Switching

In the PSTN, the switching function is performed in the central office, which contains a Class 5 switch for local calls and a Class 4 switch for long-distance calls. A Class 5 switch can cost upwards of tens of millions of dollars and is very expensive to maintain. This economy of scale has kept competitors out of the local calling market. A new technology known as softswitch is far cheaper in terms of purchase and maintenance. Potentially, softswitch enables a competitive service provider to offer its own service without having to route calls through the incumbent service provider's central office. The following pages describe softswitch.

Softswitch (a.k.a. Gatekeeper or Media Gateway Controller [MGC])

A softswitch is the intelligence in a VoIP network that coordinates the call control, signaling, and features that make a call across a network or multiple networks possible. Primarily, a softswitch performs call control. Call control performs call setups and teardowns. Once a call is set up, connection control ensures that the call stays up until the originating or terminating user releases it. Call control and service logic refer to the functions that process a call and offer telephone features. Examples of call control and service logic functions include recognizing that a party has gone off hook and that a dial tone should be provided, interpreting the dialed digits to determine where the call is to be terminated, and determining if the called party is available or busy. Such functions also include recognizing when the called party answers the phone and when either party subsequently hangs up, and recording these actions for billing.

A softswitch coordinates the routing of signaling messages between networks. Signaling coordinates actions associated with a connection to the entity at the other end of the connection. To set up a call, a common protocol must be used that defines the information in the messages and which is intelligible at each end of the network and across dissimilar networks. The main types of signaling a softswitch performs are peer to peer for call control and softswitch to gateway for media control. For signaling, the predominant protocols are SIP, SS7, and H.323. For media control, the predominant signaling protocol is MGCP.

As a point of introduction to softswitch, it is necessary to clarify the evolution to softswitch and define Media Gateway Controller (MGC) and gatekeeper, which were the precursors to softswitch. MGCs and gatekeepers (essentially synonymous terms for the earliest forms of softswitch) were designed to manage low-density (relative to a carrier-grade solution) voice networks. MGC communicates with both the signaling gateway and the media gateway to provide the necessary call-processing functions. The MGC uses either MGCP or MEGACO/H.248 (described in a later chapter) for intergateway communications.

Gatekeeper technology evolved out of H.323 technology (a VoIP signaling protocol described in the next chapter). As H.323 was designed for LANs, an H.323 gatekeeper can only manage activities in a zone (read LAN but not specifically a LAN). A zone is a collection of one or more gateways managed by a single gatekeeper. A gatekeeper should be thought of as a logical function, not a physical entity. The functions of a gatekeeper are address translation (that is, a name or e-mail address for a terminal or gateway and a transport address) and admissions control (it authorizes access to the network).

As VoIP networks got larger and more complex, management solutions with far greater intelligence became necessary. Greater call-processing power became necessary, as did the ability to interface signaling between IP networks with the PSTN (VoIP signaling protocols to SS7). Other drivers included the need to integrate features on the network and interface disparate VoIP protocols. Thus, softswitch was born.

A significant market driver for softswitch is the protocol intermediation necessary to interface H.323 and SIP networks, for example. Another market driver for softswitch is to interface between the PSTN (SS7) and IP networks (SIP and H.323). Another function for softswitch is the intermediation between media gateways of dissimilar vendors. Despite an emphasis on standards such as H.323, interoperability remains elusive. A softswitch application can overcome intermediation issues between media gateways. More information on VoIP protocols and signaling IP to PSTN is provided in following chapters.

The softswitch provides usage statistics to coordinate billing, track operations, and administrative functions of the platform while interfacing with an application server to deliver value-added subscriber services. The softswitch controls the number and type of features provided. It interfaces with the application server to coordinate features (conferencing, call forwarding, and so on) for a call.

Physically, a softswitch is software hosted on a server chassis filled with IP boards and it includes the call control applications and drivers.[5] Very simply, the more powerful the server, the more capable the softswitch. That server need not be co-located with other components of the softswitch architecture.

Other Softswitch Components

The key advantage of softswitch over its circuit-switched predecessor is that it utilizes distributed architecture. That is, its components do need to be co-located. Those components include the signaling gateway, media gateway, and application server (see Figure 6-5).

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Figure 6-5: Relationship of softswitch components

Signaling Gateway Signaling gateways are used to terminate signaling links from PSTN networks or other signaling points. The SS7 signaling gateway serves as a protocol mediator (translator) between the PSTN and IP networks. That is, when a call originates in an IP network using H.323 as a VoIP protocol and must terminate in the PSTN, a translation from the H.323 signaling protocol to SS7 is necessary in order to complete the call. Physically, signaling function can be embedded directly into the MGC or housed within a standalone gateway.

Media Gateway The media gateway converts an analog or circuit-switched voice stream to a packetized voice stream. Media gateways rank from one- or two-port residential gateways to carrier-grade platforms with 100,000 ports. The media gateway can be located at the customer's premises or co-located at the central office.

Application Server The application server accommodates the service and feature applications made available to a service provider's customers. Examples include call forwarding, conferencing, voice mail, forward on busy, and so on. Physically, an application server is a server loaded with a software suite that offers the application programs. The softswitch accesses these and then enables and applies them to the appropriate subscribers as needed.

A softswitch solution emphasizes open standards as opposed to the Class 4 or 5 switch that historically offered a proprietary and closed environment. A carrier was a "Nortel shop" or a "Lucent shop." No components (hardware or software) from one vendor would be compatible with products from another vendor. Any application or feature on a DMS-250, for example, had to be a Nortel product or specifically approved by Nortel. This usually translates into less than competitive pricing for those components. Softswitch open standards are aimed at freeing service providers from vendor dependence and the long and expensive service development cycles of legacy switch manufacturers.

Concern exists among service providers as to whether a softswitch solution can transmit a robust feature list identical to those found on a 5ESS Class 5 switch, for example. Softswitch offers the advantage of allowing a service provider to integrate third-party applications or even write their own while interoperating with the features of the PSTN via SS7. This is potentially the greatest advantage to a service provider presented by softswitch technology.

Features reside at the application layer in softswitch architecture. The interface between the Call Control Layer and specific applications is the Application Program Interface (API). Writing and interfacing an application with the rest of the softswitch architecture occurs in the Service Creation Environment.

VoIP and Softswitch Pave the Way for Vo802.11

A number of attempts have been made to deploy voice services via WLL, that is, using wireless technologies (not 802.11) to offer telephone service in underserved or third-world markets. As these services have been limited to voice services, they have not caught on with a mass market. 802.11 is different in that it is a protocol for Ethernet over a wireless medium. The building blocks for a potential alternative to the PSTN now fall into place. Not only does an 802.11 network offer a potential bypass of the PSTN for voice services, it also offers broadband Internet and its incumbent suite of services.

[5]Bill Douskalis, IP Telephony—The Integration of Robust VoIP Services (New Jersey: Prentice Hall, 2000), 9.



Wi-Fi Handbook(c) Building 802.11b Wireless Networks
Wi-Fi Handbook : Building 802.11b Wireless Networks
ISBN: 0071412514
EAN: 2147483647
Year: 2003
Pages: 96

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