The Physical Layer

1
Chapter 2

• The Physical Layer

2
Transmission of light through fiber
Attenuation depends on wavelength
CS481
Samir Chettri
Optic fibers
3
Transmission of light through fiber

• Attenuation in dB 10log (transmitted power/recd.
  Power)
• 3 bands for comm. 0.85, 1.3, 1.55 micron
• dispersion Light pulses sent down a fiber spread
  out in a manner that is wavelength dependent.
  Solution 1) separate pulses 2) or raised
  hyperbolic cosines - solitons can propagate 1000
  or more km

Optic fibers
4
Fibre Cables
Optic fibers
5
Humor

• Terrestrial fiber sheaths are normally laid in
  the ground within a meter of the surface, where
  they are occasionally subject to attacks by
  backhoes or gophers. Near the shore, transoceanic
  fiber sheaths are buried in trenches by a kind of
  seaplow. In deep water, they just lie on the
  bottom, where they can be snagged by fishing
  trawlers or eaten by sharks.

Optic fibers
6
Fiber connections

• Method 1 Terminate in connectors and end up in
  fiber sockets. Loss upto 20 of light
• Method 2 Spliced mechanically. Use a sleeve and
  align and calibrate.
• Method 3 Fuse 2 ends of fibre into solid
  connection.

Optic fibers
7
Fibre Optic networks (ring)
Optic fiber networks
8
Fibre Optic networks (star - broadcast)
Silica cylinder
Optic fiber networks
9
Fibre optic vs Copper wire
Fiber
Copper
30km
5km
Repeaters
External events (power surges)
Unaffected
Affected
Size (weight)
Light
Heavy
Security
Difficult to tap
Easier
Expense


Optic fiber
10
Wireless/Electromagnetic spectrum
Electromagnetic spectrum
11
Wireless Transmission/Radio

• Advantage Radio waves are easy to generate, can
  travel long distances, penetrate material objects
  easily and are omnidirectional.
• Disadvantage 1/r3 behavior. Absorbed by rain.
  Interference.

Wireless/Radio
12
Wireless Transmission/Radio
Wireless/Radio
13
Wireless/Microwave trans.

• gt 100MHz waves travel in straight lines.
• Repeaters are needed. High towers are
  constructed.
• Do not pass through buildings
• Multipath fading (late arriving waves are out of
  phase with original wave, therefore fading).
• Above 8GHz absorption by H2O occurs.
• Some bands are free (e.g., 2.4-2.484 GHz)

Wireless/Micro
14
Wireless/IR and mm waves

• Used in VCRs stereos
• Candidates for indoor wireless LANs e.g.,
  portable computers with IR capability can be on
  the local LAN without a physical connection.
• Dont pass through walls therefore security is
  good.
• Cant use outdoors - sunlight washes it out.

Wireless/IR mm
15
Lightwave transmission
16
Signal Transmission
Signal transmission
17
Signal Transmission

• Square waves (digital signaling) have a wide
  spectrum.
• Attenuation is frequency dependent therefore a
  large range of frequencies is undesirable for
  long distance
• Baseband (DC) is not suitable for long distance
  transmission
• Discovery - a continuous oscillating signal
  propagates further.

18
Modems

• Input serial bit stream is converted to a
  modulated carrier (and vice-versa) by MODEM
  (modulator-demodulator)
• Need to increase the of bits / sample (per baud)

4 bits/baud on 2400 baud line
19
Modems

• Patterns (phase and amplitude diagrams) like
  shown in previous slide are called constellation
  patterns.
• 6 bits/baud on a 2400 baud line (14,400 bps) is
  called V.32 bis. V.34 runs at 28,800 bps.
• Constellation pattern is complicated and small
  noise leads to large error.

Modems
20
Modems

• Some modems have compression built in - thereby
  increasing the effective data rate.
• Popular compression scheme is MNP5 which uses run
  length encoding.
• Another coding scheme is V.42 bis which uses a
  Ziv-Lempel algorithm.

Modems
21
Multiplexing

• Economies of scale.Building a trunk line is
  expensive, the cost of optic fibre is not
  (relatively speaking).
• Therefore there are many schemes for multiplexing
  many conversations over a single physical trunk.
• Two categories Frequency Division Multiplexing
  (FDM) and Time Division Multiplexing (TDM).

Multiplexing
22
Multiplexing

• Radio Each station is given a frequency and it
  broadcasts only on that frequency. Frequencies of
  radio stations are widely separated to minimize
  interference (FDM).
• Radio II Radio sends ads followed by music
  (TDM).
• Two categories Frequency Division Multiplexing
  (FDM) and Time Division Multiplexing (TDM).

Multiplexing
23
FDM
Overlap
4000Hz
Multiplexing
24
FDM

• A standard is 12, 4000Hz voice channels (3000Hz
  2, 500Hz guard bands) multiplexed into the
  60-108kHz band. This is called a group.
• 5 groups (60 voice channels) are multiplexed to
  form a super-group.
• 5-10 super-groups form a master-group.

Multiplexing
25
FDM (WDM)

• A variation on FDM for fibre-optic channels is
  wavelength division multiplexing.

Multiplexing
26
TDM
Multiplexing
27
TDM

• Codec Samples at 8000 samples/second (125
  microsec/sample) (Nyquist thm. States that we
  need sampling rate of 2 x max frequency).
• The above technique is called Pulse Code
  Modulation.
• Consider the T1 carrier (next slide).

Multiplexing
28
TDM
Multiplexing
29
TDM

• Analog signals from modems etc. are sampled in
  round robin fashion and then fed to the codec
  (rather than having 24 different codecs)
• Each of the 24 channels puts 8 bits into the
  output stream. 7 are data bits, 1 is a control
  bit. (7x8000 1x8000) 64Kbps per channel.
• One frame 24x8 1framing bit 193bits

Multiplexing
30
TDM

• There are 193 bits / 125microsec. 1.544Mbps
• 193rd bit has alternating zeros and ones
  01010101010101010.. and is used for frame
  synchronization.
• Receiver checks 193rd bit to see if it is in sync
• 24th channel also has a special sync pattern

Multiplexing
31
TDM

• Variations E1 - 32 channels (8 bit data samples)
  in a 125 frame.
• 30 channels are used for info. Two for signaling.
• 4 frames together provide 2x8x4 signaling bits

Multiplexing
32
Multiplexing T1 streams (TDM)
Multiplexing
33
TDM (statistical techniques)

• Signals have characteristics that make them
  amenable to compression through statistical
  techniques.
• DPCM (Differential Pulse Code Modulation) - the
  amplitude is not output but the difference
  between current value prev. one. Large jumps
  are not likely so perhaps 5 bits should work.
  Wild jumps lead to error.
• Var Modulation, predictive encoding

34
TDM (statistical techniques)
Variation on Differential Pulse Code
Modulation is called DELTA MODULATION
Delta modulation
One bit is transmitted telling whether the new
sample is below or above the previous one.
Multiplexing
35
Telephone networks

• LANs are ok for computers in close proximity.
  For longer distances companies prefer to use
  existing telecommunication facilities.
• Public Switched Telephone Network (PSTN) is
  therefore worth studying especially since they
  are going towards digital.
• PSTN was designed for voice communic.

Telephone networks
36
Telephone networks

• Voice communication is quite tolerant of
  transmission errors but computer-computer
  communic. needs much less error. (Read example on
  page 102 of text)
• STRUCTURE (next slide)

Telephone networks
37
Structure of the tel. network
Telephone networks
38
(No Transcript)
39
Struct. of telephone network

• Calls from caller in end office 1 to caller in
  end office 1 go through end office 1
• Calls from end office 1 to end office 2 go
  through toll offices. In a tree there is only one
  minimal route.
• Some routes are busier than others (e.g., DC
  to NY) so direct trunks are created. Therefore
  many paths exist.

Telephone networks
40
Fiber to the curb (FTTC)
Telephone networks
41
Politics of telephone system

• Please read this section on your own section 2.4.2

Telephone networks
42
SONET/SDH

• Synchronous Optical NETwork and Synchronous
  Digital Hierarchy.
• Goals of SONET
• Different carriers (companies) need to work
  together
• Unify US, Japanese and European models
• Multiplex several digital channels together
  (i.e., the T hierarchy - T1, T2 etc to gigabit/s
  lines)
• Provide support for operations, admin. maint.

SONET/SDH
43
SONET/SDH

• SONET is a traditional TDM system.
• Everything is controlled by a highly accurate
  clock (1x10E-9 accuracy) bits are sent out under
  clock control.
• SONET consists of switches, multiplexers and
  repeaters.
• Fibre from one device to another is a section.
• Between two multiplexers is a line.
• Between source and destination is a path.

SONET/SDH
44
SONET/SDH
SONET/SDH
45
SONET/SDH

• SONET puts out a frame of 810 bytes every
  125microseconds. The 8 frames/sec matches PCM
  channels used in all digital telephony systems.
• Each frame is described as a rectangle of bytes
  90 columns wide and 9 rows high.
• SONET is synchronous since frames are emitted
  whether or not there is any data to send.

SONET/SDH
46
SONET/SDH
Synchronous payload envelope starts any- where in
frame
Between two devices
Between muxes
Synchronous Payload Envelope
SONET/SDH
47
SONET/SDH

• First 3 columns are for system mgt. info.
• First 3 rows (of first 3 cols) contain section
  overhead
• Next 6 contain line overhead
• Synchronous Payload Envelope (SPE) - the user
  data can start anywhere in the SONET frame.Useful
  when 1) a dummy frame is being constructed. 2)
  payload does not fit into a frame (we will study
  this in ATM)

SONET/SDH
48
SONET/SDH

• Section, Line and path overheads contain bytes
  for operations, admin. and maint.The fields are
  described in
• Bellamy, J. Digital Telephony, NY, JohnWiley
• Multiplexing of SONET streams are called
  tributaries.
• Low speed input streams are converted to the
  basic STS-1 (Synchronous Transport Signal-1)
  SONET rate much like muxing T1 lines.

SONET/SDH
49
SONET/SDH
Multiplexing is done byte by byte, i.e., a byte
from first tributary is used then a byte from
second and a byte from third in
round-robin fashion. This is true for all levels
of the hierarchy.
SONET/SDH
50
Switching

• The act of choosing a physical copper path
  connection from transmitter to receiver is called
  circuit switching.
• In modern times the physical copper paths may
  well be microwave links.
• An end to end path needs to be set up before any
  data can be sent. For long distance communication
  long setup times (10-20sec) occur. Computer apps.
  dont like this.

51
Circuit Switching
52
SONET/SDH
SONET/SDH
53
Message Switching

• No physical copper path is established in advance
  between sender and receiver.
• Data is stored in a switching office (router) and
  forwarded one jump at a time. E.g. the old torn
  tape offices.
• There was no limit on block size which means
  routers need disks to buffer blocks.
• This limitation lead to packet switching networks

Message switching
54
Packet Switching

• Packet switching networks place a tight upper
  limit on block sizes.
• The transmission lines can only be obtained for
  millisecond intervals.
• Therefore no one person can dominate the
  transmission lines.
• Pkt switching is good for interactive traffic.
• First pkt of multi-pkt message can be fwded
  before second one has fully arrived (see fig)
• With packet switching packets are delivered in
  wrong order (sometimes) never happens with
  circuit switching.

Packet switching
55
Packet/Circuit Switching
Pkt/Circuit switching
56
Crossbar switch
Crosspoint
Q For full duplex line and no self connections
how many crosspoints are needed? (scaling
problem)
Switching
57
Space division switch
Switching
58
Space division switch

• Example Build an NxN crossbar by staging.
• Use three stages.
• First stage has (N/n) crossbars. Each crossbar
  has n x k crosspoints, n input and k output.
• Second stage has k crossbars with (N/n)x(N/n)
  crosspoints.
• Third stage is like first but with input and
  output reversed.
• Number of crosspoints 2kN k(N/n)2

Switching
59
Space division switch

• TANSTAAFLWhat happens in stage 2 when more than
  8 calls come in? You get blocking. Figure b) is
  better but requires more crosspoints. Therefore
  we have to come to some kind of compromise.
• Having large k (in second stage) reduces the
  blocking probability but increases cost.

Switching
60
Time division switch
Switching
61
Time division switch

• n input lines are scanned in round-robin order.
  Each line contributes to an input frame of n
  slots each of k bits. (T1 lines have 8 bits,
  125microsec/frame).
• Time slot interchanger takes input frames and
  outputs new frames where reordering of time slots
  occurs (using a mapping table). This goes to n
  output lines.
• The crux of the matter is the interchanger.

Switching
62
Time division switch

• Table search is linear (in number of input
  lines). This is good.
• Need to access RAM - first store n slots, then
  read them out after accessing mapping table. This
  needs to take place in 125microsec.
• Time to process a frame is 2nTmicrosec or 2nT
  125 or n 125/2T. This determines the number of
  lines given memory speed.

Switching
63
Integrated Services Digital Network (ISDN)

• ISDN is a fully digital, circuit switched
  telephone system.
• Narrowband ISDN.
• Attempted to replace the Plain Old Telephone
  Service (POTS) with a digital one suitable for
  voice and non-voice traffic.
• Lacks bandwidth by 2 orders of magnitude for
  video (i.e., non voice traffic)

ISDN
64
ISDN Arch. (home, small bus.)
ISDN
65
ISDN Arch. (home, small bus.)

• Digital bit pipe ISDN is a conceptual
  full-duplex pipe through which the bits flow
  between customer and carrier. Origin (tel.,
  video) is irrelevant.
• Digital bit pipe does TDM.
• Network Terminating Device (NT1) placed at
  customers site. Connects customers equipment to
  ISDN exchange using twisted pair.

ISDN
66
ISDN Arch. (home, small bus.)

• NT1 has a connector to which a bus connection can
  be put. Phones, terminals etc. can be put on the
  bus.
• Digital bit pipe does TDM.
• Network Terminating Device (NT1) placed at
  customers site. Connects customers equipment (up
  to 8 devices) to ISDN exchange using twisted pair.

ISDN
67
ISDN Architecture (big business)
R conn. betn non ISDN terminal and term- inal
adapter.
S interface between PBX ISDN equip
T connector between customer, NT1
U connector between NT1, exchange
ISDN
68
ISDN Architecture (big business)

• For larger concerns, NT1 is inadequate.
• Therefore we have NT2 aka PBX (Private Branch
  Exchange)

ISDN
69
ISDN Interface

• Bit pipe supports multiple channels interleaved
  by TDM. Standard channels are
• A 4kHz analog telephone channel
• B 64kbps digital PCM channel or voice data
• C 8 or 16 kbps dig. channel for out of band sig.
• D 16 kbps dig. chan for out of band signaling
• E 64 16 kbps dig. channel for ISDN signaling
• H 384, 1536, 1920 kbps digital channel

ISDN
70
ISDN Interface

• Digital bit pipe consists of standard
  combinations of A through H channels.
• Basic rate 2B 1D
• Primary Rate 23B 1D (US, Japan) 30B 1D
  (Europe)
• Hybrid 1A 1C

ISDN
71
Broadband ISDN ATM

• Telephone companies want to invent a single new
  network for the future that will replace the
  entire telephone system with a single integrated
  network.
• The new network is called B-ISDN (Broadband
  Integrated Services Digital Network) and will
  have a huge data rate.
• Underlying B-ISDN is ATM (Asynchronous Transfer
  Mode)

ISDN
72
B-ISDN ATM

• In ATM all transmission occurs in small 53 byte
  packets called cells. 5 bytes are header and 48
  bytes are payload.
• ATM networks are connection oriented but is
  implemented internally with packet switching.
• SPEED 155.52Mbps and 622Mbps (4 155Mbps
  channels). Gigabit speeds are to follow.

B-ISDN ATM
73
B-ISDN ATM
REFERENCE MODEL
B-ISDN ATM
74
B-ISDN ATM

• Standard twisted pair cannot be used (Category 5
  twisted pair can be). Therefore we need re-wiring
  or fibre.
• Space and time-division switches cannot be used
  for ATM packet switching. Therefore need new
  switches. (more on that later).
• Wide area fibre trunks can be used.

B-ISDN ATM
75
to applications
Generate packets larger than a cell
Segments packets, transmits cells and reassembles
them at other end
Transmitting end - streams of bits to PMD.
Receiving end - streams of bits from PMD to cell
stream.
Interfaces to cable. Different hardware is
required for diff. cables and fibres.
CS Convergence Sublayer
SAR Segmentation and Reassembly
TCTransmission Convergence
PMD Physical Medium Dependent
B-ISDN ATM
76
Virtual circuits vs. circuit switching
B-ISDN ATM
77
Virtual circuits vs. circuit switching

• Permanent Virtual Circuits - are requested by the
  customer and remain in place as long as customer
  pays the rent.
• Switched Virtual Circuits - setup like telephone
  calls, i.e., allocated dynamically and then torn
  down.
• ATM a route is chosen from source to dest.
  switches make table entries to route pkts on
  virtual ckt. (Fig shows H1 to H5)

Virtual circuits
78
Virtual circuits vs. circuit switching

• When pkt arrives it switch it examines pkts
  header to determine what virtual circuit it
  belongs to.
• Virtual ckt between H1, H5 means that switches
  (routers) will hold table entries for a
  particular destination - regardless of the last
  time traffic occurred (costly, but no setup
  time). App - credit card verification.

Virtual circuits
79
Transmission in ATM networks
The rate is governed by a master clock. T1 is
synchronous
Strict alternation between different sources not
adhered to. Cells arrive randomly from different
sources.
Transmission in ATM
80
Transmission in ATM networks

• ATM permits cells to be enclosed in a carrier
  such as T1, T3, SONET etc. In each case a
  published standard is available

Transmission in ATM
81
ATM switches

• We mentioned earlier that time division and space
  division switches do not work with ATM. Generic
  switch shown below

ATM switches
82
ATM switches

• Switches may be pipelined, i.e., several cells
  from one input line may be collected before being
  sent to its output line.
• Cells arrive on input line asynchronously so
  there is a master clock that marks the beginning
  of the cycle.
• Any cell (53 bytes) that arrives before the clock
  ticks is eligible for switching. If not the cell
  is made to wait for the next cycle.

ATM switches
83
ATM switches

• Cells come in at approx. 150Mbps 36E04
  cells/sec - therefore cycle time of switch is
  1/36E04 2.7777microsec.
• Switch may have from 16 to 1024 input lines. Thus
  anywhere between 16 to 1024 cells are to be
  switched every 2.7microsec.
• At 622Mbps the time is measured in nanoseconds.

ATM switches
84
ATM switches

• All ATM switches have 2 common goals
• GOAL 1 Drop cells but only in emergencies -
  1E-12 cell loss is permissible. This translates
  to 1 or 2 cells per hour.
• GOAL 2 Cells arriving at a switch in a
  particular order must leave in that order without
  exception.
• PROBLEM What does the switch do when the cells
  arriving at 2 or more input lines want to go to
  the same output port? (PTO)

ATM switches
85
ATM switches
Input queuing
Problem Head-of-line blocking. When cell is held
up all the cells behind it get held up. PTO for
another solution.
ATM switches
86
ATM switches
Output queuing
ATM switches
87
ATM switches (Batcher-Banyan)

• Read Knockout Switch on your own.

Banyan Switch
ATM switches
88
ATM switches (Batcher-Banyan)

• Routing is done by looking up the output line for
  each cell (using routing tables)
• Each switching element has two inputs and two
  outputs (0,1. Reasons given below).
• Example 6 (110) arrives at input line 0. 3 stage
  banyan switch. Binary number is read from L-R. 1
  means use lower port 0 means use upper port. Thus
  input cell 0 with 001 as output port ends up on
  output port 6.

ATM switches
89
Batcher-Banyan (Collisions)
Collisions STAGE 1 (5,7), (0,3), (6,4), (2,1)
(Resolve for 5,0,4,1) STAGE 2
(0,1), (5,4) STAGE 3 Only 1, 5
win. Depending on input we could have collisions
and therefore good or bad routing.
ATM switches
90
Batcher-Banyan (Contd)
ATM switches
91
Batcher-Banyan (Contd)

• Batcher switch placed before banyan switch.
  Element consists of 2x2 cells.
• When an element receives 2 cells it does a
  numerical comparison.
• Higher output address goes in direction of arrow.
  
• Lower output address in opposite direction
• If only one cell it goes to output port that is
  opposite to the direction of the arrow.

ATM switches
92
Batcher-Banyan (Contd)

• After leaving Batcher switch the cells are
  shuffled and passed to a banyan switch.
• This is shown two slides previously.
• A more concrete example is given on the next
  slide.

ATM switches
93
Batcher-Banyan (Contd)
ATM switches
94
Cellular Radio

• Advanced Mobile Phone System. (AMPS)
• In AMPS a geographical region is divided into
  cells.
• These cells are circular but are modeled as
  hexagonal regions.
• Frequencies are reused in cells that are not
  adjacent.
• Small cells lead to less power requirements for
  devices.

Cellular radio
95
Cellular Radio
Subdivision of basic cells
Base Station
Cellular radio
96
Cellular Radio

• At any time a phone is in one cell and therefore
  working with the base station that sits in the
  center of that cell.
• When a phone moves to a new cell ownership of the
  phone is transferred to the new bas station.
• If a call is in progress, this transfer takes
  300ms. This transfer is called handoff.

Cellular radio
97
Cellular Radio

• There are 832 full duplex channels in the AMPS
  system.
• 832 simplex channels going from 824-849MHz for
  transmission
• 832 simplex channels going from 869-894MHz for
  reception.
• Each simplex channel is 30KHz wide.
• Echo occurs when radio waves bounces off trees,
  bldgs. as well as travels in a st. line.

Cellular radio
98
Cellular Radio

• In each city 416 channels given to B-side carrier
  (i.e., ATT) and 416 channels are given to A-side
  carrier (entrant in cellular business). This is
  done to promote competition.
• There are 4 categories of channels
• Control
• Paging
• Access (for call setup and channel assignment)
• Data (for voice, fax or data)

Cellular radio
99
Cellular Radio

• SECURITY
• Anyone with an all band receiver (radio) can tune
  into and hear everything that is going on in a
  call.
• With an all band receiver connected to a
  computer, the thief can record the 32 bit phone
  number and 34 bit SN. A DB can built up of these
  numbers and used.
• Solution - use encryption. But police dont like
  this.

Cellular radio
100
Communication Satellites

• Weather balloons (metallized on the outside) were
  used as radio reflectors. US Navy used the moon!
• Artificial satellites have been used since 1962.
• Receives signals at one frequency
• Rebroadcasts at another frequency (to avoid
  interference with incoming frequency)

Comsats
101
Communication Satellites

• Geosynchronous
• Beams are usually single spatial beam that
  illuminated entire earth. Now spot beams are also
  available.
• VSAT (Very Small Aperture Terminals). These are
  low cost microstations with 1m antennas with
  1watt power output.
• Communications take place as shown in next slide
  (due to the low power).

Comsats
102
Communication Satellites
Comsats
103
Communication Satellites
Comsats
104
Communication Satellites

• Low Orbit Satellites.
• Individual low orbit satellites are not useful
  for communication satellites.
• However groups of satellites in low earth orbit
  could be useful. One such project is Iridium.
• Here the cell phones would be mobile as would the
  cells (since the satellites are moving)
• Each satellite has 48 spot beams. 66 satellites.
  So 1628 cells. Uplink and downlink on L band
  (1.6GHz).

Comsats