PDF of this tutorialThe ATM Forum has identified a cohesive set of specifications that provide a stable ATM framework. The first and most basic ATM standards are those that provide the end-to-end service definitions as described in Topic 4. An important ATM standard and service concept is that of service interworking between ATM and frame relay (a fast-growing pervasive service), whereby ATM services can be seamlessly extended to lower-speed frame-relay users. Frame relay is a network technology that is also based on virtual circuits using variable-length frame transmission between users.
ATM user network interface (ATM UNI) standards specify how a user connects to the ATM network to access these services. A number of standards have been defined for T1/E1, 25 Mbps, T3/E3, OC–3 (155 Mbps) and OC–12 with OC–48 (2.4 Gbps) in the works. OC–3 interfaces have been specified for use over single-mode fiber (for wide-area applications) and over unshielded twisted pair or multimode fiber for lower-cost, in-building applications.
The following two ATM networking standards have been defined that provide connectivity between network switches and between networks:
- broadband intercarrier interface (B–ICI)
- public network-to-network interface (P–NNI)
P–NNI is the more feature-rich of the two and supports class of service-sensitive routing and bandwidth reservation. It provides topology-distribution mechanisms based on advertisement of link metrics and attributes, including bandwidth metrics. It uses a multilevel hierarchical routing model providing scalability to large networks. Parameters used as part of the path-computation process include the destination ATM address, traffic class, traffic contract, QoS requirements and link constraints. Metrics that are part of the ATM routing system are specific to the traffic class and include quality of service-related metrics (e.g., CTD, CLR) and bandwidth-related metrics (e.g., PCR). The path computation process includes overall network-impact assessment, avoidance of loops, minimization of rerouting attempts, and use of policy (inclusion/exclusion in rerouting, diverse routing, and carrier selection). Connection admission controls (CACs) define procedures used at the edge of the network, whereby the call is accepted or rejected based the ability of the network to support the requested QoS. Once a VC has been established across the network, network resources have to be held and quality service guaranteed for the duration of the connection.
All ATM traffic is carried in cells, yet no applications use cells. So, specific ways of putting the data into cells are defined to enable the receiver to reconstruct the original traffic. Three important schemes are highlighted in Figure 3 and discussed in detail later in the tutorial.
- RFC1483, which specifies how interrouter traffic is encapsulated into ATM using ATM adaptation Layer 5 (AAL–5); AAL–5 is optimized for handling framed traffic and has similar functionality to that provided by HDLC framing in frame relay, SDLC, and X.25
- ATM LAN emulation (LANE) and multiprotocol over ATM (MPOA), which are designed to support dynamic use of ATM SVCs primarily for TCP/IP; LANE, which is a current standard that is widely deployed and will be a subset of the MPOA standard (which is targeted for standardization only in mid-1997), will be discussed later in the tutorial
- voice and video adaptation schemes that can use AAL–1, which is defined for high efficiency—for traffic that itself has no natural breaks, such as a circuit carrying bits at a fixed rate
Figure 3. Data Insertion in Cells
