Broadband Access and Service

Transmission in the telecommunications networks of today is, more and more, digital in nature, and the transmission medium of choice is fiber. "Digital," however, does no more than imply a string of 1s and 0s racing through the network. But how are these 1s and 0s to be arranged? At what speed are they to travel? What route should they take? Answers to questions such as these have taken many forms and have made for the most complicated aspect of the telecommunications business.

There has never been a scarcity of coding schemes in the industry. Starting with Morse code, going to the Baudot code, then the ASCII code, we have seen each providing for better transmission and higher quality. In this section we will discuss the most popular and important three codes.

SONET

SONET is a standard for optical telecommunications transport. The SONET standard is expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades. It defines a technology for carrying many signals of different capacities through a synchronous optical hierarchy. The standard specifies a byte-interleaved multiplexing scheme. The synchronous optical hierarchy mentioned is shown in Table 2.

Table 2. SONET Hierarchy

Signal	Bit Rate	Capacity
OC–192	9953.280 Mbps	5376 DS–1s
OC–48	2488.32 Mbps	1344 DS–1s
OC–12	622.080 Mbps	336 DS–1s
OC–3	155.520 Mbps	84 DS–1s
OC–1	51.840 Mbps	28 DS–1s
OC = optical carrier The DS–1 channel can carry twenty-four voice circuits, each called a DS–0

The SONET standards govern not only rates, but also interface parameters; formats; multiplexing methods; and operations, administration, maintenance, and provisioning (OAM&P) for high-speed transmission. We most often hear of SONET rings in which fiber strands are strung around a metropolitan area in a ring configuration. The system is designed so that transmission can take place in either direction; should there be a fault at any one location, transmission will immediately take place in the opposite direction. That is, the system is self-healing.

ATM

Asynchronous transfer mode (ATM) is a high-performance switching and multiplexing technology that utilizes fixed-length packets to carry different types of traffic. Information is formatted into fixed-length cells consisting of 48 bytes (8 bits per byte) of payload and 5 bytes of cell header. The fixed cell size guarantees that time-critical information (e.g., voice or video) is not adversely affected by long data frames or packets. Of course, if the cells were longer in length the system would be more efficient, because the header would take up a smaller percentage of the total cell.

Multiple streams of traffic can be multiplexed on each physical facility and can be managed so as to send the streams to many different destinations. This enables cost savings through a reduction in the number of interfaces and facilities required to construct a network.

ADSL

Asymmetric digital subscriber line (ADSL) is, essentially, a modem that employs a sophisticated coding scheme. This coding scheme permits transmission over copper pairs at rates as high as 6 Mbps for distances of 9,000 to 12,000 feet. Speeds of this magnitude bring to mind television signals; a 6–Mbps channel can easily handle a television movie.

ADSL succeeds because it takes advantage of the fact that most of its target applications (video-on-demand, home shopping, Internet access, etc.) function perfectly well with a relatively low upstream data rate—hence the word asymmetric. LECs are now using ADSL as an access technology for their television businesses and for Internet access.