Chapter 6. Specialized Network Services
In this chapter...
T-1—24 Voice or Data Paths over One Telephone Circuit
ISDN—Integrated Services Digital Network
Digital Subscriber Line Technology
Frame Relay—A Shared Wide Area Network Service
Gigabit Ethernet—Ethernet over Fiber in Metropolitan Areas
ATM—Asynchronous Transfer Mode
SONET—Synchronous Optical Network
Higher speed computer processors, improvements in optical technologies and investments in telecommunications in the last decade have created new, faster network services. In particular, Gigabit Ethernet shows promise as a vehicle for easily carrying data at millions of bits per second (bps) to the Internet, to data centers and between corporate sites. It has the added benefit of being easy for end users to implement because of its compatibility with most local area networks (LANs).
Frame Relay, while older and slower than Gigabit Ethernet, is used widely for Internet access and as a replacement for private lines between sites. Each of an organization's locations that wishes to use the service has a line to the carrier's Frame Relay network. On-site equipment “packages” LAN data into a format compatible with the Frame Relay network. The carrier is responsible for capacity and network reliability. Incumbent local telephone companies, data communications companies and long distance carriers, as well as competitive local exchange carriers (CLECs) sell Frame Relay service.
Digital subscriber line (DSL) is targeted at residential and small and medium-sized business customers. DSL is a low-cost, high-speed dedicated Internet access service. There are a variety of DSL services. They run at speeds ranging from 128 kilobits per second (Kbps) to 52 megabits per second (Mbps). The DSL industry has been in turmoil. Many of the vendors that provided the infrastructure for DSL have gone out of business. The largest providers of DSL are the local incumbent telephone companies. SBC, the largest DSL provider, had 767,000 lines in service by year-end 2000. SBC has pledged to spend $6 billion for DSL upgrades, and has stated the goal of making DSL available to 80% of their customers by 2004. Regional bell operating companies (RBOCs) have made DSL a cornerstone of their push into data communications and are investing heavily in it.
Although they've been available since the 1960s in carrier networks, T-1 and T-3 services are the key telecommunications technology used by the business community.
T-1 allows 24 voice or data calls on two pairs of copper, fiber, infrared or microwave. T-1's 1.544 Mbps is the most prevalent speed used in the United States by businesses for Internet access.
T-3 has the capacity for 672 channels and a speed of 44 megabits per second (Mbps). It is used to carry much of the voice traffic between central offices.
Integrated services digital network (ISDN) is newer than T-1 and T-3. However, it is not nearly as widely used. ISDN comes in two flavors: basic rate (BRI) with two channels for voice or data, and primary rate (PRI) with 23 channels for voice or data. BRI is the second most popular Internet access service in Western Europe and Japan after dialup modems. PRI is used by call centers and commercial organizations mainly for caller ID–type service.
A summary of high-speed, digital network services is listed in Table 6.1.
| Network Service | Places Typically Used | How Used |
|---|---|---|
| T-1
24 voice or data channels E-1 30 voice or data channels | Commercial organizations | 1.54 megabit per second Internet access, connections to long distance and local telephone companies for voice and data, private lines between company sites |
| Internet service providers (ISPs) | Connections to the Internet | |
| T-3
672 voice or data channels E-3 480 voice or data channels | Very large organizations | Access to long distance companies, Internet access, high-speed private lines between company sites |
| Local exchange carriers (LECs) | Tandem-to- tandem central office traffic | |
| Large ISPs | Connections to the Internet | |
| BRI ISDN Two voice or data channels and one signaling channel | Residential customers | Telecommuting, Internet access; mostly in Europe and Japan |
| Organizations | Desktop videoconferencing, Centrex telephones (see Chapter 2 for Centrex) | |
| PRI ISDN
23 voice/data and one signaling channel in the United States 30 channels voice/data and one signaling channel in Europe | Business and commercial customers | Call centers, videoconferencing, voice and data links to local and long distance providers. PRI ISDN has different speeds in the United States and Europe. |
| ISPs | Modem connections to competitive local exchange companies | |
| Digital Subscriber Lines DSL 128 kilobits per second (Kbps) to 6 megabits per second (Mbps) | Residential consumers, small and medium- sized businesses | Internet access |
| Telecommuters | Remote access to corporate files and email | |
| Frame Relay | Medium to large commercial customers | 56 Kbps to 45 megabits access to data networks for LAN-to-LAN communications and Internet access; mostly data but some voice |
| Gigabit Ethernet | Medium to large commercial customers | 1 megabit per second to 1 gigabit (Gb) access to the Internet and LAN-to-LAN connections over fiber, data only; more flexible upgrade and faster access than Frame Relay |
| Application service providers (ASPs) and content delivery network providers | 1 megabit per second to 1 gigabit links to the Internet and to customers | |
| ATM, Asynchronous Transfer Mode (ATM), 56 Kbps to 2.5 Gigabits per second | Telephone companies | Switches voice, video and data traffic on high-usage network backbone routes over fiber |
| Frame Relay networks | Switches traffic in the core of Frame Relay networks | |
| Large organizations such as major universities | Primarily to transmit voice, video and data across campuses and between LANs | |
| Synchronous optical network (SONET); up to 129,000 channels on fiber optic cable | Carrier networks | Multiplexes voice and data traffic onto fiber optic cables; provides a backup redundant path in the local loop and in carriers' backbone networks |
ATM (asynchronous transfer mode) switches carry voice, data, image and video at very high speeds of up to 30 Gigabits per second over fiber optic cabling. ATM is used mainly in Frame Relay networks, cellular networks, carrier networks and to carry DSL traffic to Internet service providers. It originally was envisioned as carrying high-speed data to desktops and for connecting LANs together within a campus environment. However, because of its cost and complexity, it never caught on for these applications. Some very large organizations do use it for specialized applications such as videoconferencing.
SONET (synchronous optical network) is a high-speed optical multiplexing service that works on fiber optic cabling. SONET runs at speeds of up to 13.22 Gigabits. It is used in network service provider networks to carry traffic from multiple customers running at different speeds. For example, SONET networks can accept T-3 traffic from customer A and T-1 traffic from customer B and transport both streams. SONET provides a way for carriers to increase the reliability of their networks. In these applications, SONET bidirectional rings carry traffic on one ring and provide a “protect” ring and multiplexer that automatically takes over if one strand of fiber is cut or if a SONET multiplexer crashes. SONET doesn't have the switching capability found in ATM. It just carries traffic between two points—for example, between one city to another or one central office to another.