Ethernet Signal Encoding Includes some material from Forouzan ‘Data Communications’

Fast Ethernet – IEEE 802.3u 100BaseT4 100BaseTX 100BaseFX medium UTP3 at least UTP5, STP Multi Mode Fibre Mode HDX 4-wire FDX 2-wire Range 100 m (seg) 200 m (net) 100/200 m 2 km FDX 412m HDX Coding 8B/6T NRZ 4B/5B MLT-3 4B/5B NRZI On-off Manchester requires 200 Mbaud for 100 Mbps, not very efficient. But it provides a transition in the middle of the bit, hence – self clocking. 4B/5B allows each 4-bit sequence (16 possible patterns) to be coded as a 5 bit symbol (32 possible), avoiding the occurrence of long sequences of 0’s or 1’s. Clocking is thus provided. But requires a baud rate of 125 Mbps for a bitrate of 100 Mbps. MLT-3 (multiline transmission, 3-level) provides good bandwidth performance, signal rate is 0.25 of the bitrate. 8B/6T: codes each 8-bit sequence (256 possible) as a pattern of 6 ternary signal elements (478 usable patterns). The patterns are chosen to provide DC balance and clocking. 100BaseT4 uses 4 pairs of wires, three of which transmit in one direction at a time at 25Maud each. The 8B/6T encoding enable this 75 Mbaud to deliver a rate of 100 Mbps. 8 bits/sec = 6 signals/sec. Fibre has high enough bandwidth to use a simple coding scheme - NRZI

Signal Encoding Coding is chosen for bandwidth efficiency and clock synchronisation Manchester (10BaseT) provides internal clocking, but is only 50% bandwidth efficient 4B/5B and 8B/6T - used in fast Ethernet together with NRZ, NRZI (Non Return to Zero- Inverted) and MLT-3 (Multi-Level Transmit, cycles through -1,0,+1,0,-1,0 …..) NRZ/NRZI are simple, efficient (signal rate = ½ bit rate) but no clocking; long strings of 0’s or 1’s could cause drift. MLT-3 is bandwidth efficient (three levels, signal rate is ¼ the bit rate), but no clocking So we combine a BW efficient code with one that helps with clocking & synchronisation.

4B/5B Each 4 bit nibble is replaced by a 5-bit symbol. 4 bits can contain sixteen 4-bit patterns, some containing 3 or 4 consecutive 0’s and 1’s, which is not good news for clocking. We replace them with 5-bit patterns – 32 of them. We select sixteen of these with no more than 2 consecutive 0’s or 1’s. Signalling rate increased to 125 Mbps – better than Manchester (100%) Synchronisation is improved.

100Base-TX implementation Two cat5 -UTP or STP pairs for TX and RX (FDX operation) The transceiver may be separate or inside NIC; responsible for TX/RX, coding/decoding and collision detect functions

Encoding and decoding in 100Base-TX MLT3 : 3 signal values +V, 0, -V If next bit = 0, no transition If next bit = 1, and current level is not 0, next level = 0 If next = 1 and current level is 0, next level = opposite of last non-zero level MLT-3 for efficiency + 4B/5B for clocking 100 Mbps  125 Mbps

100Base-FX implementation Two fibre optic cables, FDX Transceiver – external or in NIC

Encoding and decoding in 100Base-FX NRZ-I: Non-return to zero, invert on 1’s. (No change on zero’s). 4B/5B for clock synchronisation.

100Base-T4 implementation 100baseTX for 4-wire (2-pairs - same as 10baseT) - 100baseT4 for 8-wire cable (4 pairs)

Using four wires in 100Base-T4 Three pairs (25 Mbps each) are used at any instance for sending, one for collision detect. When sending, the receive only wire pair can be used for CD. 8 bits (256 possible patterns) mapped into into 6 ternary symbols (Plus, zero, minus, giving 3^6 = 729 possibilities). The mapping selects those ternary patterns which give the maximum transitions. Cat3 wires cannot operate at more than 25 Mbps. One of the unidirectional wires is used for CD, three for transmission. 8B/6B coding is used to convert 100 Mbps to 75 Mbaud stream before transmission

8B6T Each octet (256 possible patterns) is coded as a sequence of 6 ternary symbols. Ternary – 3 values (+/0/- or A B C), so 6 ternary symbols can take 3*3*3*3*3*3 = 729 values e.g. 0++0-0 Of these we choose the best 256 to represent our 8 bit strings. Best means – good clocking, lots of transitions (i.e. not to many symbol repeats) Bandwidth: As 8 bits are sent as 6 signals, we only need 75% of the bandwidth. Examples of codes: 00000000 -+00-+ 01010010 -+-+0+ 01110011 000+00

Gigabit Ethernet IEEE 802.z/ab Usually FDX with no CD; HDX defined, rarely used; Fibre or copper jumper (z) or UTP (ab) Integrated chips are available that will allow hybrid operation at 10/100/1000 Mbps by auto negotiation Same protocol & frame format as 10/100 Mbps, but different medium and transmission/coding For HDX hub operation (with CD), enhancements are needed to basic scheme: Reduced slot time (0.512 microsec), shorter range – 25m carrier extension (min. frame size of 512 bytes, 200 m) frame bursting (multiple frames sent each time). UTP/STP - each wire pair can send and receive simultaneously, using hybrids & cancellers

Gigabit Ethernet implementations SX = short wave, LX = long wave, CX = STP copper Topology: Point to point, Star, Hierarchy of stars 1000Base-X are 2-wired and use fibre or STP 1000Base-T is 4-wire (4 x250) FDX, uses Cat5 UTP

Coding 1000Base-X (2-wire) uses NRZ with 8B/10B for clocking; the resulting stream is 1.25 G One wire for send and one for receive. 1000Base-T: 4-wire with 4D-PAM5 encoding (four-dimensional, five-level pulse amplitude modulation); All 4 wires involved in transfer (input & output), at 250 Mbps. Simultaneous TX/RX possible (using hybrids and echo cancellation), hence both FDX & HDX allowed; Range 100 m 4D PAM5 = 4 dimensions (wires), 5 voltage levels (one for FEC), Pulse Amplitude Modulation. Each 8 bit word can be coded into a single signal Details in Forouzan Data Communications & Networking.

1000 Base-X SX short wave (850 nm), 250 to 500 m; 50/62.5 Multi-mode; LX long wave (1300 nm) - 500 m to 5 Km; 10 single-mode, or 50/ 62.5 Multi-mode; CX: range 25 m, single room only; shielded copper cable, high cost; not very common;

1000Base-X implementation

Encoding in 1000Base-X

1000Base-T - cat5 UTP

10 Gbps Ethernet IEEE 802.3ae adopted 2002 Fibre, single or multi-mode, FDX only Up to 40 km, useful for backbones, WANs and MANs; POPs and Local Loops LANs (R- standards), MANs & WANs (10GBase-W) Frame format and addressing the same, but CSMA/CD abandoned. Compatibility with Frame Relay and ATM http://www.ngethernet.com/ethernet_forum/forum-5.html

10G- Standards LANs WANs (over Sonet OC-192 links) SR: 26-82 m, MM fibre, connections to high speed servers, SAN LR: 10 km, SM fibre, campus backbones, MANs ER: 40 Km, SM fibre; MANs WANs (over Sonet OC-192 links) SW: MM, 300m LW: SM 10 km EW: SM 40 km Tomso, Tittel & Johnson (Guide to Networking Essentials) p261 SAN = Storage Area Networks SM = single Mode, MM = multimode S=short , L = long, E = extended range