Bit error rate testers are devices or procedures that measure the bit error rate (BER) for a specific transmission. BER is usually expressed as ten to a negative power so that, for example, a BER of 10-9 means that for every Gigabit transmitted, there is one bit sent or received in error. Bit error rate testers can use either synchronous or asynchronous communications. In synchronous communications, data is sent as frames of large blocks instead of as individual bytes. The clock is either embedded in the data stream or located on separate clock lines so that the sender and receiver remain in synchronization. Asynchronous communications send individual characters one at a time. Each character is framed by a start bit and one or two stop bits. For both synchronous and asynchronous devices, the bit pattern received is compared against a known pattern in order to identify errors. Some bit error rate testers can be controlled or programmed remotely. Others can be clocked by an external timing device. Devices with printer ports and visual indicators are commonly available.

Bit error rate testers support several protocols. T1 is a digital wide area network (WAN) protocol that transmits DS-1 formatted data at 1.544 Mbps through the telephone switching network. E1, another digital WAN transmission protocol, carries data at a rate of 2.048 Mbps. It is used mainly in Europe. Integrated services digital network (ISDN) is a protocol that allows telephone lines to carry data, voice, and other traffic. Fibre channel uses optical fiber to connect computers and peripheral devices that require high bandwidth. The most popular type is arbitrated loop (FC-AL), a fibre channel standard that supports full-duplex transfer rates at 100 Mbps. Other fiber optic protocols include synchronous optical network (SONET) and synchronous digital hierarchy (SDH). Transmission control protocol/internet protocol (TCP/IP) is a suite of communications protocols for connecting host computers. Point-to-point protocol (PPP) enables the use of IP over a serial link such as a dial-up connection. Serial line internet protocol (SLIP) also encapsulates and transmits IP packets over serial connections. Modulator-demodulator (MODEM) devices convert digital signals to analog signals. Other protocols include gigabit Ethernet, a standard for high-speed Ethernet approved by the Institute of Electrical and Electronics Engineers (IEEE).  

There are several test patterns for bit error rate testers. Quasi-random signal source (QRSS) is a pseudo-random sequence based on a 20-bit shift register that repeats every 1,045,575 bits. Digital data service (DDS) patterns are designed for testing DDS circuits with data services of 64 kbps or less. Other commonly used test patterns include all zeroes, all ones, or a sequence of 63, 511, or 2047 bits. One-in-eight (1-in-8) is an 8-bit pattern that contains a single one. 3-in-24 is a 24-bit pattern that contains three ones.  

Bit error rate testers provide several types of interface ports. RS232 is the most commonly employed interface between computer devices and modems. RS422, a balanced serial interface for the transmission of digital data, is designed for greater distances and higher baud rates than RS232. RS485 is an enhanced version of the RS422 protocol that is designed to address communication problems between multiple devices on a single data line. Other interface ports use V.24, V.35, X.21, G.703, and G.704.


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