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Lecture 1

Lecture 1. Advanced Networking CSE 8344 Southern Methodist University Fall 2003 Mark E. Allen. Welcome!. My contact info: Mark E. Allen mallen@signalwise.com 972 747 1490 phone / messages Email is a great way to reach me. Website engr.smu.edu/cse/8344

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Lecture 1

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  1. Lecture 1 Advanced Networking CSE 8344 Southern Methodist University Fall 2003 Mark E. Allen

  2. Welcome! • My contact info: • Mark E. Allen • mallen@signalwise.com • 972 747 1490 phone / messages • Email is a great way to reach me. • Website • engr.smu.edu/cse/8344 • Will contain syllabus, notes, important dates, etc.

  3. Outline for Lecture 1 • Preliminaries • Discuss syllabus • Course goals and outline for course • Get into the content

  4. Intro (cont) • Lecture format • Power point slides • Some written examples • Please ask questions! (unless it’s a tape) • NOTE: • Next two lectures will be pre-taped • Aug 29 10 AM (lecture 2) • September 5 10 AM (lecture 3) • Tapes will play at regular time also.

  5. DISCUSS SYLABUS HERE

  6. Motivation • Purpose of networking: Sharing information between people. • Data is “information” • Voice is “information” • Evolution of networks • Teletype (Morse code) was low bit rate. • Voice (analog) • Video (analog television) • FAX • Dial-up Modems and DDS circuits • High-speed Internet

  7. Voice networks Analog voice Digital trunks introduced. Digital switching Out of band SS7, AIN, etc. Wireless voice Voice over IP Data networks Mainframes connected with SNA Ethernet, Token ring, Novell IPX Ethernet wins out Internet WWW TCP/IP wins out GigE and Wireless Ethernet catching on Network evolution Convergence

  8. Growth of traffic and internet traffic Source: RHK

  9. Data and multimedia now dominate traffic on the network • Eventually the network of the future will carry all types of service. • IP looks to be the “convergence layer” of the future. • Voice, video and data communications will eventually occur over a common network.

  10. Motivation (cont) • What are we really trying to get? • 1) Convergence: Voice, data, video, etc. all on the same user terminal • 2) Low cost: If we can afford it, we’ll use it. • 3) Mobility: We don’t want to be chained to a desk. (wireless, and the internet all give us freedom to access information wherever we are.) • 4) High bandwidth: Lots of speed will enable new and useful apps. Games, virtual reality, on demand movies, video conferencing, etc) • 5) Consumers want direct access into the data networks (B to B, e-commerce, databases, etc.)

  11. The requirements drive the technology • QoS • Bandwidth, Delay, Jitter, etc. • Mobility • Cost • Power consumption • These things are all related. • More bandwidth usually consumes more power • Mobility requires low power. • Etc. .. etc.

  12. Functions of network elements • Signaling / Addressing • Allows the users to control how information flows through the network (IP, dialed digits, etc.) • Switching • Devices necessary for steering information and signaling messages around the network • Multiplexing • Allows several information “flows” to share the same medium • We will discuss this in detail

  13. Millions of disparate customers Distributed control Usage based billing 911 and public safety concerns Lots of security concerns Legacy infrastructure FCC Issues Large geography 1 “customer” Centralized decision making No billing issues Limited public safety concerns Limited legacy concerns Fewer FCC / regulatory issues. Smaller geography Public vs. Enterprise Networks

  14. The layered protocol approach Applications Transport layer Ex: TCP, UDP ... defines how data is transported “layer 4” Network layer “layer 3” Ex: IP, IPX … defines the logical structure of the network Datalink layer “layer 2” Ex: ATM, Ethernet, Token ring… defines how the media is accessed Physical layer Ex: 10base2, 10baseT, SONET… defines the voltages and physical connectors “layer 1”

  15. Limitations of OSI model • The layered model • Provided clear demarcation points for protocol developers. • Physical layer people needn’t be concerned with software. • Was intended to be the roadmap for the “OSI” protocol (never materialized) • But… • Often creates duplication of efforts (error correction, restoration, management, etc.) $$$$ • More on this later

  16. Defining dB • dB is a convenient way of describing loss and gain • dB can be added where multiplication is normally required XdB = 10 log (X) Note: 3dB = 10 log (2) 6dB = 10 log (4) 9dB = 10 log (8) Ex) 6dB 3dB G=4 G=2 Gtotal= (2)(4) = 8 = 9dB

  17. Defining dBm • dBm describes power • YdBm= 10 log(Ymw) • Ex, 15 mwatts = 11.8 dBm 30 mwatts = 14.8 dBm (note: 2X the power is 3dBm more)

  18. Types of transmission mediums • Open copper pair • Low attenuation (few hundredths of dB per km at Voice frequencies) • Takes up lots of space (not used much anymore) • Paired wire • Many pairs in a bundle (up to a few thousand) • Can be buried or put on telephone poles • Higher attenuation than Open wire • Figure 1 shows the attenuation of copper pairs vs. frequency and wire gauge • Example: T1 (~1 MHz signal) experiences 30 dB of attenuation over 1 mile on 22 gauge wire • Common in buildings and LAN installs. 100BaseT runs on twisted pair

  19. Types of transmission mediums (cont) • Coaxial cable • Good for higher bandwidth signals (several hundred MHz) for several km • Takes up much more space • Digital 50ohm used to carry DS3 signals • Analog 75ohm used to carry TV • Fiber optic cable • Extremely wide bandwidth (several THz) • Low attenuation 0.25dB per km

  20. Loops to the home • Transmission from phone to CO occurs on a single pair of wires • A “hybrid” on either side of the two wire circuits (one in the phone, on at the subscriber side of the switch) • Implemented using specialized transformers • Imperfections in the hybrid can cause echos (see figure 2) • Loading coils • Were installed extensively on long loops (3-15 mile) • Reduces attenuation at VF (~3500 Hz) but sharp cutoff at higher frequencies • Big problem for DSL installations

  21. Pair-gain systems • Used to pack several subscribers onto a single loop • Acts time division multiplexor • Commonly called “subscriber loop carriers” • Present a problem when installing DSL • Some pair gain systems use Concentration • Acts as a statistical multiplexer • The terminal that needs the line grabs from the available pool. Some probability exists that the request is blocked.

  22. Multiplexing scheme • FDM – Frequency Division Multiplexing • Several analog VF signals are mixed using different local oscillators • A5 Channel bank multiplexor was used to mux 12 voice calls into a group • See figure 3 for groups, supergroups, etc. • This scheme worked well with analog Voice channels and microwave transmission systems.

  23. Transmission impairments (cont) • Distortion • Envelope delay refers to the delay seen by a particular frequency • Loops impose non-uniform envelope delay • Voice is not severely impacted but it’s a problem for modems • Echos • Occur when there is reflection at the opposite end of the line • Normally causes by hybrid imbalance (2W to 4 Wire) • Attenuation in the circuit helps the problem • Not noticed in short circuits less than 1500 miles (10 msec of delay per 1000 mile circuit) which experience 30 msec of delay (round trip) • Old echo suppressors used active impedence device in reverse path • New echo “cans” are DSP based and use a variant of adaptive filters.

  24. Impairments (cont) • Via Net Loss (VNL) is built-in loss proportional to the length of the circuit. • Combats ringing and echo • Zero transmission level point (TLP) • 0TLP : Reference point in a circuit into the first switch • Measurements taken along the path are referenced back to 0TLP • (see examples)

  25. Digital Signals • Two Symbols: Binary SignalingSymbol is a.k.a. Bit • M Symbols: M-Ary SignalingM is usually a power of 2Log2M bits/symbol • Baud rates same? Symbol shapes similar? If yes..Bandwidth required is similarM-Ary signaling allows increased bit rate Symbols get closer together if Power fixed Receiver detection errors more likely in presence of noise • Bandwidth, Bit Rate, SNR, and BER related

  26. Example: Binary Signal • Serial Bit Stream (a.k.a. Random Binary Square Wave) • One of two possible pulses is transmitted every T seconds.Here the symbol is either a positive or negative going pulse. • When two symbols are used, a symbol is known as a ‘bit’. volts If T = .000001 seconds, then this signal moves 1 Mbps. +1 0 time -1 T

  27. Example:M-Ary Signal • One of M possible symbols is transmitted every T seconds.EX) 4-Ary signaling. Note each symbol can represent 2 bits. volts +1.34 If T = .000001 seconds, then this 1 MBaud signal moves 2 Mbps. +.45 time -.45 -1.34 T

  28. M-Ary Signaling • Bandwidth required • Function of symbols/second & symbol shape • The more rapidly changing is the symbol, the more bandwidth it requires. • An M-Ary signal with the same symbol rate and similar symbol shape as a Binary signal has essentially the same bandwidth. • The previous two slides show... • Equal Power & Equal Bandwidth Signals • M-Ary signal transfers more bits/second BUT detection errors more likely at the receiver

  29. Wired Physical Links • Untwisted Pair Cabling • Highly susceptible to EM interference • Bad choice for telecom systems • Example: Speaker Wires, Power Lines • Twisted Pair Cabling • Fairly resistant to EM interference • Bandwidth typically in 1-2 digit MHz • Examples: LAN wiring, Home telephone cables

  30. Twisted Pair Cables

  31. Wired Physical Links • Coaxial Cable • Resistant to EM interference • Bandwidth typically in 2-3 digit MHz • Example: Cable TV • Fiber Optic Cable • Immune to EM interference • Bandwidth in GHz to THz

  32. Physical Layer Ailments... • AttenuationSignal power weakens with distance • DistortionPulse shapes change with distance • Copper cablingHigh frequencies attenuate fasterPulses smear • Fiber cablingFrequencies propagate at different speedsDispersion

  33. Generating a Square Wave... 5 Hz+ 15 Hz + 25 Hz + 35 Hz 1.5 0 -1.5 1.0 0 cos2*pi*5t - (1/3)cos2*pi*15t + (1/5)cos2*pi*25t - (1/7)cos2*pi*35t)

  34. Effects of Dispersion... 5 Hz+ 15 Hz + 25 Hz + 35 Hz 1.5 0 -1.5 1.0 0 cos2*pi*5t + (1/3)cos2*pi*15t + (1/5)cos2*pi*25t + (1/7)cos2*pi*35t)In this example the 15 and 35 Hz signals have suffered a phase shift (which can be caused as a result of different propagation speeds) with respect to the 5 and 25 Hz signals. The pulse shape changes significantly.

  35. Receiver Detection • SNR tends to worsen with distance • Digital Receiver Symbol Detectors • Examine received symbol intervals (T sec.) • Decide which of M symbols was transmitted • Single Sample DetectorsSample each symbol once Make decision based on sample value • Matched Filter Detectors (Optimal)Sample each symbol effectively an infinite number of timesMake decision based on an average

  36. SNR = (Average Signal Power) Average Noise Power Binary Signal10 Bits showing

  37. SNR = 100 Sequence = 0011010111

  38. SNR = 10 Signal a sequence +1 and -1 volt pulses

  39. SNR = 1

  40. SNR = .1

  41. 4.5 0 4.5 0 20 40 60 80 100 0 k 99 Single Sample Detector: SNR = 1 Threshold is placed midway between nominal Logic 1 and 0 values. Detected sequence = 0011010111 at the receiver, but there were some near misses.

  42. 4.5 0 4.5 0 20 40 60 80 100 0 k 99 Matched Filter Detector: SNR = 1 Orange Bars are average voltage over that symbol interval. Averages are less likely to be wrong.

  43. Channel Capacity (C) • C = W*Log2(1 + SNR) bps • W = channel bandwidth (Hz) • SNR = channel signal-to-noise ratio • Maximum bit rate that can be reliably shoved down a connection • EX) Analog Modem (30 dB SNR)C = 3500 *Log2(1 + 1000) = 34,885 bps • EX) 6 MHz TV RF Channel (42 dB SNR)C = 6,000,000 *Log2(1 + 15,849) = 83.71 Mbps

  44. Why not just keep amplifying to counter-act attenuation? • Amplifiers add noise as they boost the power. • For analog signals, this degrades the signal to noise ratio (SNR) • With digital signals, the SNR (Eb/No) is degraded until the system takes errors • Low noise and multistage amplifiers are used to combat this problem.

  45. Amplifiers in series SNRin SNRout G2 G1 • A pre-amp with a low noise figure reduces the overall noise figure while providing high gain • For example, using this equation, the effective noise figure of a preamp with a gain of 20dB and noise figure of 3 dB followed by an amplifier of gain 30dB and noise figure 9 dB would be 2+(8-1)/100 = 2.07 or 3.16 dB. The total gain would be 50 dB. (make sure to use non-dB numbers in the equations) F2 F1

  46. Overview of telephony • Telecommunication networks were originally designed for voice • Analog signals of 4 kHz bandwidth • Sampled at 8 kHz with 8 bits of quantizing levels for 64kbps circuit • Digital TDM multiplexing has been the technique of choice since the late 70’s.

  47. Multiplexing Formats • Why multiplex?? • Combine several lower bandwidth signals onto a single faster channel • Saves running thousands of individual wires • Allows single carrier for several signals • Frequency division multiplexing • Multiple frequencies “stacked” • This was done in the analog days. • Each voice channel was mixed up to higher center frequency. • Doesn’t lend itself to digital technology • Requires very (spectrally) flat channels

  48. FDM Hierarchy With digital T-Carriers, this is now obsolete From: Digital Telephony Bellamy, chapter 1

  49. FDM (cont) From: Digital Telephony Bellamy, chapter 1

  50. Time division multiplexing • With TDM, every low speed signal gets a fixed amount of bandwidth in the high speed signal Low speed High speed

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