Handbook of Local Area Networks, 1998 Edition:LAN Basics
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Bit Transformation Schemes
The NRZ schemes are not suitable for LAN transmission because of the lack of synchronization. For traditional LANs, such as 10M bps Ethernet and 4M bps and 16M bps Token Ring, biphase codes are adequate. However, when data rates of 100M bps or more are needed, the biphase codes produce an unacceptably high signaling rate. For example, at a date rate of 100M bps, a signaling rate of 200M baud is required using a biphase code. To reduce bandwidth requirements, retain the synchronization characteristic of biphase codes, and achieve an effective signal spectrum shape, designers have turned to the use of bit transformation schemes. These schemes alter the input so that the desired transmission characteristics are achieved without resorting to biphase. The schemes that have found application in recent LAN designs are:
• 4B5B.
• 8B6T.
• 5B6B.
• 8B10B.
A primary object of all of these bit mappings is to structure the input bit stream so that there are frequent transitions between the two binary digits. As long as transitions are frequent, a biphase code is not needed for synchronization, and the more efficient NRZ codes can be used. Exhibit 1-5-4 shows which LAN systems use which bit mappings and which signal encoding formats.
Exhibit 1-5-4. Encoding Schemes for LANs
4B5B Code
The 4B5B code, used in combination with NRZI, is a popular choice for 100M bps transmission over optical fiber. This scheme is used for the optical fiber versions of FDDI and Fast Ethernet (100Base-FX).
For 4B5B, encoding is done four bits at a time. Each four bits of data are encoded into a symbol with five code bits, such that each code bit contains a single signal element—a block of five code bits is referred to as a code group. In effect, each set of 4 bits is encoded as 5 bits. There is a second stage of encoding: each code bit of the 4B5B stream is treated as a binary value and encoded using NRZI. Because NRZI is a differential encoding format, it aids the ultimate decoding of the signal after it has been converted back from the optical to the electrical realm.
Exhibit 1-5-5 shows the symbol encoding. Because four bits are being encoded with a 5-bit pattern, only 16 of the 32 possible patterns are needed for data encoding. The codes selected to represent the 16 4-bit data blocks are such that a transition is present at least twice for each 5-code group code. No more than three zeros in a row are allowed across one or more code groups. The encoding scheme can be summarized as follows:
Exhibit 1-5-5. 4B5B Code
• A simple NRZ encoding is rejected because it does not provide synchronization. A string of 1s or 0s will have no transitions.
• The data to be transmitted must first be encoded to assure transitions. The 4B5B code is chosen over Manchester because it is more efficient.
• The 4B5B code is further encoded using NRZI so that the resulting differential signal will improve reception reliability.
• The specific 5-bit patterns chosen for the encoding of the 16 4-bit data patterns are chosen to guarantee no more than three zeros in a row to provide for adequate synchronization.
Those code groups not used to represent data are either declared invalid or assigned special meaning as control symbols. For example, two of the patterns (codes 11000 and 10001) always occur in pairs and act as start delimiters for a frame.
The combination of 4B5B and NRZI provides an efficient, reliable transmission technique. At 100M bps, a signaling rate of 125M baud is required. By contrast, using Manchester or Differential Manchester, a signaling rate of up to 200M baud is needed.
Although 4B5B/NRZI is effective over optical fiber, it is not suitable as is for use over twisted pair because the signal energy is concentrated in such a way as to produce undesirable radiated emissions from the wire. MLT-3, which is used on both 100Base-TX and the twisted pair version of FDDI, is designed to overcome this problem. For these LANs, either shielded twisted pair or high-quality Category 5 unshielded twisted pair is used. In essence, the scheme used is 4B5B/MLT-3.
At a data rate of 100M bps, the effect of using MLT-3 is to concentrate most of the energy in the transmitted signal below 30 MHz, which reduces radiated emissions.
8B6T Code
100Base-T4 is designed to produce a 100M bps data rate over lower-quality voice grade, or Category 3, cable. The advantage of this is that in many existing buildings, there is an abundance of voice-grade cabling and very little else. Thus, if this cabling can be used, installation costs are minimized.
With present technology, a data rate of 100M bps over one or two Category 3 pairs is impractical. Instead, 100Base-T4 specifies that the data stream to be transmitted is divided into three separate data streams. Four twisted pairs are used. Data are transmitted using three pairs and received using three pairs. Thus, two of the pairs must be configured for bidirectional transmission.
As with 100Base-X, a simple NRZ encoding scheme is not used for 100Base-T4. This would require a signaling rate of 33M bps on each twisted pair and does not provide synchronization. Instead, a ternary signaling scheme known as 8B6T is used. With ternary signaling, each signal element can take on one of three values—positive voltage, negative voltage, or zero voltage. A pure ternary code is one in which the full information-carry capacity of the ternary signal is exploited. However, pure ternary is not attractive for the same reasons that a pure binary (NRZ) code is rejected: the lack of synchronization. The 8B6T code is designed to approach the efficiency of ternary and overcome this disadvantage.
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