Choosing Network Technologies: Physical Connection
In almost every physical network connectivity option, you will be using a router to act as the boundary between your network devices and the devices that belong to the telephone company. In many cases in this scenario, you may use two routers, where, as part of your contracted connection, the telephone company provides you with a router that connects directly to your router.
In other cases, you may use a router that is supporting T1, ISDN, or another connection type through a WAN Interface Card (WIC), which is an expansion card for your router, allowing your router to connect directly to the telephone company's network. The illustration shows a router that contains a T1 WIC.
Depending on your router’s capabilities and the telephone company's connection options, you may use a Data Service Unit/Channel Service Unit (DSU/CSU), which is a Layer 1 device that connects the telephone company's network to your router. In the illustration, the DSU/CSU is built into the T1 WIC, whereas in other cases, you will have separate, distinct devices connected to your router.
The DSU/CSU can be thought of as a modem in that it connects your digital device (router) to your telephone company’s network; however, unlike a modem, the connection is digital on both sides of the DSU/CSU. When the DSU/CSU is a separate device, it typically connects to your router through a serial port.
Here are some of the options to consider for connecting your network to a telephone company's network:
Cable: Many cable companies have gotten into the service-provider market, with the least expensive of their connections being their standard cable connection. In recent years, these companies have started offering faster and more reliable connection options such as fiber-based services like FiOS.
Because these companies are fairly new in the market, they tend to have more modern connection options, which may present challenges when setting up a WAN spanning borders of less technologically developed countries.
DSL: DSL technologies are fast and reliable options when newer options are not available. They have been around long enough to be available in most regions and tend to offer inexpensive connectivity options. This technology changes rapidly. Your local provider supports some DSL types, such as Asymmetric Digital Subscriber Line (ADSL) or Symmetric Digital Subscriber Line (SDSL).
T1/E1/ISDN: ISDN and T1 connectivity options are typically more readily available than even ADSL because of their level of maturity in the overall network world. T1 connections and Primary Rate Interface (PRI) ISDN connections allow you to put a full, 26-wire pair connection into place and then activate as many pairs of wires as you need, providing you with 64 Kbps per pair. This partially activated connection is called a fractional connection.
The standards on the number of pairs and the speeds vary by region. In the United States, typically you are looking at 24 pairs on a T1 connection with an 8K control channel, yielding 1.544 Mbps of throughput; whereas the European versions (E1) usually provide 32 pairs, providing a connection of 2.048 Mbps.
The North American T-Carrier signals go up to five levels, with the top end being 5,760 pairs or channels allowing 400.352 Mbps of throughput. Although these numbers get pretty high, Basic Rate ISDN (BRI ISDN) will run as little as two 64 Kbps data channels with a 16 Kbps control channel.
Serial connections: Serial connections are the oldest connection technologies and are available in areas that may be lacking other connection methods. Serial connections are available worldwide. The frame relay system is the main method of offering serial connections.
When implementing frame relay on a Cisco router, you have two choices for encapsulation methods: Point to Point Protocol (PPP) or High-Level Data Link Control (HDLC). Your choice depends on what you are using at the other end of the connection, because the same technologies must be used at both ends of the connection.