Khi các công ty viễn thông của thế giới xây dựng mạng lưới kỹ thuật số đầu tiên của họ, tốc độ truyền tải ban đầu là 64 kbps bởi vì đó là băng thông cần thiết cho một cuộc gọi thoại duy nhất. Các hạn mức tín hiệu kỹ thuật số 0 (DS0) đề cập đến các tiêu chuẩn cho một dòng duy nhất 64-kbps. | 90 Chapter 4 Fundamentals of WANs When the telcos of the world built their first digital networks the baseline transmission speed was 64 kbps because that was the necessary bandwidth for a single voice call. The term digital signal level 0 DS0 refers to the standard for a single 64-kbps line. Later the telcos starting selling data services in other words leased lines. The phone companies could sell a DS0 service at 64 kbps. However when it first came out they typically offered 56-kbps service. Why Well it turned out that the telcos needed some bits for some management overhead. They found that if they used a bit inside the actual DS0 channel occasionally the voice quality did not suffer so they defined a standard in which a switch regularly could use one of every 8 bits in the DS0 channel for its own purposes. That worked fine for voice. But for data having something else in the telco network change the bits that you sent does not work very well. At best it can cause retransmissions at worst it doesn t work. So the telco decided to just sell 7 of every 8 bits that could be sent over a DS0 and 7 8 of 64 kbps is 56 kbps. Today many telcos do not use that bit so they can offer the full 64-kbps channel. The telco offers specific increments of the DS0 channel. In the United States the digital signal level 1 DS1 standard defines a single line that supports 24 DS0s plus an 8-kbps overhead channel for a speed of Mbps. A DS1 is also called a T1 line. It also defines a digital signal level 3 DS3 service also called a T3 line which holds 28 DS1s. Other parts of the world use different standards with Europe and Japan using standards that hold 32 DS0s this type of line often is called an E1. Table 4-4 lists some of the standards for WAN speeds. Included in the table are the type of line plus the type of signaling for example DS1 . The signaling specifications define the electrical signals that encode a binary 1 or 0 on the line. You should be aware of the general idea and
