Download 1. Communication Network

CIS-532
Lecture 1
Summer of 2004
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Communication Network
• Collection of computers that are both
autonomous and interconnected
• Differs from distributed computer system
– in distributed system, presence of multiple
processors is made transparent to user whereas
in network knowledge of the multiple
processors is fundamental to user
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Digital Versus Analog
• Classification applies to data, signaling, and
transmission
• Data is “information content”
• Examples of analog data
– acoustic wave amplitude as function of time for
voice
– brightness of each pixel as function of time for
video
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Digital Data
• Examples of digital data
– binary data
– sequence of alphanumeric characters (e-mail)
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Signaling
• Electromagnetic waves used to carry data
through channel
• Analog signaling varies continuously with
time and takes continuum of values
• Digital signaling has fixed waveforms that
represent the bits 0 and 1
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Analog Signaling
• Analog signaling can be used for analog
data
– by using same waveform as data
– by modulation
• Analog signaling can be used for digital
data by modulation (modem)
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Digital Signaling
• Can be used for digital data
– by direct encoding (e.g., NRZ code)
– by more complicated encoding
• Can be used for analog data
– sampling
– quantization
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Analog Transmission
• Analog transmission means amplifiers used
– Amplification of noise is cumulative
– Only used for analog signals
– Some degradation of signal must be acceptable
• e.g., local subscriber loop for telephone
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Digital Transmission
• Repeaters used instead of amplifiers
– bits are determined and signal regenerated at
each repeater
– effect of noise eliminated at each stage unless
bit inversions
– always used when signaling is digital
– can be used for analog signaling provided
analog signal encodes digital data
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Digitization
• Nyquist Sampling Theorem
– Analog signal is uniquely determined by its
samples taken at twice its maximum frequency
• this “direction” is relevant for digital encoding of
analog data
• If 2^^N quantization levels, need N bits per
sample
• Quantization error can be regarded as noise
– SNR due to quantization is approx. 6 X N dB
where N is number of bits used to represent
each sample
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Example: Digitized Voice
• Voice grade telephone channel transmits
frequencies up to 4 kHz
– To convert to digital signal, sampling at 8 kHz
is needed
– For telephone, SNR of 48 dB is needed.
• So 8 bits per sample are required
• Generates digital bit stream at 64 kbps (PCM
channel)
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Analog signaling for digital data
– The values of an analog signal required to have
maximum frequency W may be arbitrarily
specified every 1/(2W) sec-- 2W samples/sec
can be specified
• this “direction” relates to analog signaling for digital
data
• data rate depends on number of bits per sample
– In QAM, signal constellation in plane
determines number of bits encoded per sample
– Shannon’s Theorem gives upper bound on
capacity (depends on SNR)
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Evolution of Telephone Networks
• Circuit switching as opposed to dedicated
circuits
– Switching introduces economy of scale since
traffic for many source/destination pairs can be
routed over high-capacity trunks
– Switching originally by operators, then
automated mechanical, now electronic
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Telephone Network Evol. (cont)
• Common channel signaling (CCS)
– data network used by switches to exchange
control information
– Separates call control from transfer of voice
– Together with programmable switches, permits
value-added services (e.g., call waiting, call
forwarding)
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Telephone Network Evol. (cont)
• Since 1980’s, transmission changing to
SONET (Synchronous Optical Network)
– Basic STS-1 signal has rate of 51.840 Mbps
• ISDN: digital subscriber loops and service
integration.
– Basic access is 2B + D
• B channel is full-duplex 64 kbps. Suitable for
circuit-switched connection, connection to packetswitched network, or permanent digital connection
• D channel is 16 kbps packet-switched
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Broadband ISDN
• Integration of voice, video, data in high
speed network
– ATM running over SONET
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Computer (data) Networks
• Organization of data in packets
– requires control bits (headers and trailers)
• Packet switching (store and forward)
– allows link bandwidth shared on as-needed
basis
– superior to FDM and TDM for bursty traffic
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Internet Protocol Hierarchy
• IP and (TCP/IP) can run over many physical
networks (e.g., Ethernet, Token Ring, ATM)
– allows interconnection of heterogeneous
networks
– presents uniform interface to applications
• allows development of applications that are
independent of physical network
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Multiple Access Techniques
• Common channel shared by multiple
stations
• Medium access control (MAC) required to
transform shared channel into virtual
intermittent point-to-point link
• LAN standards include Ethernet, Token
Bus, Token Ring
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Multiaccess (cont)
• MAN standards
– Fiber distributed data interface (FDDI)
• similar to token ring but faster (100 Mbps)
• has timed-token mechanism that can transmit realtime traffic (voice or video) with guaranteed delay
– Distributed Queue Dual Bus (DQDB)
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Cable Television Networks
• Current CATV uses FDM
– 69 analog TV channels, each 4.5 MHz wide
– Transmission over coaxial cable arranged as
unidirectional tree
• Fiber to curb (with cable to individual
subscriber) can increase BW and decrease
attenuation
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CATV continued
• Migration to digital transmission is
occurring to increase number of channels
• Future developments include creating LANs
for subscribers to use to send reverse traffic
(e.g., requests for movies)
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Service Integration
• Example: Asynchronous transfer mode
(ATM) networks
– Runs over a physical layer such as SONET
– 53-byte cells transmitted over virtual circuits
– Different connections can be allocated different
amounts of resources (bandwidth and buffers)
– User and network negotiate contract that
specifies user traffic characteristics and
network guaranteed QoS
– Accomodates both real-time and nonreal-time
traffic
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Economic Issues
• Economies of scale
– Due to fixed network management costs as well
as fact that cost increases slower than linearly
with data rate, cost per user decreases as
number of users increases
• Network externalities
– Value of network service to user increases as
number of users increases
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Economics cont.
• Due to economies of scale and network
externalities, there is “critical number” of
users
– below critical number, subsidy is required
• Service integration
– combining services in single network (e.g.,
BISDN) reduces cost of each service due to
shared infrastructure
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Network Structure
• Point-to-point versus broadcast
– in point-to-point, each link connects pair of
nodes
– in broadcast, all nodes share common link
(channel). All users receive each packet sent but
only those for whom it is addressed retain it
– more generally, network may support
multicasting
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Local Area Networks (LANs)
• Typically use broadcast link(s)
– require multiaccess algorithms
• Maximum distance between hosts is limited
– implies upper bound for propagation delay-which is important to multiaccess algorithms
• Examples: Ethernet, Token ring, Token bus
• MANs are similar to LANs but larger area
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Wide Area Networks (WANs)
• End systems (hosts) are interconnected via
communication subnet
• Subnet consists of switching nodes (routers)
connected by transmission lines (links)
• May be packet switched or circuit switched
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Wireless Networks
• Radio, packet radio, microwave, satellite
• May or may not involve host mobility and
time-varying network topology
– Example: cellular radio systems
– Example: wireless LANs
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Hybrid Networks
• Both wireless and wireline components
– Example: satellite-fiber networks
– Example: wired LAN on aircraft with flying
router having wireless connection to terrestrial
network
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Messages
• Message is a single unit of communication
in sense that it is useful to recipient only if
completely delivered
– Example: file in file transfer system
– Example: image in image transfer system
– Example: one line of symbols in interactive
terminal session
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Messages, cont.
• Concept of messages not very useful for
voice or video
– flow model corresponding to stream of bits is
more appropriate
– stream may be CBR or VBR
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Packets
• Messages are broken up into units of
manageable size called packets
– packets are transmitted as strings of bits
together with additional control bits
– control bits may indicate addresses, offsets, etc.
– if packets have constant length, then called cells
• Before being broken into packets, messages
may be transformed for purpose of data
compression and/or encryption
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Connectionless Versus
Connection-Oriented Service
• Services provided by a layer may be
connectionless or connection-oriented
– for transport layer refers to messages
– for network layer refers to packet
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Connectionless Service
• Messages (or packets) are independent of
each other
– analogous to postal service
– order of messages (packets) need not be
preserved
– generally not reliable, but may be made reliable
through use of acknowledgements
• analogous to certified return-receipt postal service
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Connection-Oriented Service
• Messages (packets) are part of a connection
set up (and later terminated) between
communicating hosts
– Messages are delivered in order
– Service is generally reliable: no duplication or
omission of messages
– Analogous to telephone service
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Characteristics of Traffic
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Traffic arrival rate and variability
Connection duration
Distribution of message length
Allowable delay and variability of delay
Required reliability
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Examples of Traffic Types
• Interactive terminal-to-computer sessions
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–
–
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low message rate
message length short
delay requirement moderately strict
required reliability high
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Traffic Examples, cont.
• File transfer sessions
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–
–
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message rate low
message length very long
delay requirement very relaxed
required reliability very high
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Traffic Examples, cont.
• Packetized voice
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–
–
–
concept of message not applicable
bit arrival rate moderate
delay requirement stringent (especially jitter)
required reliability low
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Circuit Switching
• When session is set up, path is chosen and
bandwidth allocated on each link (by FDM
or TDM).
– If no path with sufficient BW, call is rejected
– Advantage: once call is accepted, BW is
guaranteed; no queuing
– Disadvantage: inefficient utilization of
transmission capacity if traffic is bursty
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Packet Switching
• Store and forward
• Statistical multiplexing
– No fixed allocation of BW
– Packets from different sessions combined into
single queue for each outgoing link
– Full transmission capacity of link dedicated to
single packet
• Advantage: full utilization of link capacity
whenever traffic is present
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Connectionless versus
Connection-Oriented Routing
• Virtual circuit routing
– connection-oriented
– fixed path (but not fixed BW) assigned at start
of session; all packets follow same path
– Example: ATM
• Datagram routing
– packets in session are routed independently
– Example: IP
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