3. Ethernet

Two nodes transmitat the same time

1

Node detect therehas been a collision

2

Nodes transmit ajamming signal

3

Nodes wait a randomperiod before retransmitted

4

3.1 Ethernet (Advantages/Problems)

Advantages:Ethernet networks are easy to plan and cheap to install.Ethernet network components, such as network cards and connectors, are cheap and well supported.Uses coaxial, fibre or twisted-pair cables.It is a well-proven technology, which is fairly robust and reliable.It is simple to add and delete computers on the network.It is supported by most software and hardware systems.Available as 10Mbps (10BASE), 100Mbps (100BASE) and 1Gbps (1000BASE). Dual-speed networks can be used, such as mixed 10Mbps/100Mbps networks. Network hub negotiates the required speed.Easy upgrade for different network speed. A dual-speed network can be run, and gradually upgraded.Standardised as 1EEE 802.3.

Advantages:Ethernet networks are easy to plan and cheap to install.Ethernet network components, such as network cards and connectors, are cheap and well supported.Uses coaxial, fibre or twisted-pair cables.It is a well-proven technology, which is fairly robust and reliable.It is simple to add and delete computers on the network.It is supported by most software and hardware systems.Available as 10Mbps (10BASE), 100Mbps (100BASE) and 1Gbps (1000BASE). Dual-speed networks can be used, such as mixed 10Mbps/100Mbps networks. Network hub negotiates the required speed.Easy upgrade for different network speed. A dual-speed network can be run, and gradually upgraded.Standardised as 1EEE 802.3.

Common bus

All computers have access toa common bus at the same time

Ethernet hub

Problem:A major problem with Ethernet is that, because computers must contend to get access to the network, there is no guarantee that they will get access within a given time. This contention also causes problems when two computers try to communicate at the same time, they must both back off and no data can be

transmitted.

Problem:A major problem with Ethernet is that, because computers must contend to get access to the network, there is no guarantee that they will get access within a given time. This contention also causes problems when two computers try to communicate at the same time, they must both back off and no data can be

transmitted.

3.2 CSMA/CDCSMA/CDEthernet uses carrier sense, multiple access with collision detection (CSMA/CD). Nodes monitor the bus (or Ether) to determine if it is busy. A node wishing to send data waits for an idle condition then transmits its message. Collisions can occur when two nodes transmit at the same time, thus nodes must monitor the cable when they transmit. When a collision occurs, both nodes stop transmitting frames and transmit a jamming signal. This informs all nodes on the network that a collision has occurred. Each of the nodes involved in the collision then waits a random period of time before attempting a re-transmission. As each node has a random delay time then there can be a prioritisation of the

nodes on the network.

CSMA/CDEthernet uses carrier sense, multiple access with collision detection (CSMA/CD). Nodes monitor the bus (or Ether) to determine if it is busy. A node wishing to send data waits for an idle condition then transmits its message. Collisions can occur when two nodes transmit at the same time, thus nodes must monitor the cable when they transmit. When a collision occurs, both nodes stop transmitting frames and transmit a jamming signal. This informs all nodes on the network that a collision has occurred. Each of the nodes involved in the collision then waits a random period of time before attempting a re-transmission. As each node has a random delay time then there can be a prioritisation of the

nodes on the network.

1

2

3

4

Two nodes transmitat the same time

Nodes detect therehas been a collision

Nodes transmit ajamming signal

Nodes wait a randomperiod before retransmitted

Common bus

All computers have access toa common bus at the same time

Nodes transmit ajamming signal

Nodes wait a randomperiod before retransmitted

Two nodes transmitat the same time

Nodes detect therehas been a collision

Common bus

All computers have access toa common bus at the same time

3.3 IEEE 802 LANs (802.2/802.3)

Application

Presentation

Session

Transport

Network

Data link

Physical

Application

Presentation

Session

Transport

Network

Data link

Physical

D N T S P A D

N T S P A

T S P A

S P A

P A

A

Sender Receiver

Virtualdata flow

Actualdata flow

LLC (IEEE 802.2)Data link

PhysicalMedia access

control (IEEE 802.3)

3.4 IEEE 802.3 data frame 7 bytes 1 byte 6 bytes 6 bytes

46 to 1500 bytes

4 bytes 96 bits

10101...0101010 10101011

2 bytes

Delay FCS Length Source address

Destination address

Start delimiter Preamble

Data field (Logical Link Control)

Others:4 bytes for the CRC (32 bits) and 2 bytes for the LLC length (16 bits). The LLC part may be up to 1500 bytes long. The preamble and delay components define the start and end of the frame. The initial preamble and start delimiter are, in total, 8 bytes long and the delay component is a minimum of 96 bytes long.

Others:4 bytes for the CRC (32 bits) and 2 bytes for the LLC length (16 bits). The LLC part may be up to 1500 bytes long. The preamble and delay components define the start and end of the frame. The initial preamble and start delimiter are, in total, 8 bytes long and the delay component is a minimum of 96 bytes long.

Preamble (seven bytes) precedes the Ethernet 802.3 frame. Each byte of the preamble has a fixed binary pattern of 10101010 and each node on the network uses it to synchronise their clock and transmission timings. It also informs nodes that a frame is to be sent and for them to check the destination address in the frame.Start delimiter field (SDF) is a single byte (or octet) of 10101011. It follows the preamble and identifies that there is a valid frame being transmitted.

Preamble (seven bytes) precedes the Ethernet 802.3 frame. Each byte of the preamble has a fixed binary pattern of 10101010 and each node on the network uses it to synchronise their clock and transmission timings. It also informs nodes that a frame is to be sent and for them to check the destination address in the frame.Start delimiter field (SDF) is a single byte (or octet) of 10101011. It follows the preamble and identifies that there is a valid frame being transmitted.

Delay. The end of the frame there is a 96-bit delay period, which provides the minimum delay between two frames. This slot time delay allows for the worst-case network propagation delay.

Delay. The end of the frame there is a 96-bit delay period, which provides the minimum delay between two frames. This slot time delay allows for the worst-case network propagation delay.

Source/destination addresses (2 or 6 bytes, Most Ethernet systems use a 48-bit MAC address for the sending and receiving node. Each Ethernet node has a unique MAC address, which is normally defined as hexadecimal digits, such as:

4C-31-22-10-F1-32 (4C31 : 2210: F132)

A 48-bit address field allows 248 different addresses (or approximately 281474976710000 different addresses).

Source/destination addresses (2 or 6 bytes, Most Ethernet systems use a 48-bit MAC address for the sending and receiving node. Each Ethernet node has a unique MAC address, which is normally defined as hexadecimal digits, such as:

4C-31-22-10-F1-32 (4C31 : 2210: F132)

A 48-bit address field allows 248 different addresses (or approximately 281474976710000 different addresses).

IP TCP HTTP DataE.g.

3.5 IEEE 802.3 frame delays and timings

nssT 1001010

1

ratebit

16

msnsT 2.11008124150026617max

snsT 067.010081244626617min

sspeed

distT m 33.3

105.1

5008500

33.33100

33.3ed transmittbits ofNumber 500

ns

s

T

T

bit

m

Maximum and minimum time for a frame:

Time to transmit a single bit (for 10Mbps):

Time to propagate 500m (assuming half the speed of light):

Assuming a segment length of 500m, number of bits transmitted before a collision is detected:

500m

Speed = 1.5108

3.6 Ethernet Limitations

Parameter 10BASE5 10BASE2 10BASE-T Common name

Standard or thick-wire Ethernet

Thinnet or thin-wire Ethernet

Twisted-pair Ethernet

Data rate

10 Mbps 10 Mbps 10 Mbps

Maximum segment length

500 m 200 m 100 m

Maximum nodes on a segment

100 30 3

Maximum number of repeaters

2 4 4

Maximum nodes per network

1024 1024

Minimum node spac-ing

2.5 m 0.5 m No limit

Location of trans-ceiver electronics

located at the cable connection

integrated within the node

in a hub

Typical cable type

RG-50 (0.5” diameter)

RG-6 (0.25” diameter)

UTP cables

Connectors

N-type BNC RJ-45/ Telco

Cable impedance

50 50 100

10BASE510BASE5

RG-50 backbonecable

N-typeT-connector

9-pin D-typeconnector (AUI)

9-pin D-typeconnector (AUI)

Vampire(or bee-sting)tap

BNCT-connector

BNC connector

10BASE2

10BASE-T

Hub

RG-50 backbonecable

Twisted-paircable

RJ-45connector

10BASE5 10BASE2

10BASE210BASE-T

3.7 Ethernet types

• Standard, or thick-wire, Ethernet (10BASE5).• Thinnet, or thin-wire Ethernet, or Cheapernet (10BASE2).• Twisted-pair Ethernet (10BASE-T).• Optical fibre Ethernet (10BASE-FL).• Fast Ethernet (100BASE-TX and 100VG-Any LAN).• Gigabit Ethernet (1000BASE-SX, 1000BASE-T, 1000BASE-LX and 1000BASE-CX).

New standards relating to 100Mbps Ethernet are now becoming popular:

• 100BASE-TX (twisted-pair) – which uses 100Mbps over two pairs of Cat-5 UTP cable or two pairs of Type 1 STP cable.

• 100BASE-T4 (twisted-pair) – which is the physical layer standard for 100Mbps over Cat-3, Cat-4 or Cat-5 UTP.

• 100VG-AnyLAN (twisted-pair) – which uses 100Mbps over two pairs of Cat-5 UTP cable or two pairs of Type 1 STP cable.

• 100BASE-FX (fibre-optic cable) – which is the physical layer standard for 100Mbps over fibre-optic cables.

3.8 Ethernet hubs

10BASE-Thub

Ethernet backbone

3.9 Switches and switching hubs

LocalserverLocal

printer

Workgroupnodes

Desktop switch

LocalserverLocal

printer

Workgroupnodes

Desktop switch

Main server

Network backbone connection

Segment switches allow simultaneous communication between any client and

any server.

Segment switches allow simultaneous communication between any client and

any server.

Store-and-forward switchesminimise collisions and theycan store Ethernet frames and retransmit them when segment is quiet.

Store-and-forward switchesminimise collisions and theycan store Ethernet frames and retransmit them when segment is quiet.

3.10 802.2 and SNAP

Idle Preamble

+0.7 V

0.1 s

1 0 1 0 1

–0.7 V

Idle

PLS

PMA

MDI

MAC

Medium

AUI

MAU

PreambleStart

delimiterDestination

addressSourceaddress

Logicallink control

FCS Delay

7 bytes 1 byte 6 bytes 6 bytes 46 to 1500 bytes 4 bytes 96 bytes

10101...0101010

10101011

Length

2 bytes

Destinationservice access point (DSAP)

Sourceservice access point (DSAP)

Controlfield

DATA

1/2 bytes1 byte1 byte

PreambleStart

delimiterDestination

addressSourceaddress

Logicallink control

FCS Delay

7 bytes 1 byte 6 bytes 6 bytes 46 to 1500 bytes 4 bytes 96 bytes

10101...0101010

10101011

Length

2 bytes

1010 1010 DATA

3 bytes

1010 1010 0000 0011Organization

IDEtherType

2 bytes

SNAP header

802.2

SNAP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

DSAP

1 byte 1 byte 1/2 byte(s) 1 byte

SSAP Control Data

0Send seq. no

(0-127)P 0

Receive seq. no(0-127)

Informationframe

1 S P 0Receive seq. no

(0-127)Supervisoryframe0

RR, RNRor REJ

3.11 100BASE-4TNode Hub

Tx Rx

Rx Tx

Tx Rx

Rx Tx

Pair 1

Pair 2

Pair 3

Pair 4

Compatible with10BASE

Input Output0000 0000 -+0 0-+0000 0001 0-+ -+00000 0010 0-+ 0-+0000 0011 0-+ +0-

+

0

-

8B6T

• Cat-3 transmission (if possible, use Cat-5 with 100BASE-T). 100BASE-T uses only 2 pairs.

• Zero DC level:• DC Wander (transformer coupled).• Reduced power transmission.

• Reduced frequency content.• Contention-based (as 10BASE),

thus maximum through is only about 50Mbps.

3.12 100VG-AnyLAN

• Non contention-based. Uses Round-Robin Arbitration Method known as demand priority access method (DPAM). Maximum through put is 96Mbps.

• Nodes are connected to a hub & input ports are scanned for pending requests.A request has an in-built high or normal priority status. High: real time data; Normal:non-real-time data.

• Ethernet & Token Ring Network compatible. Allows up to seven levels of hubs (One root & six cascaded hubs). This allows a number of nodes to connect to a segment. (Limited use due to non support mechanism for data after leaving hub)