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Advanced Computer Networks: Lecture 5. This lecture will cover the following: Point-to-Point links; direct link issues in the OSI and hardware/software contexts; hardware building blocks; electromagnetic spectrum; asymmetric digital subscriber line (ADSL);...
CS716 Advanced Computer Networks By Dr. Amir Qayyum Lecture No. 5 The Big Picture Midterm exam (estimated) You are here What We Know • Networks are – Experiencing explosive growth – Providing wide range of services • It is attributed to: – General purpose nature of computer networks – Ability to add new functionality with software – High performance computers are now affordable and We Know … • Connecting mainframes over longdistance telephone lines has turned into a big business! • Lots of competing players – Computing industry – Telephone carriers – Service providers, operators, … • Global, ubiquitous, heterogeneous networking ? – Issues of connectivity, service levels, performance, … What We Have Learned • Carefully identify what we expect from a network • Costeffective connectivity – Accomplished through nested interconnection of nodes and links – Provides processtoprocess communication services – Should offer high performance using the metrics like latency and throughput • This results in a packetswitched network What is Our Approach • A layered architecture as a guideline for design • Protocols are central objects – Provides services to higherlevel protocols – Make a message exchange meaningful with peers • Implement protocols in software – Define interfaces to invoke services – Socket interface between applications and protocols – “Similar” interface within the network subsystem What Next ? Start with a simplest possible network Two nodes connected directly through some suitable medium PointtoPoint Links Reading: Peterson and Davie, Ch. 2 Outline Hardware building blocks Encoding Framing Error Detection Reliable transmission • Sliding Window Algorithm Direct Link Issues in the OSI and Hardware/Software Contexts application presentation userlevel software session transport reliability network kernel software (device drivers) data link framing, error detection, MAC physical encoding hardware (network adapter) 10 RS232 • One bit per clock • Voltage never returns to 0V (0V is a dead / disconnected line) • 15V is both idle and “1”; initiates the send by pushing to 15V for one clock (start bit) 35 RS232 • Minimum delay between character transmissions idle for one clock at –15V (stop bit) • One character leads to 2+ voltage transitions • Total of 9 bits for 7 bits of data (78% efficient) • Start and stop bits also provide framing 36 Binary Voltage Encoding • NRZ (nonreturn to zero) • NRZI (NRZ inverted) • Manchester (used by IEEE 802.3, 10 Mbps Ethernet) • 4B/5B (8B/10B) in Fast Ethernet 37 NonReturn to Zero (NRZ) • Encode binary data onto signals – e.g., 0 as low signal and 1 as high signal – voltage does not return to zero between bits • known as NonReturn to Zero (NRZ) Bits 0 1 1 1 0 0 NRZ 38 Problem: Consecutive 1s or 0s • Low signal (0) may be interpreted as no signal • High signal (1) leads to baseline wander • Unable to recover clock – sender’s and receiver’s clock have to be precisely synchronized – receiver resynchronizes on each signal transition – clock drift in long periods without transition sender’s clock receiver’s clock 39 Alternative Encodings • NonReturn to Zero Inverted (NRZI) • Make a transition from current signal (switch voltage level) to encode/transmit a “one” • Stay at current signal (maintain voltage level) to encode/ transmit a “zero” • Solves the problem of consecutive ones (shifts to 0s) 40 Alternative Encodings • Manchester (in IEEE 802.3 – 10 Mbps Ethernet) • Split cycle into two parts – Send highlow for “1”, lowhigh for “0” – Transmit XOR of NRZ encoded data and the clock • Only 50% efficient (1/2 bit per transition) 41 Different Encoding Schemes Bits 0 1 1 1 0 0 NRZ Clock Manchester NRZI 42 4B/5B Encoding • Every 4 consecutive bits of data encoded in a 5bit code (symbol) – 4bit pattern is “translated” to a 5bit pattern (not addition) • 5bit codes selected to have no more than one leading 0 and no more than two trailing 0s – 00xxx (8 symbols) and xx000 (4 symbols) are illegal – 5 free symbols (nondata) • Thus, never gets more than three consecutive 0s • Resulting 5bit codes are transmitted using NRZI • Achieves 80% efficiency 43 Binary Voltage Encoding • Problem: wide frequency range required, implying – Significant dispersion – Uneven attenuation • Prefer to use narrow frequency band (carrier frequency) • Types of modulation – – – – Amplitude (AM) Frequency (FM) Phase / phase shift Combination of these (e.g. QAM) 44 Amplitude Modulation idle idle 1 idle idle 0 idle idle time 45 Frequency Modulation idle idle 1 idle idle 0 idle time 46 Phase Modulation idle idle 1 idle idle 0 idle idle time 47 Phase Shift in Carrier Frequency 108 degrees difference in phase collapse for 108 degrees shift 48 Review Lecture 5 • Simplest possible network – 2 nodes connected directly • Building blocks – nodes and links • Nodes – workstation architecture • Links – several types, optical, wireless • Encoding – binary data into signals, RS 232 • Binary voltage encoding – NRZ, NRZI, Manchester, 4B/5B • Modulation schemes 49 ... earth channels (~2 Mbps); 155 .5? ?Mbps intersatellite channels 24 Wireless Links • Radio and infrared frequency links • 11 Mbps rates, 2.4 GHz band, distances of? ?50 150 meters – 5. 2 GHz band, >? ?55 Mbps: HIPERLAN1, IEEE ... STS-N over fiber Neighborhood optical network unit VDSL at 12.96– 55 .2 Mbps over 1000– 450 0 feet of copper Subscriber premises 21 CATV • Uses existing cable TV (CATV) infrastructure – reaches 95% of households in U.S... Appears, but may not be a single link !!! Service: Bandwidth: (bps) DS1/T1 DS3 1.5M STS1 STS3 44.7M 51 .8M 155 M STS12 STS48 622M 2.5G 18 Lastmile Links • Most economical • Home to network service provider