1. BZUPAGES.COM Telecommunications Circuits from the User’s Point of View Any circuit provided by a telecommunication service provider is made up of three components: Access Circuit Network connection Billing agreement

2. BZUPAGES.COM Access Circuits Access circuits are physical lines with circuit terminating equipment at each end. These circuits run from a user’s site to the nearest physical attachment point into the common carrier’s network. The location containing this physical attachment point is usually called a Central Office (CO). It might be pulled through the CO to another company’s Point of Presence (POP) to get access to that other company’s network. There are many different technologies for access circuits, ranging from Plain Ordinary Telephone Service (POTS) to DSL to SONET and cellular radio. These are short circuits, hopefully less than a couple of miles long.

3. BZUPAGES.COM Network Connection Network connections between the access circuits are made over high-capacity circuits that are owned and managed by the network service provider. Many options as methods of connection through their networks are offered. These can be summarized into three fundamental choices: Full period: connected all the time. Supports any kind of communications. Circuit-switched: connected for the duration of a communication session, then released. Supports any kind of communications. Bandwidth on demand or “packet-switched”: connected all the time, but supporting bursty communications rather than continuous streams.

4. BZUPAGES.COM Billing Plan There is a monthly charge for each access. Each of the types of network connections has an associated method of calculating how much the network connection costs: Ÿ Full period: billed as a monthly fixed charge. Ÿ Circuit-switched: per minute usage-sensitive charge. Ÿ Bandwidth on Demand: usage-sensitive charge based on the amount of data transmitted. A combination of access circuits with their circuit-terminating equipment, network connection, and of course, billing plan make up a circuit… which might more accurately be called a service.

5. BZUPAGES.COM Network “Cloud” We often draw pictures of carriers’ networks, like Figure 1, as clouds with sticks poking into them. Some people even get carried away with jargon and start referring to network services like Frame Relay as the “Frame Cloud”. We illustrate carriers’ networks as clouds to emphasize the idea that we don’t necessarily know what is inside the cloud, and frankly, most of the time, we don't care. When you get a circuit like a “T1” from a carrier, you are not buying a circuit… you are paying for a service. However, when ordering, and especially, troubleshooting circuits provided by network service providers, it is useful to know what is going on inside their network “cloud”.

6. BZUPAGES.COM Telecom Circuits from the Phone Company’s Point of View From the phone company’s point of view, their telecommunications network is composed of three main parts: the access network, switching centers and the transmission network. The access network is the totality of the physical equipment used to link the user to a Central Office (CO). This is often also called the outside plant. Switches are devices that connect one circuit to another for the duration of a communication session. Switching centers are the buildings that contain switches. Transmission network is the term used to generally refer to the systems that connect the COs.