Internet and Tools, Media and Serial Lines

• Packet Switching
• Internet history, measuring Internet size
• ping, traceroute, telnet, round-trip times
• Media:
  • Copper
  • Fiber
  • Wireless and Satellites
• Serial Lines:
  • Asynchronous Communication
  • Encoding
  • Baud Rates, Maximum Data Rates

Packet Switching

• telephone networks work(ed) by physically connecting telephones via copper contacts: circuit switching
• this circuit is reserved for the call whether or not anything is being said
• computers spend a lot more time saying nothing than saying something, so circuit switching is inefficient for data
• instead, computer networks are based on the idea of sending packets to and from different computers along the same wires: packet switching

Internet History

• late 1960s: ARPA (now DARPA) decided to fund research into networks that could survive nuclear attacks
• ARPAnet evolved into NSFnet to the Internet
• packet switching was picked over circuit switching because there were very few resources (56Kb/s backbone) and efficiency was paramount
• the network offered very few guarantees: packets could be
  • corrupted
  • reordered
  • lost
  • duplicated

Internet Growth

• internet size around 30M hosts connected to the Internet in 1998
• growth rate exponential from 1981 through 1998, doubling about every nine to twelve months
• how do you define a host connected to the Internet? Do modems count? systems behind firewalls?

Measuring the Size of the Internet

• automatically count the number of names assigned by various entities (60M in 2000):
  • very few hosts connected without a name
  • there may be no host for a name
  • some companies won't tell us what names they have assigned
• automatically solicit a response from a random subset of all possible IP addresses (about 30M in 2000):
  • some computers are designed not to reply to these responses ("ping"s)
  • some firewalls block these responses
  • routers and other machines with multiple interfaces (each with its own IP address) will be counted multiple times

ping

edo@maru 233-> ping www.hawaii.edu
PING www1-94.its.Hawaii.EDU
  (128.171.94.101): 56 data bytes
64 bytes from 128.171.94.101: time=5.6 ms
64 bytes from 128.171.94.101: time=0.7 ms
64 bytes from 128.171.94.101: time=0.6 ms
64 bytes from 128.171.94.101: time=0.7 ms
-www1-94.its.Hawaii.EDU ping statistics-
4 packets transmitted, 4 packets received
  0% packet loss
round-trip min/avg/max = 0.6/1.9/5.6 ms

• ping hostname
• on Solaris, ping -s hostname
• on Windows, x is alive
• determine raw connectivity

Round-Trip Times

• round-trip time is a lot easier to measure than one-way latency
• first round-trip time is slow: filling caches
• round-trip time may depend on physical distance, packet size, network bandwidth, congestion
• round-trip times tell us:
  • minimum: the latency of the uncongested network
  • average: the performance seen by applications
  • maximum: the delay variation (jitter) seen by retransmitting protocols

traceroute

edo@maru 247-> traceroute www.ifa.hawaii.edu
traceroute to galileo.IfA.Hawaii.Edu (128.171.160.7)
 1  128.171.10.1 (128.171.10.1)
      1.14 ms  1.01 ms  1.019 ms
 2  atm-to-manoa.uhnet.net (128.171.64.218)
      1.454 ms  1.037 ms  1.019 ms
 3  uhm-ifa-t3.gw.hawaii.net (128.171.68.2)
      2.21 ms  2.216 ms  1.996 ms
 4  galileo.IfA.Hawaii.Edu (128.171.160.7)
      2.653 ms *  1.634 ms

• only needed when ping fails
• if all goes well, shows intermediate hosts (routers)
• sometimes no response (e.g. second packet from Galileo)
• times match ping

telnet

• I can ping www.x.com, but can't connect to its web server
• telnet www.x.com 80, type GET / HTTP/1.0 (and two carriage returns)
• can use telnet to connect to any server that uses TCP and ASCII encoding, including the one in your project
• you must know the address and port number
• Ping exercises:
  • I recommend doing the exercises at the end of chapter 2
  • in particular, what is the largest round-trip time you can find?

Transmission Media

• transmission involves sending bits of information
• energy is needed to signal the value of bits
• most commonly, we use electromagnetic energy:
  • electrical (voltage/current) levels on wires
  • radio
  • microwave
  • infrared
  • light
• different media can carry different bit rates and have different error rates

Electrical Wire

• "copper"
• long wires act like antennas: they lose signal energy, and acquire unrelated signal energy (noise)
• to minimize antenna-like behavior, two geometric configurations:
  • coaxial
  • twisted-pair
• in both configurations, current in one direction is almost canceled by current moving in the opposite direction
• relatively high error rate, relatively low bit rate (up to about 100MHz), relatively low cost, suitable for short distances

Optical Fibers

• light reflects back into the pipe
• on/off unidirectional signaling on a single fiber (bidirectional is possible but rarely used)
• fiber types:
  • multimode (greatest dispersion)
  • multimode with low dispersion (signals traveling longer distances travel faster)
  • single-mode (least dispersion)
• relatively low error rate, high bit rate (THz), relatively high cost, suitable for long-distance land-lines

Wireless Transmission

• electromagnetic radiation can be used in ranges of increasingly higher frequency:
  • Radio
  • Microwave
  • Infrared
  • Light
• higher frequencies are more directional and (generally) more affected by weather
• higher frequencies can carry more bits/second

Satellites

• wireless transmission to and from the satellite
• a transmission from the ground to a satellite is point-to-point
• a transmission from the satellite is a broadcast to a relatively wide area
• satellites can be used to overcome the line-of-sight issues with many wireless transmissions
• satellites add delay and cost

Serial Lines

• RS-232 standard, later augmented with RS-422 standard
• asynchronous communication: receiver must be able to decode sender's signal without knowledge of the sender's clock (with synchronous communication, the sender sends the clock as well as the data)
• serial communication: one bit is sent on the wire before the next bit is sent (opposite of parallel communication)

Bit Encodings

• one encoding: no voltage is no bit (idle), positive voltage is 1, negative voltage is 0 bit
• RS-232 encoding: positive voltage is 0, negative (or zero) voltage is 1, idle sends 1
• two sides of RS-232 must agree on the number of data bits sent (e.g. 5, 8) and on the bit time (e.g. 9600 baud)
• sender sends the start bit (always 0), the n data bits, and 1 or more stop bits (always 1)
• receiver starts an accurate clock at the center of the start bit, uses it to read the data bits
• two signal wires plus ground give bidirectional communication

Bit Errors

• BREAK is a long sequence of zero bits -- multiple hundreds of milliseconds
• if a bit signal is corrupted (e.g. by lightning), the signal on the wire may not be what was intended to be sent
• if the start bit is distorted (by noise or bad wiring), the receiver may interpret the successive bits at the wrong time
• if the two endpoints fail to agree on bit time or number of bits, the communication will fail and random-looking characters will be received

Baud

• A Baud is the number of changes per second in the signal (named after Baudot)
• for RS-232 and many other systems, the number of Baud is the same as the number of bits per second
• but 10Mb/s Manchester-encoding Ethernet runs at 20MBaud
• 33Kb/s modems run at much lower baud rates (9600 baud?)
• the maximum frequency of transmission on a wire decreases with distance, increases with power used

Nyquist Theorem

• the maximum frequency of transmission on a wire decreases with distance
• frequency limiting means a signal is distorted, ideal "vertical" transitions become slanted (slower) transitions
• a signal can be decomposed into frequencies (cycles per second, Hertz): the bandwidth of a signal is the difference between the maximum and the minumum (significant) frequencies of the signal
• Nyquist Sampling Theorem: if a signal has bandwidth B and uses K signal levels, the maximum data rate is D = 2B log₂K b/s

Shannon's Theorem

• it is impossible to reach the performance of Nyquist's theorem in the presence of noise -- otherwise we could use very large K to send lots of bits with little bandwidth
• signal power S, noise power N, signal-to-noise ratio S/N
• a decibel level dB is dB = 10 log₁₀ S/N, so 20dB means the signal is 100 times more powerful than the noise
• Shannon's theorem: the capacity C of a channel with bandwidth B Hz is C = B log₂(1+S/N) b/s

Example

• Telephone circuit has a bandwidth of 3000 Hz and a S/N ratio of 30dB
• Shannon limit gives us C = 3000 log₂(1+1000) b/s =~ 30,000 b/s
• using the Nyquist theorem, this corresponds to using about 1,000 different signal levels
• in-class exercise: what channel capacity can we get if the S/N ratio is 0dB and the bandwidth is 1,000 Hz?