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Permanent Virtual Circuits

The simplest ATM designs make use of permanent virtual circuits (PVCs). In advance of the anticipated need, an administrator defines these virtual connections. This is identical to PVCs in Frame Relay.

The advantage to PVCs is that there is no signaling required for call setup, and all circuits are available for data at all times. Unfortunately, this also requires manual configuration of the circuits—a step that can become cumbersome as the network increases in size. Traditionally, the administrator must manually configure each VPI/VCI path statement at each switch in the PVC. However, vendors have created tools that can graphically define the PVC and automatically establish the path.

Most data network encapsulation using ATM is defined in RFC 1497. This RFC outlines the requirements and methods used to transport multiple protocols over ATM using SNAP. This differs from another RFC-defined methodology, RFC 1577, which defines encapsulation for IP only.

Figure 8.9 illustrates the use of RFC 1483 with a permanent virtual circuit. Note that RFC 1483 does not require the use of PVCs—SVCs are valid also.

In Figure 8.9, the PVC is defined as an end-to-end connection that does not terminate at the switch with the physical layer. In addition, the network layer is the same as frame-based, network-layer traffic—IP, for example, would start at this point. All of the traditional rules regarding subnets and routing apply. The previous layer, RFC 1483, effectively establishes the data-link layer.

FIGURE 8.9  Permanent virtual circuits

On Cisco routers, the network is associated with the PVC on a subinterface level, and designs are point-to-point.

As with Frame Relay, ATM PVCs are typically configured with two bandwidth parameters. The maximum cell rate is referred to as the Peak Cell Rate (PCR), while the amount of bandwidth available for data is called the Sustained Cell Rate (SCR). The SCR is analogous to the CIR in Frame Relay (discussed earlier in this chapter), and under the FRF.8 specifications, the two are somewhat interchangeable. (The FRF.8 and FRF.5 specifications define the methods by which ATM and Frame Relay traffic are interchanged.)

Switched Virtual Circuits

Unlike permanent virtual circuits, switched virtual circuits (SVCs) are not established in advance. Rather, the switches are responsible for dynamically establishing the circuit through the network. In most other ways, SVCs are identical to PVCs. For example, SVCs may be used for nonbroadcast multi-access network designs (point-to-multipoint) or point-to-point configurations.

Figure 8.10 illustrates the components involved in establishing a switched virtual circuit. The Q.2931 standard is used for signaling information between the switch and ATM clients, which are labeled NSAP (Network Service Access Point) A and NSAP B. The signaling between single end nodes is called UNI; switches signal each other with NNI, as described previously.

The illustration in Figure 8.10 also includes the SSCOP layer, or Service-Specific Convergence Protocol. This protocol is responsible for reassembling the cells on the signaling channel. This is different from the segmentation and reassembly process in AAL 5—the cells serviced by SSCOP are usually messages used in the management of the ATM network and not user data.

FIGURE 8.10  Switched virtual circuits

ATM Routing

There are two common methods for routing cells across ATM switches: Interim Inter-Switch Signaling Protocol (IISP) and Private Network-Network Interface (PNNI).

IISP is a static routing model that provides for a backup path in the event of primary link failure. This is somewhat limited compared to a dynamic routing protocol—IISP cannot take advantage of multiple backup paths. Designers need to remember that ATM is still a fairly new technology with many interpretations of the standards, and as a result, IISP was one of the best routing methods available.

The dynamic routing protocol, PNNI, is an improvement on the manual and static IISP. However, it is still limited in that the current standard does not support hierarchical routing and is limited in scalability as a result. PNNI provides for prefix-based routing and route aggregation while also supporting multiple alternative paths. As ATM network complexity increases, it becomes more imperative to use PNNI.

Both routing protocols support e.164 addresses, which are used in public ATM networks, and NSAP addresses, which are used in private installations. NSAP addressing is the 20-octet addressing format, while e.164 is a 10-digit number similar to phone numbers in North America. Some e.164 addresses have additional bits/digits, as shown later in the SMDS section.

The design models for ATM are very similar to those used in traditional networks. For example, configurations may follow the hierarchical model or operate in a start topology. Most ATM tariffs are quite expensive at present; however, substantial discounts may be found in local installations. Unlike most other network technologies, it is very important to avoid congestion in ATM networks. This is due to the impact of a single lost cell on the data flow—a lost cell may require 20 cells to repeat the frame. All 20 cells will be retransmitted even though only one cell was lost to congestion. This adds to the original congestion problem and results in greater data loss.

Cisco’s StrataCom Switches

In the years following the acquisition of StrataCom, Cisco struggled with developing and marketing this powerful product. As of this writing, pundits continue to criticize the product and the strategic direction presented by the company regarding this system. Nonetheless, the platform is still competing with alternative offerings, including Nortel’s Passport. The criticisms of the past may return should Cisco falter in its current efforts to link the product with the rest of the company’s offerings or should Cisco fail to add additional features to bring it in line with the competition.

However, in recent months the product has successfully competed against rivals and, more important in this context, the CID exam contains a number of questions regarding this platform. It is very important to note that the current exam objectives do not explicitly note the StrataCom product line.

The StrataCom product line provides a number of network services. These include the following:

  Cell-based trunk links are provided with either standard 53-byte ATM cells or the 24-byte FastPacket cell configurations. FastPacket cells are proprietary.
  Dial-up services are provided with the Intelligent Network Server (INS). This independent processing system supports dial-up Frame Relay, voice-switched circuits, and ATM SVCs.
  Frame Relay frame forwarding is supported. In addition, the system supports the UNI and NNI specifications.
  StrataCom switches also provide voice connections, point-to-point connections, and bandwidth control.

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