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Backbone Networks

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The backbone network is an important architectural element for building enterprise networks. It provides a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks in the same building, in different buildings in a campus environment, or over wide areas. Generally, the backbone's capacity is greater than the networks connected to it.

There are distributed backbones that snake throughout a building or campus to provide a connection point for LANs, and there are collapsed backbones that exist as wiring hubs and switches. The two topologies are illustrated in Figure B-1. A hybrid configuration ties together several collapsed backbone hubs or switches with a distributed backbone.

[Figure Backbone 1: See book]

The distributed backbone on the left in Figure B-1 shows how the network (in this case, an FDDI ring) extends to each department or floor in a building. Each network is connected via a router to the backbone network. FDDI adds fault tolerance due to its ring topology. If one of the routers fails, the rest of the network stays connected.

In the collapsed backbone shown on the right, a cable runs from each department (or floor) network to a central hub or switch, usually located in a building wiring closet or management center. The backbone is reduced to a hub or switch and the network is configured with a star- wired topology. The hub or switch uses a variety of architectural designs, such as bus, shared memory, or matrix-as discussed under "Switch Fabrics and Bus Design." A backbone is typically a network that interconnects other networks. In a switched network design, a backbone is not as clearly defined. It is usually just the high-speed switches that aggregates traffic from attached networks.

So far, our backbone has been limited to a single building. A backbone can link multiple networks in the campus environment or connect networks over wide area network links. These two approaches are pictured in Figure B-2. The fault tolerant ring topology of FDDI accommodates the campus backbone well. Another solution is Gigabit Ethernet fiber-optic links that connect to a central switch.

[Figure Backbone 2: See book]

As for wide area networks, two approaches are possible. The private network approach is pictured on the right in Figure B-2. Dedicated leased lines are installed connecting all the sites-a costly proposition, especially if the sites are far from each other, because the cost of leased lines increases with distance.

A better approach to building wide area network backbones is to use carrier and service provider networks that provide frame relay, ATM, or other similar services, as discussed in "WAN (Wide Area Network)." Also see "Internet Architecture and Backbone."

The 80/20 and 20/80 Rules

An old rule for backbones was that 80 percent of the traffic stayed in the department, while 20 percent crossed the backbone. With this model, high data throughput rates on the backbone were not a priority. If your departmental networks used 10-Mbit/sec Ethernet, you could usually get by with a 100-Mbit/sec backbone.

However, the 80/20 rule no longer applies for most networks. In fact, it has reversed due to the following:

  • Users that often communicate with one another are distributed throughout an organization rather than being in the same department.

  • Servers may be physically located at a central site, so the majority of network traffic flows to the same place.

  • Hierarchical networking schemes and centralized management naturally create a structure in which traffic flows to a central hub or switch.

  • Users access the Internet through firewall gateways, which means that all Internet traffic is funneled to a central hub or switch and then out the Internet connection.

Because of these factors, there is a need to improve the performance of the backbone or come up with different network designs. Added to that is increased traffic load put on the network by multimedia applications, including live voice and videoconferencing applications.

ATM (Asynchronous Transfer Mode) has solved the traffic problems in many networks, including carrier core networks. Multiple ATM switches can be connected together in a mesh topology with redundant load-sharing links that handle high traffic loads at the core. An example is discussed in the ATM topic under the subheading "Hybrid ATM Networks."

Another choice is to build enterprise networks with Gigabit Ethernet cores. It provides gigabit/sec (1,000-Mbit/sec) throughput on the backbone switch or between switches. It fits in well with existing Ethernet networks because the same frame format, medium access method, and other defining characteristics are retained. In most cases, a Gigabit Ethernet switch can replace older switches.

In general, switched-based building blocks are the components you need to build a high-speed hierarchical network that maintains high performance under big traffic loads. Refer to "Switching and Switched Networks" for more information on how to build the "new" networks. Also see "TIA/EIA Structured Cabling Standards."

In the wide area, many new approaches are available for connecting geographically dispersed networks. One method is to configure secure "tunnels" over the Internet in the form of VPNs (virtual private networks). Another approach is to connect with service providers that are taking advantage of optical networking technologies that significantly reduce WAN costs and improve performance and service options. Refer to "NGN (Next Generation Network)" and "WAN (Wide Area Network)" for more details.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.