1. ICOM 6115©Manuel Rodriguez-Martinez ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph.D. Lecture 17

2. ICOM 6115©Manuel Rodriguez-Martinez Lecture Objectives Describe IEEE 802 family of protocols –Ethernet –WiFi –Broadband Discuss Collisions and Binary Backoff algorithm

3. ICOM 6115©Manuel Rodriguez-Martinez Ethernet Network Issue –Same channel is shared by many host –Only one can be using it at any given time Problem –What happens is two hosts transmit at the same time?

4. ICOM 6115©Manuel Rodriguez-Martinez Transmitter Algorithm Ethernet is a Carrier Sense system –Adaptor can “listen” to signal on wire If card has a frame to send it listens to wire –If a signal is detected, then it waits and tries again later when channel is idle –Otherwise, frame is sent and card starts listening as it sends Send and listen simultaneously to detects collision

5. ICOM 6115©Manuel Rodriguez-Martinez Some issues about Frame size Maximum Ethernet frame size is 1500 bytes –Limits the amount of time using the wire –Controls amount of RAM needed to buffer up frames Minimum Frame size is 512 bits (64 bytes) –Need to detect collisions –Might need to pad frames in order to reach this size

6. ICOM 6115©Manuel Rodriguez-Martinez Transmitter Algorithm If wire is idle (no signal), then next frame F is sent –No negotiation is done Else –While line is busy (signal is detected) Wait 9.6 usec –Send next frame F imediately Listen for collision as data is being sent –CSMA/CD – Carrier Sense Multiple-Access with Collision Detection

7. ICOM 6115©Manuel Rodriguez-Martinez The issue about collision Notice that two or more hosts can see line idle at a time t Thus, two or more hosts might start sending frames concurrently –At exactly time t –Or with a time difference d In either case, they frames collide in the wire, distorting the signals

8. ICOM 6115©Manuel Rodriguez-Martinez Collisions How much can it take to detect a collision? –1 RTT

9. ICOM 6115©Manuel Rodriguez-Martinez Collisions and Jamming When a host detects a collision it –Immediately sends a 48-bit special frame runt frame – used to tell all hosts about a collision –Some cards have a red led to indicate this –Stops transmissions and waits a time T to retransmit Need to make sure –Each frame is big enough to listen to it before all bits are send –Cable is not too long for signals to propagate

10. ICOM 6115©Manuel Rodriguez-Martinez Collision Indication via jamming Signal A sends a frame F1 to B Frame F1 arrives a B B starts sending a frame F2 to A –Collision occurs B sends jamming signal

11. ICOM 6115©Manuel Rodriguez-Martinez Ethernet Length and Frame Selection Worst case scenario is when hosts are at the extremes of wire Ethernet max length is set to 2500 m Need frame to last at least 1 RTT –In order to listen to it On a 10Mbps, RTT is 51.2 usec –Minimal frame must be 512 bits (64 bytes) A B

12. ICOM 6115©Manuel Rodriguez-Martinez Frame size at higher bandwidth What happens to the minimum frame size at higher bandwidth? –It must be made larger to ensure host can listen to frame that it is currently sending. –Alternatively, we can decrease both frame size and maximum link length Example: 1Gbps Ethernet, 51.2 usec RTT –A link of 2500 m, must have a min frame size of 6400 bytes –Or, have a 640 min frame size but length cannot be longer than 250 m.

13. ICOM 6115©Manuel Rodriguez-Martinez Binary Exponential Backoff How long to wait before re-transmitting after a collision? Idea: Time is divided into slots of length 1RTT After 1 st collision, network card awaits 0 or 1 time slots before retransmitting –Randomly chooses between 0 and 1 After 2 nd collision, randomly chooses to wait 0, 1, 2 or 3 slots After 3 rd collision, randomly chooses to wait 0, 1, 2, 3, 4, 5, 6, or 7 time slots

14. ICOM 6115©Manuel Rodriguez-Martinez Binary Exponential Backoff After the i-th collision, i ≤ 10, –the sender will randomly wait for k time slots –where k is randomly chosen from the interval After 10 consecutive collisions, the maximum number of slots is fixed to 1023 After 16 consecutive collisions, network card sends an error message to the OS –Network ERROR

15. ICOM 6115©Manuel Rodriguez-Martinez Making Ethernet Go Faster Problem –Ethernet is a bus network –Given a link with N hosts and Bandwidth B, the throughput per host is N/B –More hosts means more collisions Possible Solutions: –Convert Ethernet into a point-to-point LAN Switching –Make bits shorter (higher Bandwidth)

16. ICOM 6115©Manuel Rodriguez-Martinez Switched Ethernet: Point-to-Point LAN Host can connect directly to the switch via 10BaseT Cable Alternatively, host connect to a Hub which in turn connects to the switch

17. ICOM 6115©Manuel Rodriguez-Martinez Ethernet Switch Architecture BACK PLANE Ethernet Plug-In Cards High-Speed Back Plane Interconnects the cards

18. ICOM 6115©Manuel Rodriguez-Martinez A More Detailed Architecture Back Plane A switch is just is a special-purpose computer

19. ICOM 6115©Manuel Rodriguez-Martinez Switching on Ethernet 1.An adaptor sends the Frame to the plug-in card it is attached to 2.Plug-in card checks to see if destination MAC address is connected to it –If so, the frame is copied to the port of the card –Otherwise, the frame is sent to the backplane –Backplanes moves the frame to the plug-in card of the destination Backplane usually run at Gbps –Goto 2

20. ICOM 6115©Manuel Rodriguez-Martinez Plug-in Card Types Not buffered –Each card acts like a small hub Collision are limited to the card Each Card is a collision domain Buffered Card –Adaptors (network card) send frames to a buffer on the plug-in card –Frames are taken from the buffer and moved to their destination No collisions! But buffer size limits data rate!

21. ICOM 6115©Manuel Rodriguez-Martinez Some issues Plug-in card must learn the MAC address of NICs (Network Interface Card) attached to it Switch must know the MAC address of all the NICs attached to it In all cases frames are your regular Ethernet frames of old Can be used to control traffic –Do not allow anyone to plug-in a machine

22. ICOM 6115©Manuel Rodriguez-Martinez Back Plane Issues If Back Plane acts as a bus, performance will not be that good Need a way to move frames in parallel

23. ICOM 6115©Manuel Rodriguez-Martinez Example: Crossbar Switch Input Ports Output Ports