Ethernet Surasak Sanguanpong nguan@ku.ac.th - - PDF document

Applied Network Research Group Department of Computer Engineering, Kasetsart University

1/27

Ethernet

Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan

Last updated: 27 June 2002

Applied Network Research Group Department of Computer Engineering, Kasetsart University

2/27

Ethernet and IEEE 802.3 Timeline

1976: Ethernet developed by Xerox Palo Alto Research Center, including Bob Metcalfe (who later founded 3Com) 1980: 10Mbps Ethernet specification by DEC, Intel, and Xerox 1985: becomes IEEE 802.3 standard (already widely used before this time) 1995: 100Mbps “Fast Ethernet” standardized in IEEE 802.3u (already widely used before this time) 1998: 1Gbps “Gigabit Ethernet” IEEE standard issued 1999: 10Gbps Ethernet under development

Applied Network Research Group Department of Computer Engineering, Kasetsart University

3/27

A Drawing of the First Ethernet System

http://www.ots.utexas.edu/ethernet/ethernet.html

Applied Network Research Group Department of Computer Engineering, Kasetsart University

4/27

Ethernet frame format

IEEE 802.3 PA : Preamble - 10101010s for synchronization SFD : Start of Frame delimiter -- 10101011 to start frame DA: Destination Address -- MAC address SA: Source Address -- MAC address LEN: Length -- Number of data bytes Type: identify the higher -level protocol LLC PDU+pad -- minimum 46 bytes, maximum 1500 FCS : Frame Check Sequence -- CRC-32 7 1 6 6 2 46-1500 4 8 6 6 2 46-1500 4 Ethernet calculation of the FCS 64-1518 bytes FCS PA SA LEN SFD LLC PDU DA FCS PA SA Type Data DA

Applied Network Research Group Department of Computer Engineering, Kasetsart University

5/27

Ethernet MAC address

I/G =0 Individual address I/G =1 Group address U/L=0 Global administered address U/L=1 Local administered address

Unicast : define a single destination Broadcast : FFFFFFFF each station on the network receive and accept frames multicast : a group address defines multiple recipient

I/G U/L 46-bit address

1 1 46

6 byte ethernet addr vendor card no.

3 Bytes 3 Bytes

Applied Network Research Group Department of Computer Engineering, Kasetsart University

6/27 Data link layer Physical layer

Ethernet architecture

0.5 “ Coax tap transceiver AUI cable NIC station interface data encapsulation link management encoding and decoding transmission and receipt

Applied Network Research Group Department of Computer Engineering, Kasetsart University

7/27

IEEE 802.3 Cable Types

Name Cable

• Max. Length Nodes/segment

10Base5 thick coax 500 meters 100 10Base2 thin coax 185 meters 30 10BaseT twisted pair 100 meters 1 10BaseFP fiber optic 1 km 33 passive fiber 10BaseFL fiber optic 2 km 1 point-to-point 10BaseFB fiber optic 2 km 1 point-to-point 10Broad36 coax 3.6 km ? broadband

data rate in Mbps baseband or broadband cable type or length limit 10 Base 5

Applied Network Research Group Department of Computer Engineering, Kasetsart University

8/27

10Base5

• tap : by insertion, cable does not need to be cut
• transceiver : send/receive, collision

detection, electronics isolation

• AUI : Attachment Unit Interface,

a 5-pair cable up to 50 meters long

• Use for backbone networks

maximum segment length=500m maximum number of stations per segment=100 distance between stations must be a multiple of 2.5 m maximum network distance = 2.5 km (5 segments : 1000 stations ) 0.5 inch coax vampire tap transceiver AUI cable NIC repeater DB-15 connector DB-15 connector cable terminator

Applied Network Research Group Department of Computer Engineering, Kasetsart University

9/27

10Base2

0.25 inch coax (RG58) BNC T-connector NIC

• tap: BNC connector, must splice cable
• No drop (AUI) cable
• use: for connecting workstations
• cheaper, easier to use than thick coax,

but more signal attenuation

maximum segment length=185m maximum number of stations per segment=30 minimum distance between two stations = 0.5 m maximum network distance between two stations = 925 m repeater

Applied Network Research Group Department of Computer Engineering, Kasetsart University

10/27

10BaseT

NIC hub maximum segment length = 100m

• uses 4-pair twisted pair cable (Category-3, -4, or -5)
• a hub functions as a repeater
• fewer cable problems, easier to troubleshoot than coax
• cable length at most 100 meters due to high attenuation

(with good Category-5 cable you can go longer)

• most commonly used cable today

Applied Network Research Group Department of Computer Engineering, Kasetsart University

11/27

10BaseF

• 10BaseF specification enable long distance connections with
• the use of optical fiber. Three standards are:
• 10BaseFP - 10-Mbps fiber-passive baseband Ethernet specification

using fiber-optic cabling. It organizes a number of computers into a star topology without the use of repeaters. 10BaseFP segments can be up to 500 meters long

• 10BaseFL - asynchronous point-to-point link between a station

and a hub or repeater, up to 2 km.

• 10BaseFB - synchronous point-to-point link between

repeaters, up to 2 km long, cascaded repeaters are

• allowed. Signal is retimed at each repeater

Applied Network Research Group Department of Computer Engineering, Kasetsart University

12/27

Signal Encoding

Manchester encoding ensures a transition every bit

0 : high-to-low 1 : low-to-high 0 V (high)

• 2.2 V (low)

100 ns 0 mA

• 90 mA

0 1 0 0 1 1 0 0 0 1 1

Applied Network Research Group Department of Computer Engineering, Kasetsart University

13/27

Carrier sense: station listens to media before transmitting Multiple access: multiple stations may access at same time

Carrier Sense Multiple Access

Media idle? Media idle?

YES Transmit immediately NO -- wait Listen until the media is idle, then transmit

Applied Network Research Group Department of Computer Engineering, Kasetsart University

14/27

Collision

More than one station may send a frame during

• verlapping times.

How does a station know that a collision occurred? What does the station do after a collision?

Applied Network Research Group Department of Computer Engineering, Kasetsart University

15/27

How does a node detect a collision?

Transceiver: A node monitors the media while transmitting. If the observed power is more than transmitted power + attenuated reflection of its own signal, it indicates a collision.

Transmitted signal Observed signal

Collision!

Simultaneous input on two ports Output “collision presence” on all ports

Hub: if input occurs simultaneously on two ports, it indicates a collision. Hub sends a collision presence signal on all ports.

Applied Network Research Group Department of Computer Engineering, Kasetsart University

16/27

Retransmission after a collision

Exponential backoff rule:

• Choose k = 0 or 1 at random. Wait k x 51.2 µsec.
• retransmit frame when media is idle.
• If collision, then choose k = 0, 1, 2, 3 at random
• wait k x 51.2 µsec.
• retransmit frame when media is idle.
• If collision, then choose k = 0, ..., 7 at random
• wait k x 51.2 µsec.
• retransmit frame when media is idle.
• If collision, then choose k = 0, ..., 15 at random
• wait k x 51.2 µsec.
• retransmit frame when media is idle.

. . .

• Double the wait interval until frame is transmitted or it becomes 0 - 1023.
• If frame is not transmitted within 16 attempts, give up and report failure.

Applied Network Research Group Department of Computer Engineering, Kasetsart University

17/27

Collision Detection Rules

• 1. Stations must listen to the cable while transmitting in order to

detect a collision.

• 2. A frame must be at least 64 bytes (512 bits, 51.2 microseconds)

long to ensure sender “hears” a collision before he finishes. (The transmission time must be more than the RTT.)

• 3. If a collision is detected, send a brief jamming signal and then wait

before retransmitting.

Jamming signal Jamming signal

Applied Network Research Group Department of Computer Engineering, Kasetsart University

18/27

Minimum frame size

(1) packet starts at time 0 (2) packet almost at B at t-δ (3) B send packet; collision occurs at t (4) jam signal gets back to A at 2t

A frame must take more than 2t to send to prevent the situation that the sender incorrectly concludes that the frame was successfully sent. This slot times equal 51.2 µs corresponds to 512 bit (64 bytes) The minimum frame length is 64 bytes (excluding preamble) This answers why data field must have 46 bytes minimum

A A A A B B B B A and B located at the far ends of the cable

Applied Network Research Group Department of Computer Engineering, Kasetsart University

19/27

Worst Case Collision Timing

Assume delay for repeater is 1 µsec; for transceiver 0.5 µsec.

Component Propagation Time Microsecs Five 500 meter segments 2500m/0.77c 10.8 Four repeaters 4 x 1 µsec 4.0 Nine 50 meter AUI cables 450m/0.65c 2.3 Nine transceivers 9 x 0.5 µsec 4.5 Total one-way time 21.6

2,500 meters 500 meters

Applied Network Research Group Department of Computer Engineering, Kasetsart University

20/27

Worst Case Collision Timing (cont.)

Under these assumptions, the round trip time would be 43.2 microseconds. Allowing some tolerance for equipment, IEEE chose 51.2 microseconds, equal to 512 bit-times, as the collision detection interval. This is why all frames must be at least 512 bits (64 bytes) long. 43.2 microsecond RTT 500 meters

Applied Network Research Group Department of Computer Engineering, Kasetsart University

21/27

Late Collision

A late collision occurs when sender finishes transmission

before detecting collision presence. Usual causes:

• cable too long
• too many repeaters between stations

Solution: higher layer protocol must detect packet loss and retransmit.

Applied Network Research Group Department of Computer Engineering, Kasetsart University

22/27

Interframe Gap

A sender must wait 96 bit-times between frames to give

• ther stations an opportunity to transmit.

A 96 bit-time delay is 9.6 microseconds on 10Mbps ethernet.

FRAME

96 bit-time interframe gap

FRAME

Applied Network Research Group Department of Computer Engineering, Kasetsart University

23/27

Ethernet Performance (cont.)

Tanenbaum (1996) has plotted the utilization as a function of n (number of stations) and F (average frame size):

Number of stations trying to send Ethernet channel utilization

85% 27%

Applied Network Research Group Department of Computer Engineering, Kasetsart University

24/27

Ethernet Speed and Collision Detection

So, without any delay due to repeaters or network interfaces, the largest network size in a shared media configuration is:

Ethernet Minimum Min Frame Upper Bound Speed Frame Size TX Time

• n Network Size

10 Mbps 64 byte 51.2 microsec 5,120 m 100 Mbps 64 byte 5.12 microsec 512 m 1 Gbps 64 byte 0.512 microsec 51 m Next we will see how the fast and gigabit ethernet standards address these limits. Max Network Size

Applied Network Research Group Department of Computer Engineering, Kasetsart University

25/27

Fast Ethernet

• 100 Mbps transmission rate
• same frame format, media access, and collision detection

rules as 10 Mbps ethernet

• can combine 10 Mbps ethernet and fast ethernet on same

network using a switch, or combined hub+switch

• 64 byte minimum frame ==> 5.12 microsecond xmit time
• 96 bit interframe gap ==> 0.96 microsecond
• network diameter must be smaller than 10 Mbps ethernet
• media: twisted pair or fiber optic cable (no coax)

Applied Network Research Group Department of Computer Engineering, Kasetsart University

26/27

Gigabit Ethernet

• same frame format, media access, and collision detection

rules as previous ethernet generations

• same 64 byte minimum frame and 96 bit interframe gap
• can combine 10 Mbps, 100 Mbps, and gigabit ethernet on

same network (but not on same cable) using a switch

• 200 meter network diameter (using UTP) requires a

minimum transmission time of 512 bytes on shared media

• several media types:

UTP, shielded copper short-wave fiber optics long-wave fiber optics

Applied Network Research Group Department of Computer Engineering, Kasetsart University

27/27

Gigabit Ethernet Media Types

9 micron single mode fiber 50 or 62.5 micron multimode fiber 400-500 MHz modal bandwidth 50 micron multimode 500 MHz modal bandwidth 50 micron multimode 400 MHz modal bandwidth 62.5 micron multimode 200 MHz modal bandwidth 62.5 micron multimode 160 MHz modal bandwidth 4 pair Cat-5 UTP STP 5 km 550 m 550 m 500 m 275 m 220 m 100 m 25 m 1000baseLX 1300 nm laser 1000baseSX 850 nm LED 1000baseT 1000baseCX