Computer Networks Ethernet I Professor Hui Zhang

2 Hui Zhang Aloha Network

3 Hui Zhang Original Ethernet

4 Hui Zhang High Level View  Goal: share a communication medium among multiple hosts connected to it  Problem: arbitrate between connected hosts  Solution goals:  High resource utilization  Avoid starvation  Simplicity (non-decentralized algorithms)

5 Hui Zhang Medium Access Protocols  Channel partitioning  Divide channel into smaller “pieces” (e.g., time slots, frequency)  Allocate a piece to node for exclusive use  Random access  Allow collisions  “recover” from collisions  Taking-turns  Tightly coordinate shared access to avoid collisions

6 Hui Zhang Random Access Protocols  When node has packet to send  Transmit at full channel data rate R.  No a priori coordination among nodes  Two or more transmitting nodes -> “collision”,  Random access MAC protocol specifies:  How to detect collisions  How to recover from collisions  Examples of random access MAC protocols:  Aloha  Slotted ALOHA  CSMA and CSMA/CD

7 Hui Zhang Aloha  Time is divided into equal size slots (= packet transmission time)  Node with new arriving pkt: transmit  If collision: retransmit pkt in future slots with probability p, until successful.

8 Hui Zhang Aloha  Nodes sends the message when it has data to send.  If it receives an ack, it considers the transmission completed, otherwise it retransmits after a random delay.  Simple, distributed protocol, but not very efficient  18% maximum utilization  Slotted Aloha: more efficient.  Reduces chances of collision  37% maximum utilization Central Computer

9 Hui Zhang Slotted Aloha  Time is divided into equal size slots (= packet transmission time)  Node with new arriving pkt: transmit at beginning of next slot  If collision: retransmit pkt in future slots with probability p, until successful. Success (S), Collision (C), Empty (E) slots

10 Hui Zhang CSMA/CD Broadcast technology  Carrier-sense multiple access with collision detection (CSMA/CD).  MA = multiple access  CS = carrier sense  CD = collision detection host hub

11 Hui Zhang CSMA/CD Algorithm  Sense for carrier.  If carrier present, wait until carrier ends.  Sending would force a collision and waste time  Send packet and sense for collision.  If no collision detected, consider packet delivered.  Otherwise, abort immediately, perform “exponential back off” and send packet again.  Start to send at a random time picked from an interval  Length of the interval increases with every retransmission

12 Hui Zhang Collision Detection Time ABC

13 Hui Zhang Collision Detection: Implications  All nodes must be able to detect the collision.  Any node can be sender  The implication is that either we must have a short wires, or long packets.  Or a combination of both  Can calculate length/distance based on transmission rate and propagation speed.  Messy: propagation speed is media- dependent, low-level protocol details,..  Minimum packet size is 64 bytes –Cable length ~256 bit times  Example: maximum coax cable length is 2.5 km ABC

14 Hui Zhang CSMA/CD: Some Details  When a sender detects a collision, it sends a “jam signal”.  Make sure that all nodes are aware of the collision  Length of the jam signal is 32 bit times  Exponential backoff operates in multiples of 512 bit times.  Longer than a roundtrip time  Guarantees that nodes that back off longer will notice the earlier retransmission before starting to send

15 Hui Zhang Ethernet Frame Format  Preamble marks the beginning of the frame.  Also provides clock synchronization  Source and destination are 48 bit IEEE MAC addresses.  Flat address space  Hardwired into the network interface  Type field is a demultiplexing field.  What network layer (layer 3) should receive this packet?  Is actually a length field in the standard  CRC for error checking. PreambleTypePadDestSourceDataCRC 86624

16 Hui Zhang Minimum Packet Size  Why put a minimum packet size?  Give a host enough time to detect collisions  In Ethernet, minimum packet size = 64 bytes (two 6-byte addresses, 2-byte type, 4-byte CRC, and 46 bytes of data)  If host has less than 46 bytes to send, the adaptor pads (adds) bytes to make it 46 bytes  What is the relationship between minimum packet size and the length of the LAN?

17 Hui Zhang Minimum Packet Size (more) propagation delay (d) a) Time = t; Host 1 starts to send frame Host 1Host 2 propagation delay (d) Host 1Host 2 b) Time = t + d; Host 2 starts to send a frame just before it hears from host 1’s frame propagation delay (d) Host 1Host 2 c) Time = t + 2*d; Host 1 hears Host 2’s frame  detects collision LAN length = (min_frame_size)*(light_speed)/(2*bandwidth) = = (8*64b)*(2*10 8 mps)/(2*10 7 bps) = 5.12 km

18 Hui Zhang Ethernet Physical Layer  10Base2 standard based on thin coax.  Thick coax no longer used  Nodes are connected using thin coax cables and “T” connectors in a bus topology  10-BaseT uses twisted pair and hubs.  Hub acts as a concentrator  The two designs have the same protocol properties.  Key: electrical connectivity between all nodes  Deployment is different host Hub Host

19 Hui Zhang Ethernet Technologies: 10Base2  10: 10Mbps; 2: under 200 meters max cable length  Thin coaxial cable in a bus topology  Repeaters used to connect up to multiple segments  Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

20 Hui Zhang 10BaseT and 100BaseT  10/100 Mbps rate; later called “fast ethernet”  T stands for Twisted Pair  Hub to which nodes are connected by twisted pair, thus “star topology”

21 Hui Zhang 802.3u Fast Ethernet  Apply original CSMA/CD medium access protocol at 100Mbps  Must change either minimum frame or maximum diameter: change diameter  Requires  2 UTP5 pairs (4B5B) or  4 UTP3 pairs (8B6T) or  1 fiber pair  No more “shared wire” connectivity.  Hubs and switches only  4B/5B encoding

22 Hui Zhang Gbit Ethernet  Use standard Ethernet frame format  Allows for point-to-point links and shared broadcast channels  In shared mode, CSMA/CD is used; short distances between nodes to be efficient  Uses hubs, called here “Buffered Distributors”  Full-Duplex at 1 Gbps for point-to-point links

23 Hui Zhang Traditional IEEE 802 Networks: MAC in the LAN and MAN  Ethernet defined as IEEE  Not quite identical  The IEEE 802.* set of standards defines a common framing and addressing format for LAN protocols.  Simplifies interoperability  Addresses are 48 bit strings, with no structure  (Ethernet)  (Token ring)  802.X (Token bus)  (Distributed queue dual bus)  (Wireless)

24 Hui Zhang LAN Properties  Exploit physical proximity.  Typically there is a limitation on the physical distance between the nodes, for example, –to collect collisions in a contention based network –to limit the overhead introduced by token passing or slot reservations  Relies on single administrative control and some level of trust.  Broadcasting packets to everybody and hoping everybody (other than the receiver) will ignore the packet  Token passing protocols assume everybody plays by the rules

25 Hui Zhang Why Ethernet?  Easy to manage.  You plug in the host and it basically works  No configuration at the datalink layer  Broadcast-based.  In part explains the easy management  Some of the LAN protocols (e.g. ARP) rely on broadcast –Networking would be harder without ARP  Not having natural broadcast capabilities adds complexity to a LAN –Example: ATM  Drawbacks.  Broadcast-based: limits bandwidth since each packets consumes the bandwidth of the entire network  Distance

26 Hui Zhang 802.3z Gigabit Ethernet  Same frame format and size as Ethernet.  This is what makes it Ethernet  Full duplex point-to-point links in the backbone are likely the most common use.  Added flow control to deal with congestion  Choice of a range of fiber and copper transmission media.  Defining “jumbo frames” for higher efficiency.