Transcript Fundamentals of Computer Networks

Fundamentals of Computer
Networks
Book
Andrew S. Tanenbaum, “Computer Networks”
What is a Network?
A computer network, is a collection of computers and
other hardware interconnected by communication
channels that allow sharing of resources and
information.
Uses of Computer Networks
• Business Applications
– Issue: Resource Sharing
– Goal: Make all programs, equipment, and especially data
available to anyone on the network without regard to the
physical location of the resource and the user
– Example: A group of office workers can share a common
printer
– Sharing information is more important than sharing
physical resources such as printers
Uses of Computer Networks
• Home Applications
–
–
–
–
Access to remote information.
Person-to-person communication.
Interactive entertainment.
Electronic commerce.
Uses of Computer Networks
• Mobile Users
– Notebook Computers
– Personal Digital Assistants
– Smart Phones
Network Hardware
• There are two types of transmission technology
that are in widespread use
– Broadcast links: Short messages, called packets in
certain contexts, sent by any machine are received by
all the others. If the packet is intended for the
receiving machine, that machine processes the
packet.
– Point-to-point links: To go from the source to the
destination, a packet on this type of network may
have to first visit one or more intermediate machines.
Network Hardware
Classification of interconnected processors by scale:
Local Area Network (LAN)
• LANs, are privately-owned networks within a
single building or campus of up to a few
kilometers in size.
• LANs are distinguished from other kinds of
networks by three characteristics:
– Size (Restricted in Size)
– Transmission Technology (Cables)
– Their topology
Lan Topologies
Bus
Ring
Bus Network
• In a bus network, at any instant at most one machine is the
master and is allowed to transmit. All other machines are
required to refrain from sending.
• An arbitration mechanism is needed to resolve conflicts
when two or more machines want to transmit
simultaneously.
• IEEE 802.3, popularly called Ethernet is a bus-based
broadcast network.
• Computers on an Ethernet can transmit whenever they
want to; if two or more packets collide, each computer just
waits a random time and tries again later.
Ring Network
• Each bit circumnavigates the entire ring in the
time it takes to transmit a few bits, often before
the complete packet has even been transmitted.
• As with all other broadcast systems, some rule is
needed for arbitrating simultaneous accesses to
the ring.
• IEEE 802.5 (the IBM token ring), is a ring-based
LAN.
Static & Dynamic Networks
• Depending on how the channel is allocated networks can
be further divided into static and dynamic.
• Static allocation would be to divide time into discrete
intervals and use a round-robin algorithm, allowing each
machine to broadcast only when its time slot comes up.
• Dynamic allocation methods for a common channel are
either centralized or decentralized.
– In the centralized channel allocation method, there is a single
entity, for example, a bus arbitration unit, which determines
who goes next.
– In the decentralized channel allocation method, each machine
must decide for itself whether to transmit.
Metropolitan Area Networks (MAN)
Wide Area Networks (WAN)
• A WAN, spans a large geographical area, often
a country or continent.
• It contains a collection of machines (hosts)
intended for running user programs.
Wide Area Networks (WAN)
• The hosts are connected by a communication subnet,
or just subnet for short.
• The hosts are owned by the customers (e.g., people's
personal computers), whereas the communication
subnet is typically owned and operated by a telephone
company or Internet service provider.
• The job of the subnet is to carry messages from host to
host, just as the telephone system carries words from
speaker to listener.
Wide Area Networks (WAN)
• In most wide area networks, the subnet consists
of two distinct components
– Transmission Lines
– Switching Elements.
• Transmission lines move bits between machines.
• Switching elements are specialized computers
that connect three or more transmission lines.
Routing
Wireless Networks
• Wireless networks can be divided into three main
categories:
– System interconnection (short-range radio)
– Wireless LANs.
– Wireless WANs.
Home Networks
• Home networking is on the horizon.
• The fundamental idea is that in the future
most homes will be set up for networking.
• Every device in the home will be capable of
communicating with every other device, and
all of them will be accessible over the
Internet.
Internetworks
• Many networks exist in the world, often with different
hardware and software.
• People connected to one network often want to
communicate with people attached to a different one.
• The fulfillment of this desire requires that different, and
frequently incompatible networks, be connected,
sometimes by means of machines called gateways to make
the connection and provide the necessary translation.
• A collection of interconnected networks is called an
internetwork or internet.
Network Software – Protocol
Hierarchies
• To reduce their design complexity, most networks are
organized as a stack of layers.
• Each layer is a kind of virtual machine, offering certain
services to the layer above it.
• Layer n on one machine carries on a conversation with
layer n on another machine. The rules and conventions
used in this conversation are collectively known as the layer
n protocol.
• Basically, a protocol is an agreement between the
communicating parties on how communication is to
proceed.
Network Software – Protocol
Hierarchies
Network Software – Protocol
Hierarchies
Network Software – Protocol
Hierarchies
Connection-Oriented and
Connectionless Services
• To use a connection-oriented network service,
the service user first establishes a connection,
uses the connection, and then releases the
connection.
• Connectionless service is modeled after the
postal system. Each message carries the full
destination address, and each one is routed
through the system independent of all the others.
Connection-Oriented and
Connectionless Services
Service Primitives (Operations)
Reference Models
• OSI reference model
– Protocols associated with the OSI model are rarely
used
– The model itself is actually quite general and still
valid
• TCP/IP reference model
– The model itself is not of much use
– The protocols are widely used
OSI Reference Model
• Developed by the
International
Standards
Organization (ISO)
• The model is
called the ISO OSI
(Open Systems
Interconnection)
Reference Model
OSI Reference Model
• A layer should be created where a different abstraction is
needed.
• Each layer should perform a well-defined function.
• The function of each layer should be chosen with an eye
toward defining internationally standardized protocols.
• The layer boundaries should be chosen to minimize the
information flow across the interfaces.
• The number of layers should be large enough that distinct
functions need not be thrown together in the same layer
out of necessity and small enough that the architecture
does not become unwieldy.
OSI Reference Model
• OSI model itself is not a network architecture
because it does not specify the exact services
and protocols to be used in each layer.
• ISO has also produced standards for all the
layers, although these are not part of the
reference model itself.
The Physical Layer
• Task: Transmitting raw bits over a
communication channel.
– Making sure that when one side sends a 1 bit, it is
received by the other side as a 1 bit, not as a 0 bit
– How many volts should be used to represent a 1
and how many for a 0
– How many nanoseconds a bit lasts
The Data Link Layer
• Task: To transform a raw transmission facility
into a line that appears free of undetected
transmission errors to the network layer.
– Input data is divided into data frames (typically a
few hundred or a few thousand bytes) and these
frames are transmitted sequentially
– How to keep a fast transmitter from drowning a
slow receiver in data
The Network Layer
• Task: The network layer controls the operation
of the subnet.
– How packets are routed from source to
destination
– The quality of service provided is a network layer
issue
– In broadcast networks, the routing problem is
simple, so the network layer is often thin or even
nonexistent!
The Transport Layer
• Task: Accept data from above, split it up into smaller
units if need be, pass these to the network layer, and
ensure that the pieces all arrive correctly at the other
end.
– Determines what type of service to provide to the session
layer
– The transport layer is a true end-to-end layer.
– A program on the source machine carries on a
conversation with a similar program on the destination
machine, using the message headers and control
messages.
– In the lower layers, the protocols are between each
machine and its immediate neighbors, and not between
the ultimate source and destination machines
The Session Layer
• Task: allows users on different machines to
establish sessions between them.
– Keeping track of whose turn it is to transmit
– Preventing two parties from attempting the same
critical operation at the same time
The Presentation Layer
• Task: Presentation layer is concerned with the
syntax and semantics of the information
transmitted
– Allows higher-level data structures to be defined
and exchanged.
The Application Layer
• HTTP (Hypertext Transfer Protocol)
• File transfer,
• Electronic Mail
The TCP/IP Reference Model
• Goals
– Network must be able to survive loss of subnet
hardware, with existing conversations not being
broken off
– A flexible architecture was needed since
applications with divergent requirements were
envisioned (Transferring files, realtime speech
transmission)
The TCP/IP Reference Model
TCP/IP The Internet Layer
• Permit hosts to inject packets into any
network and have them travel independently
to the destination (potentially
• Packets may arrive in a different order than
they were sent, in which case it is the job of
higher layers to rearrange them, if in-order
delivery is desired.
TCP/IP The Transport Layer
• TCP (Transmission Control Protocol)
– Allows a byte stream originating on one machine to
be delivered without error on any other machine in
the internet
– Handles flow control to make sure a fast sender
cannot swamp a slow receiver with more messages
than it can handle.
• UDP (User Datagram Protocol)
– A protocol for applications that do not want TCP's
sequencing or flow control and wish to provide their
own
Protocols in the TCP/IP model
TCP/IP Application Layer
•
•
•
•
•
TELNET (Virtual Terminal)
FTP (File Transfer Protocol)
SMTP (Simple Mail Transfer Protocol)
DNS (Domain Name System)
HTTP (Hypertext Transfer Protocol)
The Hybrid Reference Model
The Theoretical Basis: Fourier Analysis
Any reasonably behaved periodic function, g(t) with period T
can be constructed as the sum of a (possibly infinite) number
of sines and cosines
Transmission of ‘b’
• The example of the transmission of the ASCII character ''b'' encoded
in an 8-bit byte.
• The bit pattern that is to be transmitted is 01100010.
Transmission of ‘b’
Bandwidth
• No transmission facility can transmit signals without losing some
power in the process.
• If all the Fourier components were equally diminished, the resulting
signal would be reduced in amplitude but not distorted.
• Unfortunately, all transmission facilities diminish different Fourier
components by different amounts, thus introducing distortion.
• Usually, the amplitudes are transmitted undiminished from 0 up to
some frequency fc with all frequencies above this cutoff frequency
attenuated.
• The range of frequencies transmitted without being strongly
attenuated is called the bandwidth.
• In practice, the cutoff is not really sharp.
Bandwidth
• Given a bit rate of b bit/s, the time required to
send 8 bits is 8/b sec, so the frequency of the first
harmonic is b/8 Hz.
• An ordinary telephone line, often called a voicegrade line, has an artificially-introduced cutoff
frequency just above 3000 Hz.
• This restriction means that the number of the
highest harmonic passed through is roughly
3000/(b/8) or 24,000/b
The Maximum Data Rate of a Channel
• In 1924, an AT&T engineer, Henry Nyquist,
realized that even a perfect channel has a
finite transmission capacity.
• He derived an equation expressing the
maximum data rate for a finite bandwidth
noiseless channel.
H: Bandwidth
V: Signal Levels
Guided Transmission Media
• Magnetic Media
– One of the most common ways to transport data
from one computer to another is to write them
onto magnetic tape or removable media.
– It is often more cost effective, especially for
applications in which high bandwidth or cost per
bit transported is the key factor.
Guided Transmission Media
• Twisted Pair
– For many applications an on-line connection is need.
– A twisted pair consists of two insulated copper wires,
typically about 1 mm thick. The wires are twisted
together in a helical form.
– The most common applications of the twisted pair are
the telephone systems and Ethernet Networks.
– The bandwidth depends on the thickness of the wire
and the distance traveled.
Guided Transmission Media
• All of the twisted pair wiring types are often
referred to as UTP (Unshielded Twisted Pair)
Guided Transmission Media
• Shielded Twisted Pair
– Cables are often shielded in an attempt to prevent
electromagnetic interference.
– This shielding can be applied to individual pairs, or
to the collection of pairs.
Most Common Twisted-Pair Cables
Name
Type
Bandwidth
Applications
Level 1
0.4 MHz
Telephone and modem lines
Level 2
4 MHz
Older terminal systems, e.g. IBM 3270
Cat3
UTP
16 MHz
10BASE-T and 100BASE-T4 Ethernet
Cat4
UTP
20 MHz
16 Mbit/s Token Ring
Cat5
UTP
100 MHz
100BASE-TX & 1000BASE-T Ethernet
Cat5e
UTP
100 MHz
100BASE-TX & 1000BASE-T Ethernet
Cat6
UTP
250 MHz
10GBASE-T Ethernet
500 MHz
10GBASE-T Ethernet
600 MHz
Telephone, CCTV, 1000BASE-TX in the same cable. 10GBASE-T Ethernet.
1000 MHz
Telephone, CATV, 1000BASE-TX in the same cable. 10GBASE-T Ethernet.
Cat6a
Class F
Class Fa
S/FTP
Guided Transmission Media
• Coaxial Cable
– It has better shielding than twisted pairs, so it can
span longer distances at higher speeds.
Guided Transmission Media
• Coaxial Cable
– A good combination of high bandwidth and
excellent noise immunity
– Modern cables have a bandwidth of close to 1
GHz
– Coaxial cables used to be widely used within the
telephone system for long-distance lines but have
now largely been replaced by fiber optics.
– Coax is still widely used for cable television
Guided Transmission Media
• Fiber Optics
– An optical transmission system has three key components
• the light source
• the transmission medium
• the detector.
– Conventionally, a pulse of light indicates a 1 bit and the
absence of light indicates a 0 bit.
– The detector generates an electrical pulse when light falls
on it.
– By attaching a light source to one end of an optical fiber
and a detector to the other, we have a unidirectional data
transmission system that accepts an electrical signal,
converts and transmits it by light pulses, and then
reconverts the output to an electrical signal at the
receiving end.
Guided Transmission Media
• Multimode Fiber
• Single Mode Fiber
– Single-mode fibers are more expensive but are widely
used for longer distances.
– Currently available single-mode fibers can transmit
data at 50 Gbps for 100 km without amplification
Guided Transmission Media
• Transmission of Light through Fiber
Guided Transmission Media
• Fiber Cables
– In multimode fibers, the core is typically 50 microns in
diameter, about the thickness of a human hair.
– In single-mode fibers, the core is 8 to 10 microns.
Connections of Fiber Cables
• They can be connected in three different ways:
– They can terminate in connectors and be plugged into
fiber sockets.
• Connectors lose about 10 to 20 percent of the light, but they
make it easy to reconfigure systems.
– They can be spliced mechanically.
• Mechanical splices just lay the two carefully-cut ends next to
each other in a special sleeve and clamp them in place.
• Mechanical splices take trained personnel about 5 minutes
and result in a 10 percent light loss.
– Two pieces of fiber can be fused (melted) to form a
solid connection.
• A fusion splice is almost as good as a single drawn fiber, but
even here, a small amount of attenuation occurs
Transmitter and Receiver
• Two kinds of light sources are typically used to do the
signaling
– LEDs (Light Emitting Diodes)
– Semiconductor Lasers
• The receiving end of an optical fiber consists of a photodiode,
which gives off an electrical pulse when struck by light.
• The typical response time of a photodiode is 1 ns, which
limits data rates to about 1 Gbps.
Fiber Optic Networks
• Two types of interfaces are used.
– Passive Interface
– Active Repeater
A fiber optic ring with active repeaters.
Comparison of Fiber Optics and
Copper Wire
• Advantages of Fiber (Comparing fiber to copper)
– It can handle much higher bandwidths than copper.
– Due to the low attenuation, repeaters are needed only about every 50
km on long lines, versus about every 5 km for copper.
– Fiber also has the advantage of not being affected by power surges,
electromagnetic interference, or power failures
– Fiber is much lighter than copper. 1000 twisted pairs 1 km long weigh
8000 kg. Two fibers have more capacity and weigh only 100 kg, which
greatly reduces the need for expensive mechanical support systems
that must be maintained.
– Fibers do not leak light and are quite difficult to tap. These properties
gives fiber excellent security against potential wiretappers.
• Disadvantages of Fiber
– Fibers can be damaged easily by being bent too much.
– Since optical transmission is inherently unidirectional, two-way
communication requires either two fibers or two frequency bands on
one fiber.
– Fiber interfaces cost more than electrical interfaces.
Wireless Transmission
• When electrons move, they create electromagnetic waves
that can propagate through space.
• When an antenna of the appropriate size is attached to an
electrical circuit, the electromagnetic waves can be
broadcast efficiently and received by a receiver some
distance away.
• In vacuum, all electromagnetic waves travel at the same
speed, no matter what their frequency. This speed, usually
called the speed of light.
• In copper or fiber the speed slows to about 2/3 of this value
and becomes slightly frequency dependent.
• The fundamental relation between f (frequency) , λ
(Wavelength) , and c (Speed of Light in Vacuum) is
The Electromagnetic Spectrum
Radio Transmission
• Advantages
–
–
–
–
Easy to generate
Can travel long distances
Can penetrate buildings easily
Radio waves also are omnidirectional, meaning that they
travel in all directions from the source, so the transmitter
and receiver do not have to be carefully aligned physically.
• Disadvantages
– At low frequencies, radio waves pass through obstacles
well, but at high frequencies, radio waves tend to travel in
straight lines and bounce off obstacles.
– They are absorbed by rain
Radio Transmission
• In the VLF, LF, and MF bands, radio waves follow the ground.
• The main problem with using these bands for data communication is their
low bandwidth.
• In the HF and VHF bands, the waves that reach the ionosphere, a layer of
charged particles circling the earth at a height of 100 to 500 km, are
refracted by it and sent back to earth.
• Military communicates in the HF and VHF bands
Microwave Transmission
• Above 100 MHz, the waves travel in nearly straight
lines and can therefore be narrowly focused.
• Concentrating all the energy into a small beam by
means of a parabolic antenna (like the familiar satellite
TV dish) gives a much higher signal-to-noise ratio, but
the transmitting and receiving antennas must be
accurately aligned with each other.
• Before fiber optics, for decades these microwaves
formed the heart of the long-distance telephone
transmission system.
Infrared and Millimeter Waves
• Unguided infrared and millimeter waves are
widely used for short-range communication.
• The remote controls used on televisions, VCRs,
and stereos all use infrared communication.
• They are relatively directional, cheap, and
easy to build but have a major drawback: they
do not pass through solid objects.
Lightwave Transmission
Communication Satellites
Structure of the Telephone System
Structure of the Telephone System
• By 1890, the three major parts of the
telephone system were in place:
– The switching offices
– The wires between the customers and the
switching offices
– The long-distance connections between the
switching offices
Structure of the Telephone System
Structure of the Telephone System
• In summary, the telephone system consists of
three major components:
– Local loops: analog twisted pairs going into houses
and businesses.
– Trunks: digital fiber optics connecting the
switching offices.
– Switching offices: where calls are moved from one
trunk to another.
The Local Loop: Modems, ADSL, and
Wireless
The Local Loop: Modems, ADSL, and
Wireless
• When a computer wishes to send digital data over an
analog dial-up line, the data must first be converted to
analog form for transmission over the local loop.
• This conversion is done by a device called a modem.
• At the telephone company end office the data are
converted to digital form for transmission over the
long-haul trunks.
• If the other end is a computer with a modem, the
reverse conversion—digital to analog—is needed to
traverse the local loop at the destination.
The Local Loop: Modems, ADSL, and
Wireless
• Transmission lines suffer from three major problems:
– Attenuation,
– Delay Distortion
– Noise.
• Attenuation is the loss of energy as the signal propagates outward.
Energy lost depends on the frequency.
• To see the effect of this frequency dependence, imagine a signal
not as a simple waveform, but as a series of Fourier components.
• Each component is attenuated by a different amount, which results
in a different Fourier spectrum at the receiver.
• Different Fourier components also propagate at different speeds in
the wire. This speed difference leads to distortion of the signal
received at the other end.
Modems
• The square waves used in digital signals have a wide
frequency spectrum and thus are subject to strong
attenuation and delay distortion.
• To get around the problems associated with DC signaling,
especially on telephone lines, AC signaling is used.
• A continuous tone in the 1000 to 2000-Hz range, called a
sine wave carrier, is introduced.
• Its amplitude, frequency, or phase can be modulated to
transmit information.
– In amplitude modulation, two different amplitudes are used to
represent 0 and 1, respectively.
– In frequency modulation, also known as frequency shift keying,
two different tones are used.
– In the simplest form of phase modulation, the carrier wave is
systematically shifted 0 or 180 degrees at uniformly spaced
intervals.
Amplitude modulation, Frequency
Modulation and Phase Modulation
Modems Cont’d
• A device that accepts a serial stream of bits as
input and produces a carrier modulated by of
these methods (or vice versa) is called a
modem.
• The modem is inserted between the (digital)
computer and the (analog) telephone system.
Combination of Modulation
Techniques
QPSK
(Quadrature Phase Shift Keying)
QAM-16
QAM-64
(Quadrature Amplitude Modulation).
Digital Subscriber Lines
• The reason that modems are so slow is that telephones
were invented for carrying the human voice and the entire
system has been carefully optimized for this purpose.
• In the end office, the wire runs through a filter that
attenuates all frequencies below 300 Hz and above 3400
Hz.
• The trick that makes xDSL work is that when a customer
subscribes to it, the incoming line is connected to a
different kind of switch, one that does not have this filter,
thus making the entire capacity of the local loop available.
• The limiting factor then becomes the physics of the local
loop, not the artificial 3100 Hz bandwidth created by the
filter.
Potential Bandwidth as a Function of
Distance
Operation of ADSL using Discrete
Multitone modulation.
• The available 1.1 MHz spectrum on the local loop into
256 independent channels.
– Channel 0 is used for POTS (Plain Old Telephone Service).
– Channels 1–5 are not used, to keep the voice signal and
data signals from interfering with each other.
– Of the remaining 250 channels, one is used for upstream
control and one is used for downstream control.
– The rest are available for user data.
ADSL
• A 50–50 mix of upstream and downstream is
technically possible.
• However most providers allocate something
like 80%–90% of the bandwidth to the
downstream channel since most users
download more data than they upload.
• This choice gives rise to the ''A'' in ADSL.
A Typical ADSL Equipment
Configuration
DSLAM: Digital Subscriber Line Access Multiplexer
ISP: Internet Service Provider
Frequency Division Multiplexing
Wavelength Division Multiplexing
PCM (Pulse Code Modulation)
• The analog signals are digitized in the end office
by a device called a codec (coder-decoder),
producing a series of 8- bit numbers.
• The codec makes 8000 samples per second
because the Nyquist theorem says that this is
sufficient to capture all the information from the
4-kHz telephone channel bandwidth.
• This technique is called PCM (Pulse Code
Modulation).
Time Division Multiplexing
Delta Modulation
Switching
• When you or your computer places a telephone call, the
switching equipment within the telephone system seeks
out a physical path all the way from your telephone to the
receiver's telephone.
• This technique is called circuit switching. Once a call has
been set up, a dedicated path between both ends exists
and will continue to exist until the call is finished.
Message Switching
• An alternative switching strategy is message
switching.
• When this form of switching is used, no physical
path is established in advance between sender
and receiver.
• Instead, when the sender has a block of data to
be sent, it is stored in the first switching office
(i.e., router) and then forwarded later, one hop at
a time.
• Each block is received in its entirety, inspected for
errors, and then retransmitted.
Packet Switching
• With message switching, there is no limit at all
on block size, which means that routers must
have disks to buffer long blocks.
• It also means that a single block can tie up a
router-router line for minutes, rendering
message switching useless for interactive
traffic.
• To get around these problems, packet
switching was invented.
Comparison of Circuit-Switched and
Packet-Switched Networks.
First-Generation Mobile Phones:
Analog Voice
• Mobile radiotelephones were used for maritime and
military communication during the early decades of
the 20th century.
• The first system for car-based telephones:
– This system used a single large transmitter on top of a tall
building and had a single channel, used for both sending
and receiving
– To talk, the user had to push a button that enabled the
transmitter and disabled the receiver.
– In the 1950s CB-radio, taxis, and police cars often use
these push-to-talk systems.
First-Generation Mobile Phones:
Analog Voice
• In the 1960s, IMTS (Improved Mobile Telephone
System) was installed.
• It used a high-powered transmitter, on top of a
hill, but now had two frequencies, one for
sending and one for receiving.
• Push-to-talk button was no longer needed.
• Mobile users could not hear each other.
First-Generation Mobile Phones:
Analog Voice
• AMPS (Advanced Mobile Phone System), invented by Bell
Labs.
• In all mobile phone systems, a geographic region is divided
up into cells, which is why the devices are sometimes called
cell phones.
• In AMPS, the cells are typically 10 to 20 km across; in digital
systems, the cells are smaller.
• Each cell uses some set of frequencies not used by any of
its neighbors.
First-Generation Mobile Phones:
Analog Voice
• The cellular design
increases the
system capacity,
more as the cells
get smaller.
• Furthermore,
smaller cells mean
that less power is
needed, which
leads to smaller
and cheaper
transmitters and
handsets.
First-Generation Mobile Phones:
Analog Voice
• At the center of each cell is a base station to which all the
telephones in the cell transmit.
• The base station consists of a computer and transmitter/receiver
connected to an antenna.
• In a small system, all the base stations are connected to a single
device called an MTSO (Mobile Telephone Switching Office) or MSC
(Mobile Switching Center).
• In a larger one, several MTSOs may be needed, all of which are
connected to a second-level MTSO, and so on.
• The MTSOs communicate with the base stations, each other, and
the PSTN using a packet-switching network.
First-Generation Mobile Phones:
Analog Voice
• Transferring ownership to the cell getting the strongest
signal is called Handoff.
• Handoffs can be done in two ways:
– Soft handoff
– Hard Handoff
• The AMPS system uses 832 full-duplex channels, each
consisting of a pair of simplex channels.
– There are 832 simplex transmission channels from 824 to
849 MHz and 832 simplex receive channels from 869 to
894 MHz.
– Each of these simplex channels is 30 kHz wide.
First-Generation Mobile Phones:
Analog Voice
• The 832 channels are divided into four categories:
– Control (base to mobile) to manage the system.
– Paging (base to mobile) to alert mobile users to calls
for them.
– Access (bidirectional) for call setup and channel
assignment.
– Data (bidirectional) for voice, fax, or data.
• Since the same frequencies cannot be reused in
nearby cells, the actual number of voice channels
available per cell is much smaller than 832.
Second-Generation Mobile Phones:
Digital Voice
• Four different systems are in use
– D-AMPS (The Digital Advanced Mobile Phone
System)
– GSM (The Global System for Mobile
Communications)
– CDMA (Code Division Multiple Access)
– PDC (Personal Digital Cellular)
D-AMPS
• In D-AMPS, a new frequency band was made
available to handle the expected increased load.
– The upstream channels were in the 1850–1910 MHz
range, and the corresponding downstream channels
were in the 1930–1990 MHz range, again in pairs, as
in AMPS.
– In this band, the waves are 16 cm long, leading to
smaller phones.
• Many D-AMPS phones can use both the 850-MHz
and 1900-MHz bands to get a wider range of
available channels.
D-AMPS
• In D-AMPS, three users can share a single
frequency pair using time division multiplexing.
Handoff in D-AMPS
• In AMPS, the MTSO manages handoff completely without help from
the mobile devices.
• In D-AMPS, 1/3 of the time a mobile is neither sending nor
receiving.
• During the idle of the mobile telephone periods, the mobile
telephone monitors other radio channels for signal strength.
• The mobile telephone can report these measurements along with
its own received signal strength and channel back to the base
station.
• The base station can use this information along with other
information to determine if a new radio channel should be assigned
and which channel to assign the mobile telephone to.
• This technique is called MAHO (Mobile Assisted HandOff)
GSM
• GSM channels are much wider than the AMPS channels
– 200 KHz in GSM
– 30 KHz. in AMPS and D-AMPS
GSM
Also a 51-slot multiframe is also used to manage the system.
CDMA
• The key to CDMA is to be able to extract the desired
signal while rejecting everything else as random noise.
• Each station is assigned a unique m-bit code called a
chip sequence.
– To transmit a 1 bit, a station sends its chip sequence.
– To transmit a 0 bit, it sends the one's complement of its
chip sequence.
• No other patterns are permitted. Thus, for m = 8, if
station A is assigned the chip sequence 00011011, it
sends a 1 bit by sending 00011011 and a 0 bit by
sending 11100100.
CDMA
(a) Binary chip sequences
for four stations.
(b) Bipolar chip sequences.
(c) Six examples of
transmissions.
(d) Recovery of station C's
signal.
CDMA
• To see why this works, just imagine that two stations, A and
C, both transmit a 1 bit at the same time that B transmits a
0 bit.
• The receiver sees the sum
• And computes
• The first two terms vanish because all pairs of chip
sequences have been carefully chosen to be orthogonal.
Third-Generation Mobile Phones:
Digital Voice and Data (3G)
• 3G finds application in wireless voice telephony,
mobile Internet access, fixed wireless Internet
access, video calls and mobile TV.
• The basic services that the IMT-2000
(International Mobile Telecommunications-2000)
network is supposed to provide to its users are
– High-quality voice transmission.
– Messaging (replacing e-mail, fax, SMS, chat, etc.).
– Multimedia (playing music, viewing videos, films,
television, etc.).
– Internet access (Web surfing, including pages with
audio and video).
Third-Generation Mobile Phones:
Digital Voice and Data
• While waiting for the fighting over 3G to stop, some
operators are gingerly taking a cautious small step in
the direction of 3G by going to what is sometimes
called 2.5G
– EDGE (Enhanced Data rates for GSM Evolution)
• It is just GSM with more bits per baud.
• The trouble is, more bits per baud also means more errors per
baud.
– GPRS (General Packet Radio Service)
• It is an overlay packet network on top of D-AMPS or GSM.
• It allows mobile stations to send and receive IP packets in a cell
running a voice system.
• When GPRS is in operation, some time slots on some frequencies
are reserved for packet traffic.
• The number and location of the time slots can be dynamically
managed by the base station, depending on the ratio of voice to
data traffic in the cell.
4G Mobile Phones: IMT Advanced
• An IMT-Advanced system provides a
comprehensive and secure all-IP based mobile
broadband solution to laptops, wireless
modems, smartphones and other mobile
devices.
• Conceivable applications include amended
mobile web access, IP telephony, gaming
services, high-definition mobile TV, video
conferencing and 3D television.
4G Mobile Phones: IMT Advanced
• Specific requirements of the IMT-Advanced report
included:
– Based on an all-Internet Protocol (IP) packet switched network
– Interoperability with existing wireless standards
– A nominal data rate of 100 Mbit/s while the client physically
moves at high speeds relative to the station, and 1 Gbit/s while
client and station are in relatively fixed positions.
– Dynamically share and use the network resources to support
more simultaneous users per cell.
– Scalable channel bandwidth 5–20 MHz, optionally up to 40 MHz
– Seamless connectivity and global roaming across multiple
networks with smooth handovers
– Ability to offer high quality of service for multimedia support
Data Link Layer
• The data link layer has a number of specific
functions it can carry out. These functions
include:
– Providing a well-defined service interface to the
network layer.
– Dealing with transmission errors.
– Regulating the flow of data so that slow receivers
are not swamped by fast senders.
Services Provided to the Network
Layer
Service Types
• Three services that are commonly provided are:
– Unacknowledged connectionless service.
• Unacknowledged connectionless service consists of having
the source machine send independent frames to the
destination machine without having the destination machine
acknowledge them.
– Acknowledged connectionless service.
• Each frame sent is individually acknowledged.
– Acknowledged connection-oriented service.
• The source and destination machines establish a connection
before any data are transferred.
Framing
• It is up to the data link layer to detect and, if necessary,
correct transmission errors.
• The usual approach is for the data link layer to break
the bit stream up into discrete frames and compute the
checksum for each frame.
• There are four methods for framing except inserting
time gaps between them.
–
–
–
–
Character count.
Flag bytes with byte stuffing.
Starting and ending flags, with bit stuffing.
Physical layer coding violations
Character Count
Flag Bytes (Byte Stuffing)
Bit Stuffing
Physical Layer Coding Violations
• If every data bit has a transition in the middle,
it makes the receiver to locate the bit
boundaries easily.
• The combinations high-high and low-low are
not used for data but are used for delimiting
frames in some protocols.
Error Control
• The usual way to ensure reliable delivery is to
provide the sender with some feedback about
what is happening at the other end of the line.
• An additional complication comes from the
possibility that hardware troubles may cause a
frame to vanish completely. In this case, the
receiver will not react at all, since it has no reason
to react.
• This possibility is dealt with by introducing timers
into the data link layer
Flow Control
• What to do with a sender that systematically wants to
transmit frames faster than the receiver can accept
them
• Two approaches are commonly used:
– Feedback-based flow control: the receiver sends back
information to the sender giving it permission to send
more data or at least telling the sender how the receiver is
doing.
– Rate-based flow control: the protocol has a built-in
mechanism that limits the rate at which senders may
transmit data, without using feedback from the receiver
Error Correction
• Hamming Distance:
– Given any two codewords, say, 10001001 and
10110001, it is possible to determine how many
corresponding bits differ.
– To determine how many bits differ, just exclusive OR
the two codewords.
– The number of bit positions in which two codewords
differ is called the Hamming distance
Error Correction
• If two codewords are a Hamming distance d apart, it
will require d single-bit errors to convert one into the
other.
• As a simple example of an error-correcting code,
consider a code with only four valid codewords:
– 0000000000, 0000011111, 1111100000, and 1111111111
– This code has a distance 5, which means that it can correct
double errors.
– If the codeword 0000000111 arrives, the receiver knows
that the original must have been 0000011111.
– If, however, a triple error changes 0000000000 into
0000000111, the error will not be corrected properly.