Figure 3-6 Enterprise gateways on a VoIP network in Software

Drawer QR Code ISO/IEC18004 in Software Figure 3-6 Enterprise gateways on a VoIP network

Figure 3-6 Enterprise gateways on a VoIP network
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Enterprise Gateways Enterprise gateways aggregate legacy telephone infrastructures for interface with VoIP networks (see Figure 3-6). This is usually done by connecting a gateway to the trunk side of a legacy PBX. Users retain their existing handsets. This has the effect of making VoIP indistinguishable to the end user. The business need not train its staff to use new hardware or software to work their existing telephone handsets. This option also offers investment protection in that the business retains its expensive PBX and PBX-associated telephone handsets. The only thing that has changed is that the company reduces or eliminates interoffice phone bills. Enterprise gateways are usually configured in multiples of T1 or E1 cards in a single chassis that interface with the trunk side of the PBX. The T1 trunks connect to the line side of the gateway. The trunk side of the gateway is its Ethernet connection to a router if a router is not built into the gateway. Carrier-Grade Gateways The early applications for VoIP gateways were for international long-distance bypass and enterprise interoffice long distance. Success in these applications led to a demand for expanded density gateways for carrier operations (see Figure 3-7). These gateways needed to be densely populated (have enough DS0s or ports) enough to
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Softswitch Architecture or It s the Architecture, Stupid!
Softswitch Architecture or "It's the Architecture, Stupid!"
Figure 3-7 VoIP gateways in carrier-grade applications
Application Server
Data
Softswitch
IP Network
VOIP Gateway
VOIP Gateway
Telephone Class 4 or 5 Switch
Telephone Class 4 or 5 Switch
interface with Class 4 and Class 5 switches (up to 100,000 DS0s in one node with an OC-3 trunk-side interface). These switches also had to offer the reliability to interface with a circuit switch that boasted five 9s of reliability. Another requirement was that the switches be certified as being NEBS 3 compliant, a requirement for any platform to be installed in a central office. Network Equipment Building Standards (NEBS) addresses the physical reliability of a switch. It is contained in Telcordia specification SR 3580, an extensive set of rigid performance, quality, safety, and environmental requirements applicable to network equipment installed in a carrier s central office. Nearly all major carriers in North America require that equipment in their central offices or switching locations undergo rigorous NEBS testing. Tests include electrical safety, immunity from electromagnetic emissions, lightning and power faulting, and bonding and grounding evaluations. Equipment must meet physical standards as well, including temperature, humidity, altitude testing, fire resistance (usually by destructive burning), earthquake vibration resistance, and a battery of other rigid tests. NEBS compliance also means backup and disaster recovery strategies, including ensuring access to mirror sites, fire and waterproof storage facilities for critical databases and configuration backup information, and an uninterruptible power supply (UPS) to prevent network outages due to power failures.
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Softswitch Architecture or It s the Architecture, Stupid!
3
Switching
Soft switching is a break-through technology that has empowered VoIP to eclipse TDM as a telephony technology. Prior to the development of the softswitch, VoIP was handicapped by a lack of intelligence necessary to route calls across the network. Without this intelligence, an evolution to an IP alternative to the PSTN would not be possible.
Softswitch (Gatekeeper and Media Gateway Controller)
A softswitch is the intelligence in a network that coordinates 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 it is released by the originating or terminating user. 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 and finally, 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, Signaling System 7 (SS7), and H.323. For media control, the predominant signaling protocol is the Media Gateway Control Protocol (MGCP). As a point of introduction to softswitch, it is necessary to clarify the evolution to softswitch and define media gateway controllers (MGCs) and gatekeepers, 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
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Softswitch Architecture or It s the Architecture, Stupid!
Softswitch Architecture or "It's the Architecture, Stupid!"
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 local area networks where an H.323 gatekeeper would manage activities in a zone. 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 and not a physical entity. The functions of a gatekeeper are address translation (a name or email address for a terminal or gateway and a transport address) and admissions control (authorizing 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 capability 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 was born the softswitch. A significant market driver for softswitch is protocol intermediation, which is 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 softswitch function 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. Softswitch provides usage statistics to coordinate the 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 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 colocated with other components of the softswitch architecture.
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