UNIVERSITY OF HONG KONG Faculty of Engineering M.Sc.(Eng) …work6000/S02 intro to Ethernet 101117.pdf · UNIVERSITY OF HONG KONG Faculty of Engineering M.Sc.(Eng) in Building Services

UNIVERSITY OF HONG KONG Faculty of Engineering

M.Sc.(Eng) in Building Services Engineering MEBS6000 2010 Utilities Services

K.F. Chan (Mr.) Page 1 of 27 Sept 2010 All rights reserved

Brief introduction to Ethernet

- IEEE802.3

- 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.




Ethernet is a baseband medium. For the earlier versions of 10Mbps Ethernet, the

digital data is put directly on the line using Manchester Encoding




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.




CSMA / CD PROCESS

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




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

sµ25000,000,200

22500 =×

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. 51.2 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




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 0-1-2-3 slots, then, 0-1-2-3-4-…-7

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]




100 Base T4

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 2

1

8

6

3

100 ×× = 12.5ΜΗz, which can be easily handled by

Cat 3 cables.

8B6T

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.







SWITCHED ETHERNE T

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.




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

0000 11110

0001 01001

0010 10100

0011 10101

0100 01010

0101 01011

0110 01110

0111 01111

1000 10010

1001 10011

1010 10110




Nibble of data 5-bit code used to replace nibble

1011 10111

1100 11010

1101 11011

1110 11100

1111 11101

There are 3 more important code values:

11111 is used when the line is idle

00000 is used when the line is dead

00100 is a halt signal

100BaseTX employs 4B5B encoding, MLT3, thus 4

1

4

5100 ×× =31.25MHz, which is

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)




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.




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 5th 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.




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. 1000 Mbps -- 125 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.




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

2

1

4

1000× = 125MHz, which can be handled by Cat 5 cables. Full duplex transmission

only, no need for contention. Drop cable 100m.







Name Physical

Topology

Cable Type Max. length No. of nodes Remarks Duplex Block

code

Line

encoding

10 Base 2

Thin Ethernet

(Originally

802.3a)

BUS One thin

Coax

RG-58 A/U

or

RG-58 C/U,

50Ω

185m/segment,

925m/network

30 /segment Cheapest System, not

used anymore

Half 4B5B Manchester

10 Base 5

Thick Ethernet

(Originally

802.3b)

BUS One thick

Coax

RG8, 50Ω

500m/segment

2500m/network

100 /segment

1024 /network

Original version of

Ethernet, typically

used to connect cable

closets in the past

Half 4B5B Manchester

10 Base T

(Originally

802.3i)

Star

wired bus

2 pairs Cat 3,

Cat 5,

UTP or STP

100m drop

cable

1024 /network

(max. 4 levels

of hubs)

Inexpensive, easy to

install and maintain.

Half /

Full

4B5B Manchester

100 Base T4

802.3u

Star

wired bus

4 pairs Cat 3

UTP or better

100m drop

cable

1024 /network Allows Cat 3 cable;

does not allow full

Half 8B6T PAM3




Name Physical

Topology


code

Line

encoding

duplex.

100 Base TX

802.3u

Star

wired bus

2 pair

Cat 5 UTP

100m drop

cable

1024 /network Full duplex at 100

Mbps between node

and switch. Most

widely accepted fast

Ethernet.

Half /

Full

4B5B MLT3

10 Base F

(Originally

802.3j)

STAR 2 multi-mode

Fibre

500m or 2000m

depending on

version

2 station per

cable segment

1024 /network

Long distance;

uncommon.

Half /

Full

4B5B NRZI

100 Base FX

802.3u

STAR 2 multi/single

mode

Fibre

400/2000m

(half/full

duplex) / 10km

2 station per

cable segment

1024 /network

Full duplex at 100

Mbps, long run. Not

popular due to

emerging Gigabit

Ethernet on copper.

Full 4B5B NRZI

1000BaseT

IEEE802.3ab

Star 4 pairs Cat 5

UTP

100m drop

cable,

maximum 1

repeater

1 station per

drop cable

Full 8B1Q4 4D PAM5




Name Physical

Topology


code

Line

encoding

1000BaseCX

(cluster)

Star 2 pairs Cat5

STP

25m drop cable 1 station per

drop cable

Rarely used Full 8B10B NRZ

1000BaseLX

802.3z

Star 2 single

mode fibre

5km 1 station per

drop cable

Full 8B10B NRZ

1000BaseSX

(short)

802.3z

Star 2 multi-mode

fibre

550m 1 station per

drop cable

Full 8B10B NRZ

10GBase-E

802.3ae

Star 2 single

mode fibre

Up to 40km 1 station per

drop cable

Full 64B66B NRZ

10Gbase-CX4

802.3ak

Star 4 special twin

axial coaxial

cable

(twin-ax)

15m 1 station per

drop cable

Used for connecting

servers, routers,

telecom closets

full 8B10B NRZ

10Gbase-T

(10GE)

802.3an

Star 4 pairs Cat5e

or Cat6 UTP

100m 1 station per

drop cable

full 8B10B NRZ




10-Gbit Ethernet

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 ( mµ ) Maximum cable length

850nm serial Multimode fibre/50 65m

1310nm WWDM Multimode fibre/62.5 300m

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.




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.




[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.




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]




Ethernet MAC Addressing

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




FEATURES

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.




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, 3rd 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: http://grouper.ieee.org/groups/802/3/ethernet_diag.html )




Appendix 1

American wire gauge Diameter, mm Diameter, inches

19 0.910 0.036

22 0.664 0.025

24 0.511 0.020

26 0.405 0.016

28 0.032 0.012

[Table adopted from FREEMAN, Roger L. Fundamentals of Telecommunications]

Documents

UNIVERSITY OF HONG KONG Faculty of Engineering M.Sc.(Eng) …work6000/S02 intro to Ethernet 101117.pdf · UNIVERSITY OF HONG KONG Faculty of Engineering M.Sc.(Eng) in Building Services