Introduction to Computer Networking Ethernet Primer Mark Clements

22/02/2016ITCN 2 RFC 1180 What is RFC 1180 ? Was it useful ? Which parts were understandable ? Which parts were not ?

22/02/2016ITCN 3 Last week... OSI is purely a MODEL Shows steps to be followed Real stacks exist e.g. WAP, TCP/ IP Layers provide for flexibility Each layer adds a little control information for corresponding layers

22/02/2016ITCN 4 This week... Ethernet and its origins Ethernet - shared and switched Medium Access method - CSMA/ CD Reception of frames Frame formats

22/02/2016ITCN 5 Origin of Ethernet Xerox Palo Alto Research Centre developed Ethernet in the mid 1970s Designed for office use 1980 DEC, Intel & Xerox joined forces 1983 IEEE specification was launched Named after Michelson and Morley’s disproving of a luminiferous ether

22/02/2016ITCN 6 Shared Ethernet – Bus Topology  One cable formed the network  Cable was coaxial, 10 BASE 5  Nodes attached anywhere along cable/ bus

22/02/2016ITCN 7 Broadcast and Unicast Messages All messages had to be broadcast regardless whether they were intended for one or all nodes Broadcasts included ARP ARP requests translation of IP to MAC address Sent using the last available address on the network Essential for Ethernet operation

22/02/2016ITCN 8 Shared Medium Network  Coaxial cable has one conductor  Can only carry one message at a time  Bandwidth was 10 Mbit/ second  Many nodes share the cable’s bandwidth  Each computer has ALL of the bandwidth SOME of the time  Each machine has a 48 bit LOCAL address known as MAC address

22/02/2016ITCN 9 Control over Transmission  Many stations sharing one cable  Not possible to predict when any node will need to transmit  Not desirable to use polling or round-robin  time & reliability issues  Need a set of rules to govern transmission  What are rules called in general?

22/02/2016ITCN 10 Protocol = Rules A set of rules that is applied to a given situation Communication works if all nodes operate with the same set of rules or protocol – E.g. we all drive on the left Protocols are implemented in Ethernet as software e.g. firmware for a network card etc. Write a protocol for making a cup of tea

22/02/2016ITCN 11 CSMA/ CD Protocol  Carrier Sense, Multiple Access with Collision Detection is used in Ethernet  Protocol is implemented in every node  Allows a node to fail without affecting the entire network  Does not require a ‘Master’ machine  Nodes act independently of each other

22/02/2016ITCN 12 CSMA/ CD Algorithm 1. Listen to network cabling 2. Transmit if quiet else goto 1 3. When transmitting, listen to the network 4. If no other transmission is heard during your own transmission, goto 7 5. Wait pseudo random period 6. Goto 1 7. Success

22/02/2016ITCN 13 Switched Ethernet  Implemented today - shared Ethernet only found in legacy installations  The popularity of Ethernet led to massive take up in 1980s and 1990s to dominate the LAN market  Networks became overloaded with traffic  Nodes had all of the bandwidth for even less of the time

22/02/2016ITCN 14 Bridges - a solution to Congestion  Cutting network into segments helps  Segments are connected by the bridge  Layer 2 device ( Data Link Layer )  Filters messages ( frames )  Only allows those to pass that have business on other segments  Temporary solution to traffic chaos

22/02/2016ITCN 15 The Switch superseded the Bridge  Collapses network backbone into a box  Has RJ45 sockets for cable connections  Has many ports to connect PCs  Uses twisted pair cabling, Cat 3, 5, 5e, etc.  Cost of switches fell rapidly after 2000  Switches act on layer 2 MAC addresses  Switch makes a direct connection between end points involved in communication

22/02/2016ITCN 16 Ethernet Topology Evolves Now is a STAR rather than a bus topology Long cables join nodes to switches Makes CSMA/ CD unnecessary Still implemented to keep compatibility with older implementations Switches to be dealt with more fully later in the course

22/02/2016ITCN 17 Medium Access Method 1. Node wishes to transmit 2. Node listens to network 3. If network busy return to 2 else 4 4. Transmit message 5. Node listens as it transmits - if other node transmits, stop transmitting 6. Wait pseudo-random time 7. Goto 2

22/02/2016ITCN 18 Transmitting using CSMA/ CD Node A wishes to send a frame to Node B AB Ethernet Cabling A listens to cable : Quiet, so A transmits

22/02/2016ITCN 19 CS works fine but.. What if both nodes sense a quiet cable ? Legally the rule says they can transmit – therefore they do Frames propagate along cabling Both frames collide and ruin each other After a short period both nodes realise a collision has occurred and stop transmitting

22/02/2016ITCN 20 AB Ethernet Cabling Collision!! A collision and its aftermath A now knows a collision has occurred B can stop transmitting

22/02/2016ITCN 21 Post collision actions After a short time A & B stop transmission Time to stop depends on the relative positions of A and B and when they transmitted Both A and B wait a pseudo-random time After timer elapses, attempt to re-transmit If collision occurs again, wait longer random time and try again

22/02/2016ITCN 22 Algorithm lowers performance Too many nodes slow shared Ethernet Network spends too long waiting after collisions for a quiet period Switched Ethernet is the solution Implemented as Fast and Gigabit Ethernet No collisions in switched segments so long as microsegmentation is used

22/02/2016ITCN 23 Switched Ethernet Only frames for control e.g. ARP are broadcast Unicast frames are switched to their destination All nodes examine arriving frames for their own MAC address Nodes will only buffer frames with their own or the all-stations (broadcast) address Valid frames are passed up the protocol stack Any others will be ignored

22/02/2016ITCN 24 Naming of Ethernets First part is speed Second refers to signalling (baseband) Third part relates to a physical layer property

22/02/2016ITCN 25 Ethernet versus IEEE 802.3

22/02/2016ITCN 26 Frame sections Preamble for synchronisation SOF is delimiter DA & SA are addressing ( MAC ) Type relates to upper protocols Length refers to data length FCS is Cyclic Redundancy Check

22/02/2016ITCN 27 Conclusion Shared Ethernet needed rules to avoid collisions on the network - CSMA/ CD Shared Ethernet needed rules to avoid collisions on the network - CSMA/ CD Switched Ethernet does not allow for collisions so CSMA/ CD not needed Implemented for backward compatibility Switched Ethernet does not allow for collisions so CSMA/ CD not needed Implemented for backward compatibility Very little difference between Ethernet and IEEE or their respective frames Very little difference between Ethernet and IEEE or their respective frames

RFC 1180 a tutorial on the TCP/IP protocol suite 22/02/2016ITCN 28