UNIVERSITY OF HONG KONG Faculty of Engineering M.Sc.(Eng) in Building Services Engineering MEBS Utilities Services

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1 Brief introduction to Ethernet - IEEE802.3 UNIVERSITY OF HONG KONG - Widely used LAN standard, data link technology of broadband connection - Jointly developed by Xerox, Intel and Digital Equipment - Contention based protocol - Named after the luminiferous ether, through which the electromagnetic radiation was once thought to propagate. DIGITAL CODING TECHNIQUES Amplitude modulation is relatively little used in digital transmission. Normal application of carrier modulation (frequency, phase) is found in modems for data transmission along analog telephone lines. Carrier frequencies for transmission on broadband cables may reach up to 500MHz. K.F. Chan (Mr.) Page 1 of 27 Sept 2010

2 Ethernet is a baseband medium. For the earlier versions of 10Mbps Ethernet, the digital data is put directly on the line using Manchester Encoding K.F. Chan (Mr.) Page 2 of 27 Sept 2010

WITH CSMA/CD, COMPUTERS CHECK FOR CABLE TRAFFIC The method used in Ethernet for medium access co-ordination is a protocol called Carrier-Sensing Multiple Access with Collision Detection.

3 WITH CSMA/CD, COMPUTERS CHECK FOR CABLE TRAFFIC The method used in Ethernet for medium access co-ordination is a protocol called Carrier-Sensing Multiple Access with Collision Detection. The units decide independently when to access the line and start transmission. The transmitting unit constantly listens to the channel (Carrier Sense). If any other units had also sensed a free channel and had attempted transmission at the same time, both units would detect that their signals are garbled and immediately stop transmitting (Collision Detection). To ensure that all units detect that message collision had taken place, after the interruption, both units put a short 32 bit noisy jam signal on the line. K.F. Chan (Mr.) Page 3 of 27 Sept 2010

4 CSMA / CD PROCESS UNIVERSITY OF HONG KONG MINIMUM FRAME LENGTH If a station tries to transmit a very short frame, it is conceivable that a collision occurs, but the transmission completes before the noise burst gets back at 2t. The sender will then incorrectly conclude that the frame was successfully sent. To prevent this situation from occurring, all frames must take more than 2t to send. MAXIMUM CABLE LENGTH For 10Base5 Ethernet, the IEEE specification suggests that the maximum length between the most remotely connected points is 2500m, and each segment should not exceed 500m, maximum 4 repeaters. Similar limits exist for other versions of K.F. Chan (Mr.) Page 4 of 27 Sept 2010

5 Ethernet. MINIMUM FRAME LENGTH Speed of electric field in cables is about 200,000 km/s. For signal to reach the most remote point, burst and the burst received by the sender, the time required is = 25µs 200,000,000 Because it is necessary to have 4 repeaters in a 2500m bus, each repeater will introduce a few micro-seconds delay, the worst case time delay in Ethernet is taken as 50 s. For a bit rate of 10Mbps, this is equal to 500 bits. Adding a safety margin of 12 bit, minimum frame size becomes 512 bits or 64 bytes s is also called slot time. IEEE suggested the minimum frame must take at least 50 s to send, say 51.2 s. At a transmission rate of 10Mbps, (each bit 0.1 s) this time corresponds to 64 bytes (512 bits). Frames with fewer bytes are padded out to 64 bytes. (message data shorter than 46 bytes padded with empty character). Maximum frame size is 1518 bytes. This minimum frame size requirement is used in all 10 and 100Mbps versions of Ethernet. Though the minimum frame size is different, a minimum frame size (512 bytes) exists for the 1000Mbps Ethernet too. ETHERNET FRAME For the 64 byte version, format of an Ethernet frame is as below: FRAME SIZE/ CABLE LENGTH/ TRANSMISSION RATE As the network speed goes up, the minimum frame length must go up or the maximum cable length must come down, proportionally. For a 2500m LAN operating at 1 Gbps, the minimum frame size would have to be 6400 bytes. Or the minimum frame size could be 640 bytes and the maximum distance between any 2 stations K.F. Chan (Mr.) Page 5 of 27 Sept 2010

6 reduced to 250m. In reality the standard is set at 200m maximum drop cable length and 512 bytes minimum frame size for Gigabyte Ethernet running on Cat 5 cables, full duplex. These restrictions are becoming increasingly painful as we move toward gigabit networks. In practice, preamble and start of frame delimiter is not counted in the minimum of 64 bytes requirement. Exponential back off The 50 s is called time slot in Ethernet. If a transmitting unit does not detect a collision for the duration of the first time slot, it is safe. But if 2 units have to wait after a signal collision, they do so for a random time period of 0 or 1 time slots (0 to 50 s) before they attempt a new transmission. The probability of a new collision is now 50%. If a new collision occurs, the range from which the waiting period is selected at random is increased by powers of 2 to slots, then, slots and so on, up to a maximum of 1023 time slots (50ms). In that case a collision still occurs, the units assume that the problem has a different cause and report the situation to the higher layers. The first time slot after a successful transmission is reserved for immediate acknowledgement from the receiver to the transmitter. CSMA/CD LAN relationship to the OSI model [Adopted from FREEMAN, Roger, Fundamentals of Communications] K.F. Chan (Mr.) Page 6 of 27 Sept 2010

100 Base T4 UNIVERSITY OF HONG KONG In the old days, there were a lot of voice and data installations running on Cat 3 structured cablings. Cat 3 cables were rated up to 16MHz.

7 100 Base T4 UNIVERSITY OF HONG KONG In the old days, there were a lot of voice and data installations running on Cat 3 structured cablings. Cat 3 cables were rated up to 16MHz. It was with Cat 3 cables in mind that 100BaseT4 was designed. 100BastT4 is half duplex only, uses all 4 pairs in Cat 3 cable (3 pairs in each direction), fan out, 8B6T encoding, PAM3 (Pulse Amplitude Modulation 3 levels, 2 bits per clock cycle), thus Cat 3 cables = 12.5ΜΗz, which can be easily handled by B6T The signaling used in 8B6T (8 bit 6 trit) encoding is ternary, which means that the signal can have one of three possible values. A positive, negative, or zero voltage correlates to the three possible signal stages. Information on an 8B6T encoding scheme is grouped together in 8-bit data blocks where each chunk of data is mapped to a code that consists of a code group of 6 ternary symbols. The reasons why this type of encoding to a code group of 6 was done to provide synchronization and to produce dc balance on the transmission media. When the average voltage on the line is zero, then dc balance has been obtained. To produce the dc balance, the combination of signals grouped as a collection of 6 contains either an equal number of positive and negative values or contains one extra positive voltage value. The 8B6T encoding mechanism is used on 100BaseT4. K.F. Chan (Mr.) Page 7 of 27 Sept 2010

8 K.F. Chan (Mr.) Page 8 of 27 Sept 2010

9 SWITCHED ETHERNET UNIVERSITY OF HONG KONG All 10Mbps Ethernet and 100BaseT4 Ethernet do not require a switch for operation computers or devices are connected directly to the bus and contend for time slot to transmit. All 10Mbps Ethernet are logically bus in topology although 10BaseT and 100BaseT4 are physically star in topology, logically they are still bus in topology. However, 100BaseTX, all Gigabit and 10G Ethernet are switched computers and devices are individually connected to a switch which acts as a central device regulating contentions. Switched Ethernet is thus star in topology physically. 100 BASE TX FAST ETHERNET Cat 5 structured cable is rated up to 100MHz. If Manchester encoding is used for 100Mbps transmission, the line frequency will be 200MHz, exceeding the capacity of Cat 5 cables. It is with this in mind that 100BaseTX was designed. 100BaseTX Fast Ethernet employs MLT-3. MLT-3 cycles through a set of voltage levels (-1, 0, +1) to indicate an 1. The signal stays the same when transmitting a 0. It thus takes four 1 s to generate a complete cycle of change in voltage, thus reducing the baud rate to one quarter. K.F. Chan (Mr.) Page 9 of 27 Sept 2010

10 There is one problem left with Multiple Level Transitions with three levels (MLT-3) and NRZI: if a large number of 0 s are sent in a row, there would be no change in voltage. The receiving clock could thus become unsynchronized. To solve this, the 4B5B encoding is used. This encoding simply creates a table of all the possible values of a nibble (i.e. half of a byte) and maps those values to a corresponding 5-bit value, where every 5-bit values include at least two 1 s. This means that no combination of actual data values will ever allow more than 3 bit-times to pass without transitioning the voltage. 4B5B ENCODING Nibble of data 5-bit code used to replace nibble K.F. Chan (Mr.) Page 10 of 27 Sept 2010

11 Nibble of data UNIVERSITY OF HONG KONG bit code used to replace nibble There are 3 more important code values: is used when the line is idle is used when the line is dead is a halt signal BaseTX employs 4B5B encoding, MLT3, thus 100 =31.25MHz, which is 4 4 easily accommodated by Cat 5 cabling. It is capable of full duplex transmission, no need for contention, traffic is controlled by switches, and uses 2 pairs out of 4 in the Cat 5 cable. 100Base FX NRZI but not MLT3, 4B5B encoding, duplex transmission. 100Mbps Fast Ethernet still uses 512 bit minimum frame size but drop cable length limited to 100m (thus maximum distance between 2 stations becomes 200m, instead of 2500m) K.F. Chan (Mr.) Page 11 of 27 Sept 2010

Gigabit Ethernet Gigabit Ethernet on fibre The 4B5B NRZ encoding used on Fast Ethernet fibre was not adopted for Gigabit Ethernet over fibre because of its lack of DC balance.

12 Gigabit Ethernet Gigabit Ethernet on fibre The 4B5B NRZ encoding used on Fast Ethernet fibre was not adopted for Gigabit Ethernet over fibre because of its lack of DC balance. Maintaining DC balance is important because if a transmitter sends more 1 s than 0 s, it will result in heating by the laser beam, thus higher error rates. Just as data is doubly encoded on Fast Ethernet, a pair of block and line encoding is adopted for Gigabit Ethernet over fibre. The block encoding is 8B10B. Note that it will result in 1.25GHz. 8B10B encoding This is an IBM patented encoding method. In the 8B10B encoding scheme, each chunk of 8 bits of data is mapped to a 10 bit code group. The 8B10B method provides more error detection capabilities than 4B/5B and ensures that sufficient clock information is present in the serial data stream in case the sender and the receiver drift out of synchronization. The 8B10B encoding mechanism is used for Fibre Channel and the different Gigabit Ethernet implementations except those using twisted pair cables. K.F. Chan (Mr.) Page 12 of 27 Sept 2010

13 1000BaseT Minimum frame size 512 bytes instead of 64 bytes. Drop cable limited to 100m (with maximum 1 repeater between 2 stations) thus maximum distance between 2 stations becomes 400m not 2500m or 200m. 4D-PAM5 Gigabit Ethernet running on twisted pair 1000 BaseT uses four dimensional, Pulse Amplitude Modulation 5 level (PAM5). The encoding methodology takes advantage of the techniques used to provide high-speed communication over Cat 5 copper cables. The implementation of 4D-PAM5 uses 4 twisted-pair links where each link provides a 250Mbps data rate. Each of the Cat 5 cables function at full duplex so signal can be transmitted in both directions at the same time. On each of the cables, 2 bits represents a symbol and the PAM5 encoding scheme uses 5 different voltage levels. 4 of the 5 voltage levels are used to encode the 2-bit data chunks and the 5 th is used for error correction. 4D-PAM5 is very complex and incorporates scrambling techniques to improve the signal quality by producing balanced patterns of ones and zeros. K.F. Chan (Mr.) Page 13 of 27 Sept 2010

The data stream appears as a constellation, whereby a digital word is represented by an electrical signal that has a distinctive amplitude and phase relative to a marker signal.

14 The data stream appears as a constellation, whereby a digital word is represented by an electrical signal that has a distinctive amplitude and phase relative to a marker signal. The advantages of this type of coding are: more bits per bandwidth i.e Mbps MHz The disadvantages are: very complex electronics greater sensitivity to noise Just as data is doubly encoded on Fast Ethernet, a pair of block and line encoding method is also used on 1000Base T the Gigabit Ethernet over copper. The signal is first encoded using 8B1Q4 and then the 4D PAM5. The 8B1Q4 (8 bits plus 1 bit to 4 quinary symbols) encoding method converts each group of 8 data bits to 4 quinary symbols. Each quinary symbol is then line encoded using 4D PAM5 a system using 5 voltage levels. As 2 bits are represented in each quinary symbol, and the clock rate is set at 125MHz, this gives 250Mbps data per twisted pair and therefore 1000Mbps for the whole cable. K.F. Chan (Mr.) Page 14 of 27 Sept 2010

15 IEEE802.3ab dual duplex 5-level pulse amplitude modulation [Adopted from FRENZEL, Louis E. Jr. Principles of Electronic Communication Systems] NEXT FEXT 1000BaseT divides the signal across all four pairs of Cat 5 UTP, and transmits in both directions simultaneously down each pair. 2 bit per pair in one clock cycle, thus = 125MHz, which can be handled by Cat 5 cables. Full duplex transmission 4 2 only, no need for contention. Drop cable 100m. K.F. Chan (Mr.) Page 15 of 27 Sept 2010

16 K.F. Chan (Mr.) Page 16 of 27 Sept 2010

17 Name 10 Base 2 Thin Ethernet (Originally 802.3a) 10 Base 5 Thick Ethernet (Originally 802.3b) 10 Base T (Originally 802.3i) 100 Base T u Physical Topology BUS BUS Star wired bus Star wired bus Cable Type Max. length No. of nodes Remarks Duplex Block code Line encoding One thin 185m/segment, 30 /segment Cheapest System, not Half 4B5B Manchester Coax 925m/network used anymore RG-58 A/U or RG-58 C/U, 50Ω One thick 500m/segment 100 /segment Original version of Half 4B5B Manchester Coax 2500m/network 1024 /network Ethernet, typically RG8, 50Ω used to connect cable closets in the past 2 pairs Cat 3, 100m drop 1024 /network Inexpensive, easy to Half / 4B5B Manchester Cat 5, cable (max. 4 levels install and maintain. Full UTP or STP of hubs) 4 pairs Cat 3 100m drop 1024 /network Allows Cat 3 cable; Half 8B6T PAM3 UTP or better cable does not allow full K.F. Chan (Mr.) Page 17 of 27 Sept 2010

18 Name 100 Base TX 802.3u 10 Base F (Originally 802.3j) 100 Base FX 802.3u 1000BaseT IEEE802.3ab Physical Topology Star wired bus STAR STAR Cable Type Max. length No. of nodes Remarks Duplex Block Line code encoding duplex. 2 pair Cat 5 UTP 100m drop cable 2 multi-mode 500m or 2000m Fibre depending on version 2 multi/single 400/2000m mode (half/full Fibre duplex) / 10km Star 4 pairs Cat 5 UTP 100m drop cable, maximum 1 repeater 1024 /network Full duplex at 100 Mbps between node and switch. Most widely accepted fast Ethernet. Half / Full 4B5B MLT3 2 station per Long distance; Half / 4B5B NRZI cable segment uncommon. Full 1024 /network 2 station per Full duplex at 100 Full 4B5B NRZI cable segment Mbps, long run. Not 1024 /network popular due to emerging Gigabit Ethernet on copper. 1 station per Full 8B1Q4 4D PAM5 drop cable K.F. Chan (Mr.) Page 18 of 27 Sept 2010

19 Name 1000BaseCX (cluster) 1000BaseLX 802.3z 1000BaseSX (short) 802.3z 10GBase-E 802.3ae 10Gbase-CX ak 10Gbase-T (10GE) 802.3an Physical Topology Star Star Star Star Star Star UNIVERSITY OF HONG KONG Cable Type Max. length No. of nodes Remarks Duplex Block 2 pairs Cat5 STP 2 single mode fibre 2 multi-mode fibre 2 single mode fibre 4 special twin axial coaxial cable (twin-ax) 4 pairs Cat5e or Cat6 UTP 25m drop cable 5km 550m Up to 40km 15m 100m 1 station per drop cable 1 station per drop cable 1 station per drop cable 1 station per drop cable 1 station per drop cable 1 station per drop cable code Line encoding Rarely used Full 8B10B NRZ Used for connecting servers, routers, telecom closets Full 8B10B NRZ Full 8B10B NRZ Full 64B66B NRZ full 8B10B NRZ full 8B10B NRZ K.F. Chan (Mr.) Page 19 of 27 Sept 2010

20 10-Gbit Ethernet UNIVERSITY OF HONG KONG The newest version of Ethernet is 10-Gbit Ethernet (10GE), which permits data speeds up to 10Gbps over fibre optic cable. The defining IEEE standard is 802.3ae. As with Gigabit Ethernet, there are several versions, but all use 8B/10B coding. Laser wavelength Cable type/size ( µ m ) Maximum cable length 850nm serial Multimode fibre/50 65m 1310nm WWDM Multimode fibre/ m 1310nm WWDM Single mode fibre/9 10km 1310nm serial Single mode fibre/9 10km 1550nm serial Single mode fibre/9 40km Three of the five variations use serial data transmission. The other two use wide-wavelength division multiplexing (WWDM) which is also known as coarse wavelength division multiplexing (CWDM) it is like frequency division multiplexing and divide the data into four channels and transmit it simultaneously over four different wavelengths of infrared light near 1310nm. It is hard to believe that a 10Gbps pulse signal could be carried over a copper cable, given the huge attenuation and distortion that the cable capacitance, inductance, and resistance can cause. Yet, today there is one copper version of 10GE, now available. Called 10Gbase-CX4, this version of Ethernet is standardized by the IEEE standard 802.3ak. The cable is a special twin axial coaxial cable (called twin-ax) that contains two conductors inside the outer shield. Four of these coaxial cable assemblies are combined to make a cable. The data to be transmitted is divided into four parallel paths that transmit at 3.125Gbps. The encoding is 8B/10B. This gives an actual data rate of 2.5Gbps, four paths of an aggregate of 10Gbps. The range is limited to roughly 15m. This is sufficient for connecting several servers, routers, Ethernet switches, and other equipment in wiring closets, data centres, or server farms, where the equipment are located close to each other. Another copper version of 10GE has been in development for several years. Designated 802.3an or 10Gbase-T, it is designed to use the four pairs of conductors in Cat5e or Cat6 UTP. The range is 100m. Because of the severe cross talk that occurs in UTP at 10Gbps, extensive DSP filtering and equalization is employed. K.F. Chan (Mr.) Page 20 of 27 Sept 2010

21 The speed and distance capabilities make 1-Gbps and 10-GE attractive for MAN applications. The 40-km version even makes 10-Gbps Ethernet appropriate for some WAN applications. It may eventually replace the more complex and expensive SONET now common in most MANs and WANs. Ethernet in the First Mile Also known as Ethernet Passive Optical Network (EPON), EFM is the IEEE standard 802.3ah. It is a version of Ethernet designed to be used in fibre optic networks that connect homes and businesses to high speed Internet services. The first mile, also called the last mile, is a term used to describe the relatively short connection from a home or office to a local terminal or connection point that distributes data via a fibre optic link. The EFM system uses the standard Ethernet protocols at a speed of 1.25Gbps. It permits up to 32 users per connection, and the maximum range is about 20km. Power over Ethernet PoE is an addition to Ethernet LANs that is used to deliver dc power to remote devices connected to the network. Specifically, it suppliers about 48V of unregulated direct current over two of the twisted pairs in a CAT5 UTP cable. This eliminated the need for some devices on the LAN to have their own power supply, and it eliminates the need for some remote device to be near a power supply socket outlet. Some examples of applications are wireless access points used to extend the LAN and Voice over Internet Protocol (VoIP) telephones which are rapidly replacing standard switched analogue phones. There are numerous industrial applications as well. K.F. Chan (Mr.) Page 21 of 27 Sept 2010

22 [Adopted from FRENZEL, Louis E. Jr. Principles of Electronic Communication Systems] The above figure shows a common PoE arrangement. A 48V dc power supply is connected to the centre taps of the I/O transformers in the Ethernet NIC (Network Interface Card). These transformers carry the serial Ethernet data. Both wires in each pair carry the direct current. The wires in the twisted pairs are effectively in parallel for direct current. The direct current does not interfere with the data. On the receiving end, transformers accept the signal and pass it along to the NIC circuitry in that device as usual. The dc voltage is captured from the centre taps. This dc voltage is then translated to another dc level by a dc-dc converter or a voltage regulator. Voltage of 24, 12, 6, 5 and 3.3V are common. This voltage powers the interface circuits at that end of the cable, thereby eliminating the need for a separate ac power supply line. The choice of 48V was based on the fact that the wires in Cat5 cable are very small, usually gauge 28 (see Appendix 1). Smaller wires have higher dc resistance and so can produce a rather large voltage drop along the cable. By keeping the voltage high, the line current is less for a given amount of power consumption in the load, thereby producing much less of a voltage drop. In practice, the maximum range is only 100m, and the voltage can usually be anything from 44 to 57V as the dc supply is unregulated. K.F. Chan (Mr.) Page 22 of 27 Sept 2010

23 The maximum allowed current is 550mA, although the current is usually held to a value of 350mA or less. At 48V, this translates to a maximum current consumption of 16.8W. The standard states that the maximum desirable load is 15.4W. Most loads consume much less than that. Power over Ethernet is designed to work with all UTP versions of Ethernet including 10, 100 and 1000Mbps systems. Only two pairs are used. The dc power is applied to the cable with a separate piece of equipment called an injector. Sometimes the direct current is supplied inside a hub or switch. Different versions of the standard vary with the pairs defined to carry the direct current and which pins on the RJ45 jacks are used. Some companies offer variations that supply 12V instead of 48V. [This section of material adopted from FRENZEL, Louis E. Jr. Principles of Electronic Communication Systems] K.F. Chan (Mr.) Page 23 of 27 Sept 2010

Ethernet MAC Addressing UNIVERSITY OF HONG KONG The Ethernet medium access control address consists of two parts: 1. The first set of octets: Define the Unique Manufacturers ID 2.

24 Ethernet MAC Addressing UNIVERSITY OF HONG KONG The Ethernet medium access control address consists of two parts: 1. The first set of octets: Define the Unique Manufacturers ID 2. The second set of octets: Define the Serial Number of the Network Interface Card (NIC) Multicast Addressing Communicate with many devices on a network simultaneously Broadcast Addressing Meant to be heard by all stations on the network K.F. Chan (Mr.) Page 24 of 27 Sept 2010

25 FEATURES UNIVERSITY OF HONG KONG Ethernet s concept is flexible and open Some companies complete Ethernet-based communication packages which may also implement higher level services in the OSI hierarchy Ethernet is non-deterministic DRAWBACK Theoretically there is no upper bound to the time it may take to access the medium and transfer a message. This can be serious drawback for industrial REAL-TIME applications where it is necessary to know exactly the worst-case performance in advance. REMARKS On heavily trafficked networks collisions are fairly common however a collision rate greater than 5% of all traffic is unusual and may mean a problematic Network Interface Card or poor cabling on the network. K.F. Chan (Mr.) Page 25 of 27 Sept 2010

Remarks on development of LAN standards The same thing has happened with LAN s as with all other products for communication and automation technology.

26 Remarks on development of LAN standards The same thing has happened with LAN s as with all other products for communication and automation technology. In the beginning it was hoped that only one general LAN standard for office and industrial applications would be selected and everyone would follow it. When it was time to make a decision, there were 3 competing and mutually incompatible technology each backed by DIX, GM and IBM. The committee charged with defining the standard could not agree on any one of them. In the end it was decided 3 different standards would be better than no standard at all. This led to today s IEEE802.3 CSMA/CD, IEEE802.4 Token Bus and IEEE802.5 Token Ring. Time has not allowed us to look at them all in detail. As IEEE802.3, commonly called Ethernet, is commonly used in LAN and broadband connection and also adopted by BACnet, it is studied in this course. This set of lecture material is mostly taken from 1) TANENBAUM, A S. Computer network, Prentice Hall, NJ (2003) 2) HALSALL, Fred. Computer networking and the Internet, Addison-Wesley, Harlow (2005) 3) FRENZEL, Louis E. Jr. Principles of Electronic Communication Systems, 3 rd Ed., McGraw Hill, NY (2008) This diagram was hand drawn by Robert M. Metcalfe and photographed by Dave R. Boggs in 1976 to produce a 35mm slide used to present Ethernet to the National Computer Conference in June of that year. On the drawing are the original terms for describing Ethernet. (Source: ) K.F. Chan (Mr.) Page 26 of 27 Sept 2010

27 Appendix 1 American wire gauge Diameter, mm Diameter, inches [Table adopted from FREEMAN, Roger L. Fundamentals of Telecommunications] K.F. Chan (Mr.) Page 27 of 27 Sept 2010

Ethernet Technologies

Ethernet Technologies CCNA 1 v3 Module 7 NESCOT CATC 1 10 Mbps Ethernet Legacy Ethernet means: 10BASE5 10BASE2 10BASE-T Common features are: frame format timing parameters transmission process basic design