1 CS716 Advanced Computer Networks By A. Wahid Shaikh.
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Transcript of 1 CS716 Advanced Computer Networks By A. Wahid Shaikh.
1
CS716
Advanced Computer Networks
By A. Wahid Shaikh
Lecture No. 5
3
The Big Picture
You are
here
Midterm exam
(estimated)
4
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
5
and We Know …
• Connecting mainframes over long-distance 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, …
6
What We Have Learned
• Carefully identify what we expect from a network
• Cost-effective connectivity– Accomplished through nested interconnection of
nodes and links
– Provides process-to-process communication services
– Should offer high performance using the metrics like latency and throughput
• This results in a packet-switched network
7
What is Our Approach
• A layered architecture as a guideline for design
• Protocols are central objects– Provides services to higher-level 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
8
What Next ?
Start with a simplest possible network
Two nodes connected directly through some suitable medium
9
Point-to-Point Links
Reading: Peterson and Davie, Ch. 2
OutlineHardware building blocksEncodingFramingError DetectionReliable transmission
• Sliding Window Algorithm
10
Direct Link Issues in the OSI and Hardware/Software Contexts
transport
network
data link
physical
session
presentation
application
user-level software
kernel software (device drivers)
reliability
framing, error detection, MAC
encoding hardware (network adapter)
11
Hardware Building Blocks
• Nodes– Hosts: general-purpose computers– Switches: typically special-purpose hardware– Routers (connecting networks): varies
• Links– Copper wire with electronic signaling– Glass fiber with optical signaling– Wireless with electromagnetic (radio, infrared,
microwave) signaling
12
Nodes – A Workstation Architecture
CPU(processor)
Cache $
MemoryI/O bus
Networkadaptor
to network
finite memory (implies limited
buffer space)
Device driver managing network adaptor which is using system’s I/O bus
Memory access much slower
than CPU speed
memory bus
13
Links
• Physical media– twisted pair cable– coaxial cable– optical fiber– space
• Media is used to propagate signals
• Signals are electromagnetic waves of certain frequency, traveling at speed of light
14
Electromagnetic Spectrum
Radio Infrared UVMicrowave Gamma ray
f (Hz)
FM
Coax
Satellite
TV
AM Terrestrial microwave
Fiber optics
X ray
100
104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016
102 106 108 1010 1012 1014 1016 1018 1020 1022 1024104
Wavelength = speed/frequency = 2 x 108 / 300 = 667 meters
15
Signals Over a Link
• Signal is modulated for transmission– varying frequency/amplitude/phase to
receive distinguishable signals
• Binary data (0s and 1s) is encoded in a signal– make it understandable by the receiving
host
16
Bits Over a Link
• Bit streams may be transmitted both ways at a time on a point-to-point link– full-duplex
• Sometimes two nodes must alternate link usage– half duplex
17
Which Link to Use ?
• Cables– same room / building / site
Cable Typical Bandwidths Distances
Cat-5 twisted pair 10-100 Mbps 100 m
Thin-net coax 10-100 Mbps 200 m
Thick-net coax 10-100 Mbps 500 m
Multimode fiber 100 Mbps 2 km
Single-mode fiber 100-2400 Mbps 40 km
insulation
braided conductor
copper core
coax
twisted pair
glass core (fiber)
glass clading
plastic jacket
18
Leased Lines
• Across city / country
• Dedicated link from the telephone company
• Appears, but may not be a single link !!!
Service: DS1/T1 DS3 STS-1 STS-3 STS-12 ... STS-48
Bandwidth: 1.5M 44.7M 51.8M 155M 622M ... 2.5G
(bps)
19
Last-mile Links
• Most economical
• Home to network service provider
• To take benefit of an existing network
Service: POTS ISDN xDSL CATV
Bandwidth: 28.8 - 56 K 64 - 128 K 16 K - 55.2 M 20 - 40 M
(bps)
20
ADSL(Asymmetric Digital Subscriber Line)
• Connects the subscriber to the central office via the local loop
• Bandwidth depends on length of local loop
Centraloffice
Subscriberpremises
1.554– 8.448 Mbps
16– 640 Kbps
Local loop2.74 – 5.48 Km
21
VDSL(Very high data rate DSL)
• Connects the subscriber to the optical network that reaches the neighborhood
• Runs over short distances
• Symmetric
Centraloffice
Neighborhood opticalnetwork unit
STS-N
over fiber
Subscriberpremises
VDSL at 12.96– 55.2 Mbps
over 1000– 4500 feet of copper
22
CATV
• Uses existing cable TV (CATV) infrastructure– reaches 95% of households in U.S.
• Single CATV channel has bandwidth of 6 MHz
• Can be used in asymmetric way
• Currently achieves on a single channel:– 40 Mbps downstream (100 Mbps theoretical capacity)– 20 Mbps upstream
• Multiple access on shared channel (IEEE 802.14)
23
Optical Communication
• Higher bandwidths• Superior attenuation properties• Immune from electromagnetic
interference• No cross-talk between fibers• Thin, lightweight and cheap (the fiber, not
the optical-electrical interfaces)
24
Wireless Links• Satellite links
• Provide a grid of medium and low orbit satellites– Geosynchronous satellite 600-1000 Mbps
– Low Earth Orbit (LEO) array ~400 Mbps
• Targeted at voice communication modems
• Teledesic supports 1440 16 kbps satellite-to-earth channels (~2 Mbps); 155.5 Mbps intersatellite channels
25
Wireless Links• Radio and infra-red frequency links
• 11 Mbps rates, 2.4 GHz band, distances of 50-150 meters– 5.2 GHz band, > 55 Mbps: HIPERLAN-1, IEEE
802.11a
• Bluetooth piconets: Infrared links, 1 Mbps, 10 meters
26
Encoding
27
Point-to-Point Links
• Reading: Peterson and Davie, Ch. 2
• Hardware building blocks• Encoding• Framing• Error Detection• Reliable transmission
– Sliding Window Algorithm
28
Encoding
• Signals propagate over a physical medium– modulate electromagnetic waves
– e.g., vary voltage
• Encode binary data onto signals that propagate
Signalling component
Signal
Bits
Node NodeAdaptor Adaptor
29
Encoding
• Problems with signal transmission– Attenuation: signal power absorbed by medium
– Dispersion: a discrete signal spreads in space
– Noise: random background “signals”
modulator demodulatora string
of signals
Digital data (a string of symbols)
Digital data (a string of symbols)
30
Advantages of Digital Transmission over Analog
• Reasonably low-error rates over arbitrary distances– Calculate/measure effects of transmission
problems
– Periodically interpret and regenerate signal
• Simpler for multiplexing distinct data types (audio, video, e-mail, etc.)
31
Advantages of Digital Transmission over Analog
• Examples of modulators-demodulators (modems)
• Electronic Industries Association (EIA) standard RS-232(-C)
• International Telecommunications Union (ITU) standard V.32 96 kbps modem
32
RS-232(-C)
• Communication between computer and modem
• Uses two voltage levels (+15V, -15V), a binary voltage encoding
• Data rate limited to 19.2 kbps (RS-232-C); raised in later standards
33
RS-232(-C)
• Characteristics
• Serial: one signaling wire, one bit at a time
• Asynchronous: line can be idle, clock generated from data
• Character-based: send data in 7- or 8-bit characters
34
RS-232 Timing Diagram
+15
-15
volt
age
Idle start 1 0 0 1 1 0 0 stop idle
time
35
RS-232
• 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)
36
RS-232
• 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
37
Binary Voltage Encoding
• NRZ (non-return to zero)
• NRZI (NRZ inverted)
• Manchester (used by IEEE 802.3, 10 Mbps Ethernet)
• 4B/5B (8B/10B) in Fast Ethernet
38
Non-Return 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 Non-Return to Zero (NRZ)
Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
39
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
40
Alternative Encodings• Non-Return 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)
41
Alternative Encodings• Manchester (in IEEE 802.3 – 10 Mbps
Ethernet)
• Split cycle into two parts– Send high--low for “1”, low--high for “0”– Transmit XOR of NRZ encoded data and the
clock
• Only 50% efficient (1/2 bit per transition)
42
Different Encoding Schemes
Bits
NRZ
Clock
Manchester
NRZI
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
43
4B/5B Encoding
• Every 4 consecutive bits of data encoded in a 5-bit code (symbol)– 4-bit pattern is “translated” to a 5-bit pattern (not addition)
• 5-bit 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 (non-data)
• Thus, never gets more than three consecutive 0s• Resulting 5-bit codes are transmitted using NRZI • Achieves 80% efficiency
44
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)
45
Amplitude Modulation
idle idle 1 idle idle 0 idle idle
time
46
Frequency Modulation
idle idle 1 idle idle 0 idle
time
47
Phase Modulation
idle idle 1 idle idle 0 idle idle
time
48
Phase Shift in Carrier Frequency
108 degrees difference in phasecollapse for 108 degrees shift
49
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