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ITGD4103 Data Communications and Networks

Lecture-12: Ethernet

week 12- q-2/ 2008

Dr. Anwar Mousa

University of Palestine InternationalFaculty of Information Technology

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Contents:1.Ethernet

2.Fast Ethernet

3.Gigabit Ethernet

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Ethernet

• The Ethernet protocol is by far the most widely used. Ethernet uses an access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection).

– This is a system where each computer listens to the cable before sending anything through the network.

– If the network is clear, the computer will transmit.

– If some other node is already transmitting on the cable, the computer will wait and try again when the line is clear.

– Sometimes, two computers attempt to transmit at the same instant. When this happens a collision occurs.

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Ethernet

– Each computer then backs off and waits a random amount of time before attempting to retransmit.

– With this access method, it is normal to have collisions.

– However, the delay caused by collisions and retransmitting is very small and does not normally effect the speed of transmission on the network.

• The Ethernet protocol allows for linear bus, star, or tree topologies.

• Data can be transmitted over wireless access points, twisted pair, coaxial, or fiber optic cable at a speed of 10 Mbps up to 1000 Mbps.

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Ethernet

• Ethernet was developed by the Xerox Corporation's Palo Alto Research Centre (known colloquially as Xerox PARC) in 1972 and was probably the first true LAN to be introduced.

• In 1985, the Institute of Electrical and Electronic Engineers (IEEE) in the United States of America, produced a series of standards for Local Area Networks (LANs) called the IEEE 802 standards.

• These have found widespread acceptability and now form the core of most LANs.

• One of the IEEE 802 standards, IEEE 802.3, is a standard known as "Ethernet".

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Ethernet • This is the most widely used LAN technology in the

world today. – Although IEEE 802.3 differs somewhat from the

original standard (the "blue book" defined in September 1980) it is very similar, and both sets of standards may be used with the same LAN.

• The IEEE standards have been adopted by the International Standards Organisation (ISO), and is standardised in a series of standards known as ISO 8802-3.

• ISO was created in 1947 to construct world-wide standards for a wide variety of Engineering tasks.

• Adoption of ISO standards allows manufacturers to produce equipment which is guarented to operate anywhere it is finally used.

• ISO standards tend to be based on other standards (such as those produced by the IEEE), the only problem is that the ISO standards tend to be issued later, and are therefore less up to date.

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Ethernet • The simplest form of Ethernet uses a passive bus

operated at 10 Mbps. • The bus is formed from a 50 Ohm co-axial cable which

connects all the computers in the LAN. • A single LAN may have up to 1024 attached systems,

although in practice most LANs have far fewer. • One or more pieces of coaxial cable are joined end to end

to create the bus, known as an "Ethernet Cable Segment".• Each segment is terminated at both ends by 50 Ohm

resistors (to prevent reflections from the discontinuity at the end of the cable)

• and is also normally earthed at one end (for electrical safety).

• Computers may attach to the cable using transceivers and network interface cards.

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An Ethernet LAN consisting of three computers joined by a shared coaxial cable

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Ethernet• Frames of data are formed using a protocol called

Medium Access Control (MAC),

• and encoded using Manchester line encoding.

• 100 Mbps networks may operate full duplex (using a Fast Ethernet Switch) or half duplex (using a Fast Ethernet Hub).

• 1 Gbps networks usually operate between a pair of Ethernet Switches.

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Ethernet

The types of media segments supported by Ethernet are:

• 10B5 Low loss coaxial cable (also known as "thick" Ethernet) • 10B2 Low cost coaxial cable (also known as "thin" Ethernet) • 10BT Low cost twisted pair copper cable (also known as Unshielded

Twisted Pair (UTP), Category-5) • 10BF Fibre optic cable • 100BT Low cost twisted pair copper cable (also known as Unshielded

Twisted Pair (UTP), Category-5) • 100BF Fibre Fast Ethernet • 1000BT Low cost twisted pair copper cable (also known as

Unshielded Twisted Pair (UTP), Category-5) • 1000BF Fibre Gigabit Ethernet • 10000BT Category 6 (Unshielded Twisted Pair (UTP), Category-6) • 10000BF Fibre 10 Gigabit Ethernet

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Ethernet• There is also a version of Ethernet which operates

fibre optic links at 40 Gbps and at 100 Gbps. • Many LANs combine the various speeds of operation

using dual-speed switches– which allow the same switch to connect some ports to one

speed of network, and other ports at another speed. – The higher speed ports are usually used to connect switches

to one another.

• Did you know? The original work on Ethernet at Xerox PARC used 75 Ohm coaxial

cable, and operated at 3 Mbps.• The main reason for developing Ethernet was to share very

expensive printers. • Robert Metcalf (who went on to found 3COM Corp) was working at

Xerox PARC, and is often considered the "father" of Ethernet.

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A Chronology of Ethernet//• 1972 - Ethernet used at Xerox PARC• 1980 - Consortium of DEC, Intel and Xerox announced the Blue Book• 1982 - Version 2 of the Blue Book issued.• 1982 - ISOC RFC 826 definition of the address resolution protocol for Ethernet• 1984 - ISOC RFC 894 definition of IP network using Ethernet links• 1985 - IEEE 802.3 (slightly incompatible with v2)• 1988 - IEEE published a collection of supplements.• 1988 - ISOC RFC 1042 definition of IP network using IEEE 802.3/LLC links.• 1989 - ISO 802.3a Ethernet for thin coaxial cable (10Base2).• 1990 - IEEE 802.3i Ethernet over CAT-5 Unshielded Twisted Pair (10BaseT).• 1990 - IEEE 802.1D Ethernet Bridging.• 1993 - 10BT Hubs and Bridges have become a common component in LANs, and start

replacing 10Base2/10Base5.• 1993 - IEEE 802.3j defines Ethernet over Fibre (10BaseF).• 1993- IEEE 802.1D MAC Layer Bridges (ISO 10038).• 1995 - IEEE 802.3u defines Fast Ethernet (100BaseTX, 100BaseT4, 100BaseFX).• 1998 - 100BT Fast Ethernet has become a common component in LANs (100BaseT4 was

not widely adopted).• 1998 - Full-duplex mode supported in Fast Ethernet.• 1998 - IEEE 802.3z defines Gigabit Ethernet over Fibre (some years later in 802.3 ab

over UTP).• 2001 - IEEE 802.11 (wireless) and Gigabit Ethernet have become common LAN

components.• 2006 - 10 Gigabit Ethernet over Category-6 (10000BT) UTP is available in commercial

products.

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Fast Ethernet • To allow for an increased speed of transmission, the

Ethernet protocol has developed a new standard that supports 100 Mbps.

• This is commonly called Fast Ethernet.

• Fast Ethernet requires the use of different, more expensive network concentrators/hubs and network interface cards.

• In addition, category 5 twisted pair or fiber optic cable is necessary.

• Fast Ethernet is becoming common in schools that have been recently wired.

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Fast Ethernet • The Unshielded Twisted Pair (UTP) cable provides a low cost

Ethernet technology, that supports 10BT. • The UTP cabling system uses a RJ-45 connector and 100

Ohm unshielded twisted pair cabling. • This connects the computer directly (i.e. using a point to point

link) to a wiring hub which acts as a media repeater. The maximum distance of a 10BT link is 100 m.

• 100BASE-T uses the same cabling to provide 100 Mbit/s Ethernet in either a half-duplex (using CSMA/CD) or full-duplex form (switched).

• 100BASE-TX runs over two pairs of wires in Category 5 unshielded twisted pair cable.

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Fast Ethernet //• Like 10BASE-T, the normal pairs are coloured orange and green

pairs (using pins 1, 2, 3 and 6 of the RJ-45 connector). This cable has a bandwidth of less than 100 MHz.

• A Manchester encoded waveform would require 200-400 MHz of bandwidth, far in excess of that offered by the cable.

• A scheme using 4B5B binary encoding therefore generates a series of 0 and 1 bits clocked at 125 MHz; the 4B5B encoding provides DC equalisation and spectrum shaping.

• 4B5B encoding works by mapping each group of four bits (one 1/2 of a byte) to one group of 5 bits.

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Fast Ethernet// • Since there are (2^5) 32 possible combinations of 5 bits, and there

are only (2^4) 16 combinations of 4 bits one half the patterns are unused.

• The chosen set of 16 5-bit patterns are those with the most transitions, this ensures clocking information is present in the signal (for locking the receiver DPLL). This results in a bandwidth increased of 25%.

• Cross-Talk requirements (i.e. to limit RF Emission) led to the need for a scrambler, this randomises the bit stream to prevent the same set of byte values generating a completely repetitive pattern, which would have stong signal components at some characteristic frequencies.

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Fast Ethernet //• The data is finally sent as a 3-level physical waveform known as

MLT-3.• MLT-3 cycles through a set of voltage levels {-1, 0, +1}, to

indicate a 1-bit. • The signal stays the same when transmitting a 0 bit. It takes four 1

bits to generate a complete cycle, this the maximum fundamental frequency is reduced to one fourth of the baud rate.

• This scheme of 45/5b with MLT-3 encoding leads to a waveform of 31.25 MHz, well within the specification for Unshielded Twisted Pair Cabling.

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Fast Ethernet//

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Gigabit Ethernet • The most recent development in the Ethernet standard is

a protocol that has a transmission speed of 1 Gbps. • Gigabit Ethernet is primarily used for backbones on a

network at this time.• it could be used for workstation and server connections

also.• It can be used with both fiber optic cabling and copper.

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Gigabit Ethernet //• The original 10 Mbps Ethernet used Manchester

encoding, while Fast Ethernet (at 100 Mbps) used a 4b/5b MLT code.

• Gigabit Ethernet utilises five levels and 8b/10b encoding, to provide even more efficient use of the limited cable bandwidth,– sending 1 Gbps within approx 100 MHz of bandwidth

(i.e. the capacity of a UTP Cat5e cable).• The interface uses 8b/10b encoding of each byte of data

to generate a 10 bit code that is scrambled and converted into a physical layer signal using a 5-level Pulse-Amplitude-Modulation (PAM).

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Gigabit Ethernet //

•