TECHNOLOGY OPTIONS
Dial-up Modems: Dial-up modems are now gasping for their last few breaths. Most external modems have now been replaced by internal ones. In terms of specs, 56 kb speeds with V.92 technology and V.PCM enhancement remain the dial-up’s last bastions. V.92 offers features like Internet call notification (ICN), streamlined call handling and modem-on-hold. V.PCM Upstream reaches upstream speeds up to 48 kbps and lets the user choose either the fastest downstream speed (up to 53 kbps) or a ‘balanced’ connection of downstream and upstream communication.

l DSL Modems: A DSL modem facilitates simultaneous voice and data transmission over the same line. It does this by splitting the regular copper telephone line into two frequency ranges. The frequencies below 4 KHz are reserved for voice, and the range above that is used for data. The splitter, consequently, allows simultaneous access to the line by the telephone and the computer.

Since a large number of DSL protocols have been developed, they are collectively referred to as xDSL, including HDSL, SDSL, ADSL, and VDSL. High-bit rate DSL (HDSL) provides T1 or E1 connections over two or three twisted-pair copper lines respectively. Unlike most other forms of DSL HDSL is not a typical consumer service, it is used mostly to replace traditional T1/E1 connections, such as connecting PBXs to telcos.

A Single-pair High-speed Digital Subscriber Line (SDSL) is similar to HDSL, but provides T1 or E1 connections over a single twisted-pair copper line. Assymetric DSL (ADSL) is a form of DSL in which the bandwidth available for downstream connection is significantly larger than for upstream. Although designed to minimize the effect of cross talk between the upstream and downstream channels, this is suited for Web browsing and client-server applications as well as some emerging applications such as video on demand. Very-high bitrate DSL (VDSL) is similar to ADSL but provides higher speeds at reduced lengths.

l Cable Modems: Cable modems convert Internet data into a radio frequency (RF) signal that can be transmitted over co-axial cable. Cable is a shared medium and so transmission speeds will slow down as more users log on. Cable networks are inherently hierarchical in nature and thus require two paths, one for downstream and one for upstream. This requires either a second cable plant for upstream or a second frequency band allocated onto the existing system. Cable modems separate digital and analog traffic and enable a PC to send and receive data using the coaxial cable sharing its downstream and upstream frequencies with the Cable TV channels. A cable modem interacts with a CMTS installed at the location of the cable operator. The CMTS in turn interfaces with the CATV frequency spectrum. Cable modems use various technologies like the TDMA based DOCSIS standard or the more robust and modern SCDMA-based Terayon proprietary technology.

l Ethernet-to-the-Home (ETTH): This Ethernet-based technology may soon replace the need for modems. Like standard Ethernet, the bandwidth is 10 Mbps for home users, but can reach 50 Mbps using symmetrical data connectivity as for enterprise customers. While this technology is Ethernet-based, it has been designed to run over existing coaxial networks with minimal changes to the existing infrastructure. The customer end connection is a standard RJ45 ethernet socket/plug, which can be connected straight to the customer’s PC without a modem or adaptor. Unlike DSL, ETTH can run over a long distance using amplifiers.

l Leased-line Modems: Leased line modems are analog and not digital. These special modems are used on lines leased from the service provider or sometimes on just a long direct wire hookup. They often will also work as ordinary modems, but go into leased-line mode when the AT command and L1 is given. Leased-line modems often use a ‘dumb mode’ where they ignore AT commands, disable result reporting, etc. One type of leased line used two pairs of wires (one for each direction) using V.29 modulation at 9600 baud. Some brands of leased-line modems are incompatible with other brands.

l ISDN Modems: The ISDN modem connects the computer to a ISDN line. The transmission is purely digital.

l LAN Modems: The concept of shared Internet access has resulted in LAN modems. These modems have a built-in IP router, 4-port 10BASE-T hub and voice ports. In effect, this device allows the user to instantly create a 10Mbps LAN for the SOHO, and share a printer, modem or fax.