CS 453 Computer Networks Lecture 14 Medium Access Control Sublayer

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 Other important networking standards… IEEE – Wireless LAN (WiFi) IEEE – Bluetooth IEEE – 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

Ethernet 10Base5 Early Ethernet Used thick, heavy, stiff yellow coax cable(RG-8) Vampire tap transceiver Transceiver cable 50 meters Twisted pair Interface in the computer From: From:

Ethernet 10Base2 Thinnet Used thin (RG-58) coax cable BNC T- connectors No drop cables Thin coax snaked around computer to computer to printer,… From:

Ethernet 10Base2 Very difficult to maintain and troubleshoot “Cable breaks” common occurrence Took whole segments offline From: From: Computer Desktop Encyclopedia, © 1998

Ethernet 10BaseT 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

Ethernet 10BaseT Remember the bus topology of 10Base5 and 10Base2… Hub is a bus-in-a- box Easy installation Simple troubleshooting Simple maintenance Ethernet Hub

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

Ethernet 100BaseT 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 encoding Manchester 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 encoding Differential 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 = v = 0 Other LAN technologies use Differential Manchester Encoding

Ethernet MAC Sublayer Protocol General Ethernet Frame description From: 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

Ethernet Frame 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 address 6 bytes – MAC address of sending device

Ethernet Frame 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

Ethernet Frame 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

Ethernet Frame specifications Payload size Min 46 bytes – if legitamate frame is less than 46 bytes Pad field is used to fill it out 64 bytes If receiver receive smaller payload – its trash When a collision is detected detector puts an alarm on the channel If 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 channel A 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

Ethernet Frame 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 Algorithm When a 2 nd collision occurs… All colliding stations randomly… …backoff and wait 0,1,2, or 3 time slots, and retry If a 3 rd 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 n th collision… All competing stations backoff and randomly wait 2 n -1 time slots then retry After 10 successive collisions ( = 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, ) 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 long Applications easily outstripped the 10 Mbps (there is trend here!!!) IEEE committee tackled this and came up with a few faster versions of Ethernet Standard is an extension to 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