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

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

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

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

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

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

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

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

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

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

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

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:

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:

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?

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

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.

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

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 MHz Cat 5 – 100 Mhz –Fiber 25,000 GHz

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

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

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

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!

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

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)