Figure 2-5 ATM cell format at a UNI and b NNI.
The last header field is the Header Error Check HEC field which allows error detection and error correction in the header. No error checking or correction applies to payload data as it passes through the
network. They are left to higher level protocols at the end nodes since the ATM switch has no knowledge of the user data content.
IV. Signaling in ATM Networks
A. Introduction
There are a variety of technologies available to adapt to these changes; however, ATM is the most promising. Designed for a wide range of traffic types, including voice, video and data, ATM is best
qualified for broadband ISDN, and it can scale rapidly to meet the growing demand for bandwidth.
Until recently, ATM networks have been run using permanent virtual circuits PVCs. End customers of any future broadband ISDN network, however, will want bandwidth on demand. This only becomes
possible with the introduction of switched virtual circuits SVCs.
Permanent virtual connections: For permanent virtual connections virtual path or virtual channel, the service contract is defined at the user end-points and along the route that has been selected for the
connection using the provisioning capabilities of the network vendor.
Switched virtual connections: Switched virtual connections enable the network to allocate bandwidth on demand. End users make their service request to the network using functions provided by the end
system for accessing the ATM user-to-network interface UNI.
ATM networks with SVC capabilities have advantages over today’s PVC-only networks. An end customer with SVCs only pays for the connection time and the bandwidth that is used. While there will
always be customers who require PVC services, SVC services will offer economies in many cases depending on the length of time of the connection and tariff rates.
Consequently, the customer base is expected to grow rapidly. Carriers will be able to be more flexible in the way they operate the networks, and their costs will be lower since less administrative intervention
will be necessary. Driven by the obvious market and administrative advantages, carriers are demanding SVC-enabled ATM networks.
Despite these obvious business drivers there are a number of significant technical challenges posed by SVCs, especially in the area of signaling and routing. Information for connection control, traffic
management and carrier-specific service features must be transferred between exchanges using signaling. The successful integration of high speed ATM networks with the various legacy data and
telephone networks will place high demands on signaling throughout the ATM network. Routing is also critical for traffic control and will play an important role in protecting against traffic overload in SVC-
enabled ATM networks.
In summary, if carriers are to meet the demand for switched bandwidth services, their core and network access ATM equipment will need to be able to handle all widely used signaling interfaces while
accommodating interworking between the individual variants.
This section presents the signaling systems that are currently in use. It also provides a glimpse into the complexity of future networks and the demands that this will place on signaling in general.
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Copyr ight © CRC Pr ess LLC
by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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B. Signaling Mechanisms
