1. ICOM 6115©Manuel Rodriguez-Martinez ICOM 6115 – Computer Networks and the WWW Manuel Rodriguez-Martinez, Ph.D. Lecture 8

2. ICOM 6115©Manuel Rodriguez-Martinez Lecture Objectives Polish the edges –RTT and bandwidth Signaling Physical Technologies

3. ICOM 6115©Manuel Rodriguez-Martinez Network Latency Latency – how long it takes a “typical” packet of data to travel from one end of the link to another –Measure in terms of time –Ex. Transcontinental delay of fiber link is 24 milliseconds Round-trip-time (RTT) – time for a message to reach one end of the network and travel back –2 X latency of network

4. ICOM 6115©Manuel Rodriguez-Martinez Network Latency and RTT Sender Receiver Sender Receiver One way latency RTT

5. ICOM 6115©Manuel Rodriguez-Martinez Network Latency (aka Delay) There are three major components for latency –Propagation delay – time for packet to move over wire, fiber or air –Transmit time – time to put bits on the wire –Queuing Time – time spent by message store in queues at routers/switches

6. ICOM 6115©Manuel Rodriguez-Martinez Components of Network Latency Latency = Propagation + Transmit + Queue Propagation = Distance / Effective Speed of Light on medium Transmit = Packet Size / Bandwidth Queue = average time for packets at queue on router/switch before getting forwarded

7. ICOM 6115©Manuel Rodriguez-Martinez Delay x Bandwidth product Consider a network link as a hollow pipe –To move bits from host A to host B Bandwidth –Diameter of the pipe Delay –Length of pipe (time for 1 bit to travel it) Delay x Bandwidth products –Number of bits that can be fit into the pipe during 1 latency

8. ICOM 6115©Manuel Rodriguez-Martinez Delay x Bandwidth Product Delay x bandwidth gives the bit volume of the pipe Bandwidth Delay

9. ICOM 6115©Manuel Rodriguez-Martinez Why is this important? Network must be used efficiently Sending one bit at a time is out of the question! –Need to send groups of bits Frames Delay x bandwidth products tells us how many bits we can send to fill the pipe –If we get to transmit try to fill the pipe

10. ICOM 6115©Manuel Rodriguez-Martinez RTT and Delay x Bandwidth RTT is 2 x delay In 1 delay we can send delay x bandwidth bits If sender expects a reply from receiver it will take a another delay for those bits to arrive We can send another pack of bits just after the reply arrives –We can send 2 delay x bandwidth worth bits before processing the first acknowledgement

11. ICOM 6115©Manuel Rodriguez-Martinez Bandwidth versus Latency Relative importance –1-byte: 1ms vs 100ms dominates 1Mbps vs 100Mbps –25MB: 1Mbps vs 100Mbps dominates 1ms vs 100ms Infinite bandwidth –RTT dominates Throughput = TransferSize / TransferTime TransferTime = RTT + 1/Bandwidth x TransferSize 1-MB file to 1-Gbps link as 1-KB packet to 1-Mbps link

12. ICOM 6115©Manuel Rodriguez-Martinez Question Calculate the total time required to transfer a 1000MB file if the network has a RTT of 100 ms, packet size of 1KB of data, bandwidth of 1.5Mpbs and no routers, and packets can be send continuously. Answer:

13. ICOM 6115©Manuel Rodriguez-Martinez Questions Suppose we have a 1000Mpbs channel? –How long are the bits? –Answer: 1 nanosecond Consider a 10Mbps with a –Answer:

14. ICOM 6115©Manuel Rodriguez-Martinez Question Suppose we have a 1KM copper wire, with a propagation delay of 200 ms and 10Mbps bandwidth –How many bits can we fit on the wire?

15. ICOM 6115©Manuel Rodriguez-Martinez Signals OK, this is EE domain … Need to encode bit into signals –Voltage – cooper wire –Light pulses – fiber –Radio waves – air

16. ICOM 6115©Manuel Rodriguez-Martinez Another view of bandwidth Each signal will be made up of components of various frequencies If a signal has a range of frequencies f1 through f2 –Bandwidth is f2 – f1 –Ex. If a signal has frequencies 10Hz through 60Hz, then the bandwidth will be 50Hz –Telephone has 3kHz of bandwidth In this view (communications guys) our “bandwidth” is called data rate.

17. ICOM 6115©Manuel Rodriguez-Martinez Bandwidth and Data Rate Henry Nyquist proved that –A signal with bandwidth H can be reconstructed if the medium is sampled with a frequency 2H. Noiseless channel If the signal has V discrete levels then –Data rate = 2H log_2 V bits/sec –Data rate is proportional to the bandwidth So, we will call data rate the bandwidth

18. ICOM 6115©Manuel Rodriguez-Martinez A few numbers Bandwidth and data rates for a binary signaling scheme –Telephone - 3kHz – gives 6000 bps Need to user more tones to encode more bits –Go faster –Cooper Cat 3 - 16 MHz Cat 5 – 100 Mhz –Fiber 25,000 GHz

19. ICOM 6115©Manuel Rodriguez-Martinez Signal-to-Noise ratio Data Channels are not noiseless Signal-to-noise ratio –How much of the signal is noise –Decibel (dB) – 10 log_10 S/N, where S/N is the signal-to-noise ratio Claude Shannon proved the following –Maximum Data rate = H log_10 (1 + S/N) bits/sec Telephone has 3kHz bandwidth and 30dB S/N –Maximum data rate = 30,000 bps

20. ICOM 6115©Manuel Rodriguez-Martinez Guided Media Tape or a DVD Twisted Pair Cable Coaxial Cable Fiber Optics

21. ICOM 6115©Manuel Rodriguez-Martinez Tape Store data in tapes, pack your tapes, and carry them around –This is a silly example, but illustrates difference between bandwidth and latency One tape can hold hundreds of gigabytes –UItrium – 200 gigabytes –1 box of 1000 tapes can hold 200 Terabytes, or 1600 terabits (1.6 petabits) –Ship the box by 24hours FedEx – 86,400 sec –Bandwidth of this link: 19Gbps From San Juan to Mayaguez (2 hr drive) – 200 Gbps

22. ICOM 6115©Manuel Rodriguez-Martinez Problem with Tape Latency –It takes 1 day to see any data by 24 hr FedEx –It takes 2 hours to see any by driving from SJ to Mayaguez Many times bandwidth is not the issue, is latency!

23. ICOM 6115©Manuel Rodriguez-Martinez Unshielded Twisted Pair Cooper Cables are twisted to control radiation of waves out cable Use by telephone, and in many LANs (Ethernet) Most common cables are Cat. 3 and now Cat. 5 (most popular currently) –New comers (Cat 5e, Cat 6 and Cat 7) CAT 3 – 16MHz CAT 5 – 100MHz

24. ICOM 6115©Manuel Rodriguez-Martinez Coaxial Cable Can span longer distances (shielding) –75 ohm – Cable TV –50 ohm – Digital Transmission (old days Ethernet) Bandwidth – 1GHz (modern day)