Bandwidth

The following set of notes corresponds to the Bandwidth module of the Internet Design course, providing students an outline of the information they are expected to understand. The corresponding reading for this module is chapter one of i-Net+ Study Guide by David Groth, et. al., pages 34-43.

Speed Abbreviations

• Kbps = Kilo-bits per second
• Mbps = Mega-bits per second.
• Gbps = Giga-bits per second.

Bandwidth Technologies and Link Types

• DDS, or digital data service, is a dedicated digital connection providing up to 56Kbps throughput.
• A T1 provides a 1.544Mbps digital connection through two pair of UTP wires or fiber-optic cabeling. The cable is divided into 24 separate data channels, with each channel delivering 64Kbps. A full T1 connection uses all 24 channels, while a fractional T1 connection will use only a portion of the 24 channels.
• T3 connections provide 44.736 Mbps transfer rates using 672 channels (equivalent to 28 T1 lines). T3 connections are used primarily for the backbone of the Internet and Internet service providers.
• Asynchronous Transfer Mode (ATM) is a transfer technology that can use a variety of cabling options that can support data rates from 25Mbps up to 622Mbps. With ATM, data is broken down into small, fixed-size packets called cells, then sent along a fixed channel or route.
• ISDN (Integrated Services Digital Network) uses regular telephone wiring (Plain Old Telephone Service) with a digital signal to provide transfer rates of 64Kbps (with one line, or "B channel) to 128Kbps (with two lines). ISDN requires an ISDN terminal adapter on the host computer.
• DSL (Digital Subscriber Lines) also use regular telephone wiring to transmit data digitally. Using special modulation schemes, DSL lines are able to reach transfer rates downstream of 9Mbps and higher (some claim 32Mbps), and upstream transmissions from 64Kbps to more than 1Mbps. DSL lines can only run up to 20,000 feet from a central telephone station, so service areas are limited.
• DSL comes in a variety of formats, including HDSL (High Data-Rate Subscriber Lines), SDSL (Symmetric Digital Subscriber Lines), and ADSL (Asymmetric Digital Subscriber Lines), which are collectively referred to as xDSL.
• SONET (Synchronous Optical Network), also known as SDH (Synchronous Digital Hierarchy), is a standard for transmitting multiple transmission types over fiber-optic cables using multiplexing and fixed frame sizes. SONET uses from 1 to 768 channels, which are designated Optical Character (OC) lines. Transfer speeds go from 51.84Mbps (OC-1), up to multi-Gbps (OC-768).
• Public Switched Telephone Network (PSTN), also known as Plain Old Telephone System (POTS), is using the regular telephone lines to send data in analog format. This method requires a modem on the host computers and is capable of data transmission rates of 56Kbps.
• Frame Relay is a transfer method that uses virtual circuits and variable packet sizes to send data over a network. Frame relay does not require a direct connection between the two communicating computers, instead it can use whatever lines are available between computer A and B and establish a virtual circuit, thus making it a very cost-efficient technology.
• X.25 is similar to frame relay but includes some built-in error correction and flow control.