Evaluating atm network topologies
A comparison among three common atm topologies shows where network designers can derive significant cost savings.
Sam Lisle and Paul Havala Fujitsu Network Communications Inc.
Asynchronous Transfer Mode (atm) infrastructure deployment among large public carriers remains in its infancy. The topologies of many initial carrier atm networks have been designed to suit a specific application or tailored to handle a small number of switches.
In some cases, atm switches are connected in a mesh of nonredundant interfaces. This topology has relatively low initial equipment costs, but has its drawbacks. For instance, it must support survivability either through the atm network management system (or other atm layer function), which cannot provide 50-msec restoration after facility failures, or through 1+1 protected Synchronous Optical Network (sonet) line interfaces, which provide quick restoration but double the fiber consumption.
In many cases, an underlying sonet ring connects the atm switches. There is more electronic equipment cost in these topologies, but the sonet ring provides fast facility protection, bandwidth grooming capabilities, and substantial fiber savings in many scenarios. Moreover, the sonet ring may also transport non-atm traffic.
In a variation of this topology, atm-capable add/drop multiplexers (adms) can offer more efficient sonet ring usage, and atm switches with integrated ring functionality can provide vast efficiencies.
This article examines in more detail the relative equipment costs, survivability benefits, and bandwidth grooming capabilities of these network topologies for interconnecting atm switches. Specifically, the article will discuss
point-to-point topologies, including mesh, partial mesh, and star,logical point-to-point connectivity over a physical sonet/atm ring,sonet or atm ring functionality inte-grated directly in atm switches.
These topologies, summarized in the table, are not mutually exclusive. In general, networks employ a mix of these different methods for atm element interconnection.
Point-to-point topologies
In point-to-point topologies, oc-n interfaces directly connect switches without an intervening transport network. Partial mesh networks have direct connections between most pairs of switches. Figure 1 shows a single-tier partial mesh interoffice network, along with a two-tier network.
In a single-tier network, all swit-ches have end-users connected directly to them. In larger interoffice networks, adding multiple tiers can improve the manageability of the network and reduce the number of switch hops required for a connection. In the two-tier network in Fig. 1, there are still five atm switches with direct user connections, but the interconnections among the switches funnel through a tandem switch.
The tandem switch may have direct user connections, but its main purpose is to interconnect smaller switches. A multiple-tier topology reduces the number of direct fiber interconnections and the total amount of fiber in the network. This topology uses smaller switches, close to the customer, to aggregate traffic for trunking toward the rest of the network, improving port use on the tandem switch.
To enhance the survivability of a multi-tier network, each end-office switch can home to more than one tandem switch. If one tandem switch fails, the traffic can be rerouted through the remaining tandem switch.
In the access network, point-to-point topologies are in a star, where all atm customer equipment homes to the atm switch in the central office. Figure 2 illustrates single- and dual-homed star topologies in the access network.
Relative equipment costs
The equipment cost of an atm network consists of the atm switching equipment, transport equipment, and fiber and cable plant.
In the interoffice network, a single-tier mesh system requires no transport network elements and, therefore, no transport network equipment costs. However, the cost of the fiber and cable plant can be high. Further, if 1+1 sonet line automatic protection switching (aps) is provided, the fiber investment doubles.
In Equation 1, consider N end-office switches separated by an average distance of D, connected in a full mesh. The amount of optical fiber required is given by:
where P=2 if no sonet line aps is used on the link and P=4 if 1+1 sonet line aps is used. As an example, if eight atm switches separated by an average distance of 6 mi are linked using 1+1 sonet line aps, 672 fiber-mi are required. By contrast, networks built with sonet transport or atm switches with integrated ring capability require only 96 fiber-mi.
In a multiple-tier network where each end-user switch connects to others through a tandem switch, savings can be realized in the fiber plant, but additional equipment (the tandem switch) is required.
In Equation 2, consider N end-office switches and T tandem switches all separated by an average distance of D connected in a two-tier network. If the tandem switches are connected in a full mesh using 1+1 sonet line aps and the end-office switches all have a single connection to one tandem switch, the amount of optical fiber required is given by:
As an example, if eight atm end-office switches separated from the tandem switch by an average distance of 6 mi are linked through the tandem switch using 1+1 sonet line aps, only 192 fiber-mi are required. Because there is only one physical link from each end-office switch toward the network (rather than many physical links from each end-office switch), the line rate of end-office switch ports may have to be greater than in the single-tier case.
In the access network, most end-user traffic homes to a single atm switch in the end office. Star topologies can be deployed without a transport network if an oc-n interface is used between the atm customer premises equipment (cpe) and the atm end-office switch. Although eliminating the transport network may be attractive, this topology requires the end-user to purchase an OC-3 (155-Mbit/sec) interface card on the cpe and an OC-3 access (switch port presence) rate. For customers desiring DS-3 (44.736-Mbit/sec) or DS-1 (1.544-Mbit/sec) cell-relay service, an intervening transport network will be required.
Also, as in the interoffice network, fiber consumption can be quite high. In Equation 3, consider a single end-office switch receiving traffic from N atm cpe at an average distance, D, from the end office. The amount of required fiber scales linearly with the number of cpe:
where¥= 2 if the oc-n interface is unprotected or P=4 if 1+1 line aps is used.This topology can waste atm switch port capacity. In general, the traffic from each atm cpe will be less than the access line rate. If each end-user is connected directly to the atm switch, then the entire switch port is consumed on the switch regardless of how much traffic is on the port. An atm multiplexing function in the access network could alleviate this problem by concentrating atm traffic from many users onto a single atm switch port.
Survivability attributes
Many atm switches rely on centralized or distributed atm connection rerouting for survivability, rather than on sonet or atm layer aps. Current automatic connection rerouting performance can require hundreds of milliseconds or more for restoration to occur. Further, since protection capacity is not always reserved, the rerouting scheme may not guarantee enough spare capacity in the network to restore all failed traffic without preempting other traffic. The restoration time will also increase in proportion to the number of atm switches.
As atm switches begin to support 1+1 line aps, 50-msec aps will be provided. However, line aps provides its largest survivability benefit only if the working and protection links are diversely routed between each pair of switches. This may not be possible if fiber is limited. Also, 1+1 line aps implemented directly on atm switches could consume an additional port on the atm switching fabric, thereby reducing the effective switching capacity of the network element. This depends on the specific atm switch architecture.
In the access network, star topology survivability poses even more problems. atm cpe with 1+1 protected oc-n interfaces could force considerable cost onto the customer (or onto the service if the cpe is bundled with the service). Much customer premises equipment does not even offer this capability today. With no intervening transport network or 1+1 line aps, one solution is to rely on rerouting methods of restoration and deploy another port on the cpe that homes to either a second port on a single atm switch or a second atm switch altogether.
Bandwidth management attributes
Direct mesh connections of atm switches provide limited bandwidth management options. For example, assume that the partial mesh interoffice network in Fig. 1 is interconnected using OC-3 links. If the bandwidth between switches A and B exceeds OC-3 capacity, the network provider must install another link between the two switches, upgrade the existing link to OC-12 (622 Mbits/sec), or route A to B traffic through a third switch, C, that interfaces with A and B.
Further, if the line interfaces that connect two switches do not closely match the capacity demands over the link, then the atm switch ports are inefficiently used, and additional atm switch port hardware is required. For example, if switch A is delivering 75 Mbits/sec of traffic directly to both switch B and switch C, then two OC-3 ports on switch A will be needed.
Logical point-to-point topologies using sonet/atm rings
atm switches are often connected in a logical point-to-point fashion with the links riding over sonet physical rings. An example of this is when a DS-3 connection is desired between two atm switches and the DS-3 rides over a sonet unidirectional path-switched ring (upsr). Typical loop and interoffice topologies are shown in Fig. 3.
The sonet adms can manage bandwidth at the sts path layer, where they interconnect switches using sts-1 (50.12-Mbit/sec), sts-3c (150.336-Mbit/sec), and sts-12c (601.334-Mbit/sec) pipes. atm adms also may manage bandwidth at the atm virtual path (VP) layer and interconnect switches with atm pipe sizes that are customized for the precise amount of bandwidth required to interconnect the atm switches.
Relative equipment costs
This atm network topology requires additional equipment in the transport network. However, the fiber savings can be significant.
In Equation 4, consider N atm switches/cpe all separated by an average distance of D connected over a sonet ring. The amount of optical fiber required is given by
where¥= 2 for a 2-fiber ring and P=4 for a 4-fiber ring. As an example, if eight atm switches separated by an average distance of 6 mi are linked with a 2-fiber sonet ring, only 96 fiber-mi are required. Compare this with the 672 fiber-mi required to interconnect these switches in a point-to-point mesh (refer to the example following Equation 1). Figure 4 further illustrates how fiber mileage varies with the number of switches for mesh and ring configurations.
Because of the additional cost of the transport equipment, a sonet ring may cost more than a point-to-point configuration for small numbers of switches in close geographic proximity. However, sonet rings are already widely deployed for time-division multiplexing transport. Interconnecting atm switches with unused bandwidth on existing rings is perhaps the most inexpensive strategy.
If atm VP processing is introduced in the ring, then some of the transport network equipment cost can be offset by the atm multiplexing function that occurs in the ring. atm transport allows the network provider to customize the transport pipe to the traffic being transported. The efficiency of atm when transporting services that do not match the sonet bandwidth hierarchy (e.g., 10- or 100-Mbit/sec local area network transport), coupled with the ability of atm to statistically multiplex bursty traffic, provides a tighter packing of data services onto the ring. When compared with sonet transport for data transport applications, this can result in smaller rings (e.g., an OC-12 ring where sonet transport requires OC-48) or more nodes on a ring--both of which translate into cost savings for the network provider.
atm transport also provides cost savings at the switches. Recall that an access network built by directly connecting the cpe with the atm end-office switch can waste atm switch port capacity if the links are not fully utilized. atm multiplexing allows several end-users to share the same atm switch port on the end-office switch, which allows the service provider to make better use of the atm switching resources (see Fig. 5).
This topology also saves atm switch hardware in the interoffice network. In most cases, the atm switches and the sonet transport nodes will be collocated, allowing the network provider to configure the atm switch and the sonet multiplexer with less expensive short-reach oc-n, or perhaps DS-3, interfaces.
Survivability attributes
Interconnecting atm switches with sonet adms provides 50-msec aps for all physical layer failures. However, no protection switching is provided by the sonet ring for atm layer defects. The interconnection between the atm switches/cpe and the sonet ring may optionally be unprotected.
atm adms that add and drop atm VPs must provide protection at the VP layer. Both itu-t (International Telecommunication Union-Telecommunication) and T1S1.5 are developing standards for VP layer protection. The drafts of these standards describe atm layer trigger mechanisms and switching protocols that allow the atm layer to perform 50-msec restoration following a physical or atm layer failure. Both groups expect to complete their standards this year.
Bandwidth management attributes
sonet rings dramatically enhance the bandwidth management capabilities of a network over point-to-point connections with direct fiber. If additional bandwidth is required between atm switches, new fiber is not needed as long as bandwidth remains on the ring. The incremental cost is limited to additional interface cards on the atm switch and the sonet adm.
For data services, atm VP rings add another dimension to transport bandwidth management. Just as sonet adms can create any sts path layer topology, so can atm adms create any logical topology at the atm VP layer. However, the ability of atm VPs to carry statistically multiplexed traffic at virtually any bit rate allows the network provider to customize the transport to match the application. This can offer tremendous gains in network efficiency and flexibility for nontraditional, bursty services such as local area network transport and cell relay.
If the same ring can transport atm VPs as well as traditional VT-mapped DS-1s and sts-mapped DS-3s, then all transport bandwidth can be managed in its native form.
atm switches with integrated ring functionality
By integrating ring functionality directly onto atm switches and atm cpe, the economic benefits of ring topologies can be realized without deploying separate, dedicated transport network elements.
In the interoffice network, atm end-office switches can be interconnected using integrated sonet upsr or bidirectional line-switched ring technology (see Fig. 6). This approach can be deployed in a single-tier network in which all switches on the interoffice ring are end-office switches or in a two-tier arrangement in which one switch in the ring is a tandem switch that interfaces to another tandem switch on another ring.
In the access network, the same topology can be realized by integrating a sonet upsr or atm VP ring into the atm switch in the end office and in some of the atm cpe (see Fig. 7). Since not all cpe will have integrated ring capability, some equipment will continue to stand alone and interface either directly to the atm end-office switch or to an atm adm or sonet adm in the access network. Enhancing atm adm equipment to include adaptation interfaces and functions allows the cpe function to be absorbed by the access network.
In both the access and interoffice networks, integrating ring functionality in atm switches and cpe provides the fiber cost savings, survivability, and bandwidth management benefits of networks built using dedicated sonet/atm transport equipment. The overall cost of the network is reduced by integrating the transport functions directly into the switch. u
Sam Lisle is manager of the OC-12 transport product group and Paul Havala is a senior product planner of atm and sonet transport products, both at Fujitsu Network Communications in Richardson, TX.