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ATM Interworking Quality of Service is also an issue. Depending upon a service provider’s backbone capabilities, a level of service available in one technology is not necessarily available in another. Remember, not all networks are created equal. The inequality of the networks is due to the implementation of ATM before standardized specifications, and service providers and OEMs stampeding to be "first-to-market" with ATM products and services. Service providers bought ATM switches several years ago from different OEMs that utilized proprietary designs in their equipment. Now, some service providers are finding it necessary to trash their previous ATM backbone to install ATM Forum standards compliant equipment. Anyway, check with each service provider; after all, there are not that many companies offering ATM services—yet. Determine which providers can offer the interworking required. As the ATM Forum standards effort progresses, technology interworking solutions will become as easy as booting up a computer.

A term that is occasionally seen in the ATM literature is "tunneling." Tunneling refers to the encapsulation of Frame Relay frames into ATM cells. The Frame Relay frame, including data and signaling information, is treated just like any other bit stream received, and completely encapsulated. The frame signaling information remains embedded in the ATM cells along with the data. The signaling for the ATM backbone network comes from the CPE aggregator, concentrator, or ATM switch.

The interface between a variable length frame-based technology and ATM is called a Data Exchange Interface (DXI). (See page 136.) The DXI is responsible for converting the variable length frame into the fixed length ATM cell and vice versa. The actual piece of equipment that performs the conversion is variously called a CSU/DSU, concentrator, or aggregator.

Quality of Service

When a user desires ATM service, the user must contract with an ATM service provider to carry the user’s data traffic over the service provider’s ATM network. One of the several choices the user must make when contracting ATM service is the level of Quality of Service (QoS) the user desires. Of course, network design requirements dictate the particular Quality of Service the user chooses. Remember, however, a contract that is legally binding must be negotiated and signed before any service provider will haul data over its ATM network. Contract lengths vary depending upon service provider and user and the amount of traffic expected. Typically, a service provider requires the user to sign a three-year contract. And we are discussing a contract that costs the typical user anywhere from several thousand dollars per month to $50,000 plus per month. It is not a good thing to be stuck in a contract that does not really provide the Quality of Service the network requires for optimum performance. Renegotiating the contract is always a possibility but the loser most likely is the user.

Automatic rerouting of links after link failure takes time, from 30 seconds for a small number of switches in the backbone to several minutes for a large number of switches. Rerouting occurs if there is a trunk or port failure or if the bit error rate exceeds predetermined thresholds for an extended period of time, usually two minutes or more. Rerouting is a function of switch manufacture. Since the service providers have installed ATM switches from various switch manufacturers, rerouting time varies from service provider to service provider.

Switched Virtual Circuits

Switched virtual circuits (SVCs) is a user on-demand connection. SVCs are only established when one party desires to transmit to another party, and disconnect occurs when the parties have completed the transmission. ATM layer service class agreements and negotiation between network elements require complex handling by the hardware and software interface components. ATM layer class and traffic descriptors for SVCs are the same as those for PVCs. The SVC connection is referred to as the switched virtual circuit connection, or SVCC. The SVP connection is referred to as the switched virtual path connection, or SVPC.

Two extremely important service features are necessary to make SVCs attractive: screening lists and source address authentication. Without these security features, no network manager worth their pay could allow an SVC connection to their network. Imagine the havoc if an unauthorized connection were allowed.

When the network receives a connection request, the source address authorization is checked. The calling party number in the setup message must match the ATM port address. If the two do not match, the connection is refused.

SVCs allow connections between two or more UNIs, or DXIs/FUNIs. The connection is made when the calling party signals the called party, and disconnect occurs when either party signals the other party of the end of the connection. Signaling is performed in-band and all data transfer occurs in real time. The in-band signaling guarantees the same circuit control over the integrity of the transmission as that provided by PVCs.

Switched virtual circuits (SVCs) offer the user PVCs on demand. The user establishes and tears down circuits using Q.2931 (ATM Forum’s UNI 3.1) signaling. Traffic descriptors and service classes prescribed for PVCs also are applicable for SVCs. The calling party can send the called party, in SETUP, the called party subaddress, calling party subaddress, AAL parameters, Broadband High Layer Information, and Broadband Low Layer Information. The called party can send the calling party, in CONNECT, AAL parameters and Broadband Low Layer Information.

SVCs support point-to-point connections, point-to-multipoint connections (also called multicasting), and multipoint-to-multipoint connections. SVCs also support simplex point-to-point, duplex point-to-point, and simplex point-to-multipoint transmission.

The calling party’s source address is authenticated. An added feature of SVCs is the ability to provide address screening to users. Address screening provides an imperative security feature that allows users to control who connects to their network. With address screening, only those connection requests with valid source addresses are accepted. Address screening lists using allowed or disallowed decisions can be kept. A calling party address list may contain up to 256 allowed/disallowed addresses. Also, a called party address list may contain up to 256 allowed/disallowed addresses. When a call is placed, the calling party switch determines if the address called is allowed or disallowed and only places the call if the called address is on the allowed screening list. The call is blocked if the address is on the disallowed list. When the called party receives the request to connect, the called party compares the calling party address to its allowed or disallowed screening list and accepts or denies the connection based upon the results of the screening.


Figure 7-10.  World wide ATM revenue

Currently, only two long-haul service providers, GTE and AT&T, offer SVCs but all the ATM players plan to offer SVCs by the end of 1998.


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