1Ethernet

Lecture # 12

Computer Communication & Networks

Ethernet

History

↗ Ethernet is the most widespread LAN technology

↗ Ethernet was developed at Xerox (1973) inspired by ALOHAnet which

Robert Metcalfe had studied as part of his PhD dissertation

↗ Standardized by Xerox/DEC/Intel in 1978

↗ Ethernet was officially accepted as IEEE standard 802.3 in 1985

↗ The original Xerox Ethernet operated at 3Mbps and used thick coax

cable

↗ Ethernet networks up to 10Gbps now exist (switched Ethernet)

2Ethernet

Ethernet & IEEE 802.3

What the IEEE standard covers- Physical layer andinterface to the link layer. IEEE 802.2 is the Link layer standard.

Differences between Ethernet and 802.31-There are some electrical and connector differences; most equipment uses IEEE 802.3.

2-There is difference in the header. DIX uses TYPE, 802.3 uses LENGTH. SInce the frame is limited in size, the two coexist.

3-Most people use the DIX format.3Ethernet

IEEE & OSI

PHY

MAC

LLC 2- Data link

1- physical

LLC = Logical Link ControlMAC = Media Access ControlPHY = Physical

4Ethernet

Ethernet

Why Ethernet became so popular

↗Easy to understand, implement, manage, and maintain

↗Low-cost network implementations

↗Topological flexibility for network installation

↗Successful interconnection and operation of products, regardless of manufacturer

5Ethernet

Frame FormatEthernet and IEEE 802.3 Frame Format

6Ethernet

Ethernet

↗ Preamble:Starts with 7 bytes of 1010101 to get timing synchronized↗ SFD: indicate the frame is starting (10101011) , 1- byte↗ PDU or Frames: headers (addresses + length/type)↗ 802.3 standard substitutes length with type field↗ The Length/EtherType: In 802.3 it indicates the number of bytes of data in the frame

from 0 to 1500 bytes. Frames must be at least 64 bytes long, not including the preamble, so, if the data field is shorter than 46 bytes, it must be compensated by the Pad field (padding bytes).

↗ Type field is the first thing in data portion↗ Data (46-1500), add bits if necessary to get to 46 bytes.↗ CRC-32 FCS for error checking

Preamble SFDDestination

AddressSource Address

Length Data + Pad FCS

7 1 6 6 2 446-1500

Frame Format (Ethernet & 802.3)

Type

7Ethernet

Ethernet

Ethernet Addresses

↗MAC address, short for Media Access Control address, a hardware address that uniquely identifies each node on a network↗When a manufacturer creates a network capable piece of hardware they will assign the MAC address which will usually begin with a code that is tied to the manufacturer↗It will be unique to every device, even two devices of the same type.↗A device’s MAC address is composed of six pairs of hexadecimal numbers↗The numbers are separated by colons as in the following example:

↗ 6E:51:F5:C1:11:00↗Once assigned, the MAC address is used to ensure that each device connected to the LAN is unique

8Ethernet

Ethernet

Ethernet Address

↗The first three octets identify the organization that issued the identifier and are known as the Organizationally Unique Identifier (OUI)↗The following three octets are assigned by that organization in nearly any manner they please, subject to the constraint of uniqueness

e.g. in the address 00:A0:C9:14:C8:29, the prefix 00A0C9 indicates the manufacturer is Intel Corporation

9Ethernet

Ethernet

Ethernet Addresses Types

↗Unicast; unique, 48-bit address assigned to each adapter↗Examples:

↗ 01:23:45:67:89:AB↗ 08:00:07:A9:B2:FC

↗Broadcast: all 1s↗Multicast: first bit is 1

10Ethernet

Normal Ethernet Operation

A

B

D

Data

C

Address mismatchpacket discarded

Address mismatchpacket discarded

Address matchpacket processed

Send datato node D

Transmitted packet seenby all stations on the LAN

(broadcast medium)

11Ethernet

Ethernet Collisions

A

B C

D

Collision

Data transmission for A Data transmission for C

12Ethernet

Ethernet

• Two stations (2 and 4) communicating using a single bus

How Ethernet Works

13Ethernet

Ethernet

How Ethernet Works

Shared Media Problem

↗Problem: demands can conflict, e.g., two hosts send simultaneously↗Solution is a medium access control (MAC) algorithm

Need method to moderate access

↗Fair arbitration↗Good performance

14Ethernet

Ethernet

Need for an Access Method

↗Whenever multiple users have unregulated access to a single line, there is a danger of signals overlapping and destroying each other↗Such overlaps which turn signals to noise are called Collisions↗As traffic increases on multiple-access link, so do collisions↗Such a network therefore needs a mechanism to coordinate traffic, minimize the number of collisions and maximizes the number of frames that are delivered successfully↗The access mechanism used in Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

15Ethernet

Ethernet

Ethernet MAC – CSMA/CD

(Carrier Sense Multiple Access/Collision Detection)

↗ Multiple access

↗ Nodes send and receive frames over a shared link

↗ Carrier sense

↗ Nodes can distinguish between an idle and busy link

↗ Collision detection

↗ A node listens as it transmits to detect collision

16Ethernet

Ethernet

Ethernet MAC – CSMA/CD

MAC Algorithm

↗ If line is idle (no carrier sensed)

↗ Send immediately

↗ Upper bound message size of ~1500 bytes

↗ Must wait 9.6µs between back-to-back frames

17Ethernet

Ethernet

Ethernet MAC – CSMA/CD

MAC Algorithm

↗ If line is busy (carrier sensed)

↗ Wait until the line becomes idle and then transmit immediately

↗ Called 1-persistent (special case of p-persistent)

↗ If collision detected

↗ Stop sending data and jam signal

↗ Try again later

18Ethernet

Ethernet 19

Ethernet MAC (CSMA/CD)

Packet?

Sense Carrier

Discard Packet

Send Detect Collision

Jam channel b=CalcBackoff();

wait(b);attempts++;

No

Yes

attempts < 16

attempts == 16

• Carrier Sense Multiple Access/Collision Detection

Ethernet 20

Ethernet’s CSMA/CD (more)

Jam Signal: make sure all other transmitters are aware of collision; 48 bits;

Exponential Backoff: • If deterministic delay after collision, collision will

occur again in lockstep• If random delay with fixed mean

– Few senders needless waiting– Too many senders too many collisions

• Goal: adapt retransmission attempts to estimated current load– heavy load: random wait will be longer

Ethernet

Switched Ethernet

↗ No need Of MAC

↗ No collision because of the micro level segmentation

↗ Switches performs the switching between hosts

↗ Today’s Ethernet LANs are built on switch topologies

21Ethernet

Ethernet

Ethernet Flavors

10Base5 10BaseT 10baseFL 100BaseTX 100BaseFX

Media Thick coaxRG-8, RG-11

TIA/EIA UTP Cat 3,4,5,5e,6(2 pair)

62.2/125 mMulti-modeFiber

TIA/EIA UTP Cat 5 and up (2 pair)

62.2/125 mMulti-modeFiberOr Single mode fiber

Speed 10 Mbps 10 Mbps 10 Mbps 100 Mbps 100 Mbps

Max Seg. Length

500 Meters 100 Meters 2000 Meters 100 Meters 400/2000 Meters (full/half duplex), 10 Km

22Ethernet