CS 453CS 453Computer NetworksComputer Networks

Lecture 14Lecture 14

Medium Access Control Medium Access Control

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Ethernet

Ethernet is by far the most popular LAN technology

I refer to as technology because it is a collection of… Protocols Hardware interfaces Cabling standards…

Ethernet

Ethernet standardize by IEEE As IEEE 802.3

Other important networking standards… IEEE 802.11 – Wireless LAN (WiFi) IEEE 802.15 – Bluetooth IEEE 802.16 – Wireless MAN (WiMax)

We will look at these later

Ethernet

Let’s backup and look at some Physical Layer issues

Ethernet cabling

From: Tanenbaum, 2003, pg. 271

Ethernet10Base5

Early Ethernet Used

thick, heavy, stiff yellow coax cable(RG-8)Vampire tap transceiverTransceiver cable

50 meters Twisted pair Interface in the

computer

From:http://en.wikipedia.org/wiki/10BASE5

From:

www.ozkankaradavut.com/aktel/products.htm

Ethernet10Base2

Thinnet Used thin (RG-58)

coax cable BNC T-connectors No drop cables Thin coax snaked

around computer to computer to printer,…

From: http://en.wikipedia.org/wiki/10BASE2

Ethernet10Base2 Very difficult to

maintain and troubleshoot

“Cable breaks” common occurrence

Took whole segments offline

From: http://en.wikipedia.org/wiki/10BASE2

From: Computer Desktop Encyclopedia, © 1998

Ethernet10BaseT

What most of us know as Ethernet

Uses twisted pair copper cabling

T means twisted pair

Cables connect to hubs

No serial chain connections

Ethernet Hub

Ethernet10BaseT

Remember the bus topology of 10Base5 and 10Base2…

Hub is a bus-in-a-box

Easy installation Simple

troubleshooting Simple

maintenance

Ethernet Hub

Ethernet10BaseF Like 10BaseT but use Fiber cable Very good noise immunity Very secure Good range

Ethernet100BaseT FastEthernet – 100 Mbps

1000BaseT GigabitEthernet – 1000 Mbps

10GigE

100GigE

Ethernet – Bit encoding

We need a way to electrically encode a bit stream on a transmission medium

Binary encoding of a bit stream often involves generating +5 volt for a 1 and 0 volts for 0

This a bad idea because devices can’t distinguish between a binary 0 and a dropped bit

If we use two other voltages we still have a problem with the possibility that the sender and receiver will get out of sync regarding the bit boundaries

Ethernet – Bit encodingManchester Encoding A bit transmission period has two parts 1 = high to low transition during bit period 0 = low to high transition during bit period

From: Tanenbaum, 2003, pg. 275

Ethernet – Bit encodingDifferential Manchester Encoding

A bit transmission period has two parts 1 = no transition during bit period 0 = transition during bit period

Ethernet Manchester Encoding +0.85v =1 -0.85v = 0

Other LAN technologies use Differential Manchester Encoding

Ethernet MAC Sublayer Protocol

General Ethernet Frame description

From: http://en.wikipedia.org/wiki/Ethernet#Ethernet_frame_types_and_the_EtherType_field

Preample: 62 bits –alternating 0,1 announces frame

SFD: 2 bit sequence 11 – Start of Frame Delimiter

Destination Address – 6 bytes

Source Address – 6 bytes

Ethertype – defines protocol or length of data packet

Payload – data packet – minimum size=46 bytes, maximum size=1500 bytes

Frame Check Sequence – Cyclic Redundancy Check on Frame

EthernetFrame specifications Destination address

6 bytes

High order bit 0=normal address 1=Group address (multicast)

Bit 46 (high order -1) Determines if address is local or global

Source address6 bytes – MAC address of sending device

EthernetFrame specifications Destination address

If bit 47 is set to 1 the address is Multicast address

Going to a designated group of addresses

If all destination address bits are set to one address is a broadcast address

Going to all addresses in the ethernet

EthernetFrame specifications Type field

<=1500 defines length of payload packet

>1500 defines of the protocol for handling the packet (what process gets the data)

IP protocol is Type = 2048

EthernetFrame specifications

Payload sizeMin 46 bytes – if legitamate frame is less than 46 bytes Pad field is used to fill it out 64 bytesIf receiver receive smaller payload – its trashWhen a collision is detected detector puts an alarm on the channelIf station A starts a transmission and later B starts a transmission (because of propagation delay) and causes a collision, B puts an alarm on the channelA must be able to “hear” the alarm before it completes the transmission of its frame, and abort…Otherwise, it believes that the transmission was successful

Therefore, the outgoing frame must be long enough to be in transmission at Station A at the worst case time that it could “hear” a collision signal triggered by Station B

EthernetFrame specifications Payload size

This is true for 10 Mbps (10basex) networks

As network speeds get greater

Minimum packet sizes must be get bigger

Or the cable length must get smaller (to reduce propagation delay)

Ethernet: Dealing with Collisions

Ethernet uses CSMA/CD

So what happens when a collision occurs?

Binary Exponential Backoff Algorithm

When a collision occurs… All stations trying to use the channel… … backoff and randomly wait either … 0 or 1 ( like a coin toss) time slots and try

again

Ethernet: Dealing with Collisions

Binary Exponential Backoff AlgorithmWhen a 2nd collision occurs… All colliding stations randomly… …backoff and wait 0,1,2, or 3 time slots, and

retry

If a 3rd collision occurs All colliding stations randomly backoff and… …wait 0,1,2,…,7 time slots and retry

Do you see the pattern in algorithm?

Ethernet: Dealing with Collisions

Binary Exponential Backoff Algorithm On the nth collision… All competing stations backoff and randomly

wait 2n-1 time slots then retry After 10 successive collisions (210 -1 = 1023)

… ….declare defeat and pass the problem to the

Network layer

Ethernet: Dealing with Collisions

Binary Exponential Backoff Algorithm This turns out to work pretty well If the protocol just used the “flip-a-coin”

algorithm ( 0, 22 -1)

If there were 100 competing stations

A frame could not flow until 1 station picked 0 and every other station picked 1…

That could take forever (remember each station is “flipping a coin”)

Ethernet: Dealing with Collisions

Binary Exponential Backoff Algorithm Alternatively, if the algorithm based the

number of wait periods on the number of stations N…

Then the average wait time would be high This would introduce large delays

Ethernet Switches

Hubs – bus-in-a-box All ports on a hub represent one collision

domain

Switches Ports are ganged in small groups (like 4) If frames is destined for port in same group it

is switched there If not, it is placed on a high speed backplane

and routed to another port in another group

Ethernet Switches

Switches Some switches are buffered And have groups of one This means that the collision domains are one And therefore, collisions are not possible, at least in

the group In this case all frames are buffered and move by high

speed backplane to the destination port

Switches can support full duplex transmit/receive Not possible with CSMA/CD

Fast Ethernet

10Base?? Was not fast enough for very longApplications easily outstripped the 10 Mbps (there is trend here!!!)IEEE 802.3 committee tackled this and came up with a few faster versions of EthernetStandard is an extension to 802.3 called IEEE 802.3u or FastEthernet

Fast Ethernet

One constraint with the design of FastEthernet was that it work with existing Cat3 and Cat5 UTP cables

Why Cat3?

For Cat3 –100BaseT4 Unable to run 200 Megabaud (Manchester encoding)

for 100 meters Bumped up the bandwidth from 20 Mhz (standard

Ethernet) to 25 Mhz Requires four Twisted Pairs Dropped Manchester Encoding

Fast Ethernet

100BaseTx Requires Cat5 Only needs two twisted pairs Uses 4B/5B encoding rather than Manchester Full Duplex – 100 Mbps each way

100BaseFx Similar to 100BaseTx but uses fiber One fiber each way – full duplex Long range Good security Expensive