VSC) is the heart of Cisco's Open Packet Telephony. It is a software application running on general purpose and special fault-tolerant computer platforms. This flexibility permits the creation of a reliable network of any size. An enterprise network can easily be upgraded to the scale and grade of a national carrier.
When signaling is received through ISDN-type connections, it is directed to the VSC by gateways. Any to Any signaling protocol conversion technology allows the VSC to understand and communicate with many different types of networks and telecommunications equipment.
These protocols are stored in the protocol library within the VSC. New protocols are constantly being developed to reflect customer needs for communicating with new networks. As a result of this ability to interface with a great variety of protocols, one VSC can simultaneously speak to several gateways interfacing networks with different signaling protocols. It can also control calls initiated on a PBX in Asia and terminated on a IP phone in the US without the need for any additional equipment. For each call, the VSC generates the necessary billing and usage statistics. The VSC also provides a set of tools for enhanced system management as shown in Figure 6.
When the VSC receives the initial call processing request, it immediately begins call control functions. As shown in Figure 6, there are a large variety of switching networks and protocols available for its use. To handle a call request, the VSC begins an intricate control process requiring the management of many parameters. Call originating messages are converted into the universal call model. This is the key to the VSC multi-protocol conversion capability. Call data is then analyzed using sophisticated routing algorithms. The VSC communicates with the gateways and PSTN switches on both the originating and terminating ends of the call to assure that the proper call variables are established and maintained. It also maintains routing and bearer circuit tables in its memory so that it can make hundreds of routing decisions per second while still assuring that these decisions conform with existing telecom network regulations.
An OPT network may employ multiple VSCs to match customer requirements for optimal traffic flow and robustness. VSCs communicate over C-ISUP. The network uses a reliable transport protocol to deliver C-ISUP messages between VSCs.
The software architecture of the Cisco VSC2700 is shown in Figure 6.
Call processing is the heart of the VSC and consists of a number of subcomponents which are shown in Figure 7.
The Call Manager is the component that creates and manages all call instances that exist on the VSC platform. If a message associated with an existing call instance arrives at the call manager then the message routes to the existing call instance. If no call instance exists then a new one is created. In this case a newly created call instance consists of an originating call model processed by originating call control (OCC - state machine that defines the originating protocol) and a universal call model (UCM - internal generic state machine). The terminating call model processed by the terminating call control (TCC) is created dynamically once a routing decision has been made and the terminating signaling channel and protocol has been determined.
Following receipt of the initial setup message the OCC unpacks the message and stores all data in call context component (an area of memory allocated for all parameterized information contained in the voice signaling protocols). The OCC then sends an internal signal to the UCM to start call processing which causes the analysis and routing functions to be invoked. These functions return a routing result identified as being either locally managed by this VSC or remotely managed by another VSC.
If the routing result indicates local management, the route identifier is resolved by determining the protocol assigned to that route. The appropriate TCC starts invoking the connection plane management. If the routing result indicates remote management, a C-ISUP TCC is started to merge the C-ISUP signaling with the remote VSC.
All protocol conversion functions are performed in the UCM automatically as a result of the interactions of OCC and TCC with the generic UCM.
This software function enables the VSC to perform analysis and modification functions on received address digits, both calling and called party numbers. These functions are table driven from data tables located on the VSC platform, the formatted data files are created by provisioning tools from structured input files (created by the administrator). The following basic capabilities are provided:
The route selection results from the analysis function, in this release, provides a single option. It is not possible to specify multiple options or alternate routes. This capability will be added in the next VSC release. For the initial installations of the Cisco VSC2700, the route is directed to a device controlled by the current VSC or to another VSC for continuing processing.
Connection plane management is the software function that provides access from the
Cisco VSC2700 to the following voice gateways:
Essentially the UCM (VSC internal call model) provides the following instructions to the gateways:
These signals are mapped into SGCP commands and issued to the gateway which is expected to respond.
The VSC has a number of management processes that run to perform a number of tasks, these are summarized below:
The first OPT application as shown in Figure 8, is a configuration designed to be used by enterprise or small carriers. This configuration uses Cisco's Catalyst family of ATM multiservice switch routers acting as TDM to ATM gateways. By including the Cisco VSC2700 in the network, this configuration now has the intelligence to handle an ATM campus network with dozens of buildings and tens of thousands of phone extensions. This network can connect both private branch exchange (PBX) and local-area networks (LANs) to the public switched telephone network (PSTN).
Between the Cisco VSC2700 and the Cisco Catalyst 8510 MSR, Cisco Catalyst 8540 MSR, or
Cisco Catalyst 5500 ATM switches, communication is performed using SGCP. SGCP uses IP transport. This can be an overlaid ATM network or an independent network. The communications network in Figure 8, is considered a campus/metropolitan-area network (MAN) ATM.
The customer realizes savings by maintaining only one multiservice network. They still have the flexibility to provide bandwidth for their voice on demand sites. Having the capability of dynamic bandwidth allocation saves significant resources by reducing the number of required PBX interfaces and in some cases eliminating the need for a tandem switch altogether.
In Figure 9, the customer's PBX equipment and PSTN central office switch are connected to the VSC network by ISDN primary rate interface (PRI) comprised of 23 bearer (B) channels and one data (D) channel for T1 networks. In an E1 network, there are 30B channels and 1D channel. The signaling link from the gateway to the VSC is comprised of only D channels because the D channel carries the call information and signaling. Only the D channels route through the gateways to the VSC, the call controller. For the initial VSC application, the gateway units are:
T1s or E1s enter the Cisco gateways and the signaling D channel is separated from the B channels and delivered to its controlling VSC directly or indirectly by soft PVCs in the ATM network. The B channels are then packetized and sent into the ATM network. An SGCP command to the gateways causes the establishment of a bearer connection between them. If a routing decision is made to route to another VSC, the communication between the VSCs is conducted via C-ISUP.
The number of VSCs on the network is determined by required network performance (number of calls processed per second) and the network's topology. The number of interfaces, PBX to gateway and gateway to ATM network is determined by expected traffic volume. There is no need to have dedicated connections for establishing a path to another location. Consolidated data streams from each site can be inserted into an ATM network through a Cisco router with an ATM interface.
Cisco Connection Online (CCO) is Cisco Systems' primary, real-time support channel. Maintenance customers and partners can self-register on CCO to obtain additional information and services.
Available 24 hours a day, 7 days a week, CCO provides a wealth of standard and value-added services to Cisco's customers and business partners. CCO services include product information, product documentation, software updates, release notes, technical tips, the Bug Navigator, configuration notes, brochures, descriptions of service offerings, and download access to public and authorized files.
CCO serves a wide variety of users through two interfaces that are updated and enhanced simultaneously: a character-based version and a multimedia version that resides on the World Wide Web (WWW). The character-based CCO supports Zmodem, Kermit, Xmodem, FTP, and Internet e-mail, and it is excellent for quick access to information over lower bandwidths. The WWW version of CCO provides richly formatted documents with photographs, figures, graphics, and video, as well as hyperlinks to related information.
You can access CCO in the following ways:
For a copy of CCO's Frequently Asked Questions (FAQ), contact cco-help@cisco.com. For additional information, contact cco-team@cisco.com.
Cisco documentation and additional literature are available in a CD-ROM package, which ships with your product. The Documentation CD-ROM, a member of the Cisco Connection Family, is updated monthly. Therefore, it might be more current than printed documentation. To order additional copies of the Documentation CD-ROM, contact your local sales representative or call customer service. The CD-ROM package is available as a single package or as an annual subscription. You can also access Cisco documentation on the World Wide Web at http://www.cisco.com, http://www-china.cisco.com, or http://www-europe.cisco.com.
If you are reading Cisco product documentation on the World Wide Web, you can submit comments electronically. Click Feedback in the toolbar and select Documentation. After you complete the form, click Submit to send it to Cisco. We appreciate your comments.
Posted: Fri Oct 29 14:49:59 PDT 1999
Copyright 1989-1999©Cisco Systems Inc.