Choosing the right bandwidth to connect to the backbone is a piece of cake—once you know which ingredients youre looking for.
First, youll need to know whether your customers traffic is bursty, whether you have a high proportion of end users or youre a portal (which determines how much traffic will stay in your network), and what type of applications or services you offer (real-time applications like videoconferencing or Voice over IP require special handling).
The chart on the following page ("Speeds and Feeds") outlines the key points for each technology.
The options available to connect to the backbone are numerous, confusing and expensive, but we can try to simplify it. Its important to understand the difference between speeds and technologies. T1 is a speed. OC is a speed. DSL, Frame Relay, ATM, Packet over Sonet and Ethernet are physical-layer technologies Just to make your technical people earn their money, you can combine technologies. For example, frame relay is often carved out from a T1, which rides on HDSL copper.
Perhaps the most common connection at the low end is the trusty T1 line (or a fractional T1, which is a provisioning of only some of a T1s 24 64-bit channels). A T1 is just the speed, though—the technology is HDSL, the daddy to the widely available ADSL, SDSL, etc. T1 and its big brother, T3 (or DS3), generally run over copper wire.
Also on the low end is frame relay, a shared, packet-switching protocol, which is great for inexpensively connecting branch offices together over long distances, but its not really suited for a service provider to connect to the backbone because of the inefficiencies in encapsulating IP within frame relay.
Many users would never notice the difference, but if youre running a real-time application like a VPN or videoconferencing, youll see inferior quality of service.
Stepping up to even higher bandwidth levels comes ATM, with a maximum limit of 622Mbps. Basically, if you have ATM, you should go with ATM, but its complicated and expensive.
Fiber is only now becoming widely available (thus dropping its price), which makes Packet over Sonet (whose speed is defined by Optical Carrier) a viable backbone connector. Sonet (synchronous optical network) defines rate and interface connections over a fiber-optic line.
Fiber is resilient, meaning if a line is cut, the traffic is automatically routed along another path. It also has fewer problems with interference, making it an excellent transport medium over long distances.
Last, but not least, is Gigabit Ethernet. While not widely available, Ethernet is running on a majority of LANs, which makes it easy to connect to an Ethernet interface on the backbone. The problem is that theres a distance limitation to Ethernet, which usually requires co-location of the routers and switches.
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