The Provider Edge: Layer 1, Layer 2, and the PSTN in .NET

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The Provider Edge: Layer 1, Layer 2, and the PSTN
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tools used by POP wholesalers Just as dial servers can either be operated by wholesalers or by a service provider, so can gateways The operator of a gateway, commonly a broadband wholesaler or CLEC, has to manage tens or hundreds of thousands of subscriber accesses as well as sort out the aggregate data streams from these subscribers and send them to the operator s own customers, which are ISPs and other service providers Efficient human interfaces and interfaces for carrier-specific scripts are essential for commercial success Service selection gateways (SSGs) connect data subscribers to ISPs and other data providers A more general term, media gateways (MGs) also provide conversion to and from nondata services such as voice and video MG is the more general term According to RFC 2805, MGs map or translate the media mapping between potentially dissimilar networks, one of which is presumed to be a packet, frame or cell network The most obvious MG application is between telephony switched circuit network (SCN) facilities, such as local loops, trunks, and channel banks, and an IP packet network Note that MG functions are perfectly applicable to situations where all interfaces are packetized, such as digital conference bridges Never forget the song lyrics from Cabaret: Money makes the world go round Reducing the labor cost of circuit provisioning is the first motivation for service gateways Once gateways were introduced, it quickly became
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GATEWAYS AND THE INTELLIGENT EDGE In addition to the simple conversion and management of services, gateways may themselves provide high-touch application services in a context often called the intelligent edge High-touch means that the gateway looks deeper into packets than do typical routers, and considers application layer content High-touch functions tend to fall into two categories: those that are primarily concerned with application session establishment and those that involve inspection of every packet Web caching, content-based redirection to servers, and possibly assignment of traffic to VPNs are session-oriented Firewalling, virus scanning, interactive voice response, encryption, and voice/video conferencing involve every packet An obvious application is placing a multicast server at the edge to service video users in a local area High-touch services usually require specialized server cards in the gateway or directly attached to it A gateway that performs high-touch services will also need to retain much more per-user state than a router, requiring much more memory and greatly complicating failover In other words, high-touch services are user-oriented rather than address-oriented
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obvious that they were natural homes for value-added data and voice services Another realization is that a gateway is most flexible when it is agnostic to the particular access technology Gateways can be modular devices that can accept dial access, ISDN, or broadband access technologies such as xDSL, cable, fixed wireless, free-space optical, and cellular Be aware that traditional layer 2 techniques connecting the access provider to the ISP are not necessarily the most flexible now available See Scalability with MPLS in 10
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Gateway Architecture
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Grooming can certainly be part of the functionality of a gateway, as can routing These gateways, however, have more functionality than either traditional DACSs or traditional routers Standard routers are connectionless, while the very essence of a service gateway is that it monitors connection state Conventional routers do not need to keep track of individual users and individual user sessions The Multiservice Forum (MSF) has been established to deal with implementation issues for distributed switching systems including backbones of ATM, frame, and IP, and access technologies including analog, TDM, xDSL, wireless, and cable Its architecture is layered into five planes (Figure 714) The planes are discussed later in this section, along with certain functions that span planes One of the issues the MSF architecture deals with is scalability, and one of the fundamental approaches for making gateways scalable is separating control and management from switching As long as there is sufficient redundancy for carrier-grade reliability, there need not be full management on every dataprocessing element
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STATES Communications protocols can be hard-state, soft-state, or stateless A hard-state protocol (for example, Q931 or TCP) has explicit connection and disconnection phases, during which resources are committed A soft-state protocol (for example, IGMP or RSVP) has an implied connection setup in the sense that the first message sent in the protocol creates a connection The resources affected by the protocol monitor the connection for periodic retransmissions of the protocol message, and, if no messages are received before a timer expires, they assume a disconnection and release resources Stateless protocols (for example, IP or UDP) do not imply any resource commitment Protocol messages are handled by processing elements, which retain no knowledge of having handled previous knowledge A processing element, however, must retain knowledge of the existence of a connection established by a hard- or soft-state protocol
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