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ECE 4371, Fall, 2012 Introduction to Telecommunication Engineering/Telecommunication Laboratory

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Title: ECE 4371, Fall, 2012 Introduction to Telecommunication Engineering/Telecommunication Laboratory


1
ECE 4371, Fall, 2012Introduction to
Telecommunication Engineering/Telecommunication
Laboratory
  • Zhu Han
  • Department of Electrical and Computer Engineering
  • Class 22
  • Nov. 25th, 2012

2
Multiaccess vs. Point-to-point
  • Multiaccess means shared medium.
  • many end-systems share the same physical
    communication resources (wire, frequency, ...)
  • There must be some arbitration mechanism.
  • Point-to-point
  • only 2 systems involved
  • no doubt about where data came from !

3
Internetwork
  • Connection of 2 or more distinct (possibly
    dissimilar) networks.
  • Requires some kind of network device to
    facilitate the connection.

Net A
Net B
4
Comparison
  • Speed and Range

5
ISO/OSI Reference Model
  • To address the growing tangle of incompatible
    proprietary network protocols, in 1984 the ISO
    formed a committee to devise a unified protocol
    standard.
  • The result of this effort is the ISO Open Systems
    Interconnect Reference Model (ISO/OSI RM).
  • The ISOs work is called a reference model
    because virtually no commercial system uses all
    of the features precisely as specified in the
    model.
  • The ISO/OSI model does, however, lend itself to
    understanding the concept of a unified
    communications architecture.

6
ISO/OSI Reference Model
  • The OSI RM contains seven protocol layers,
    starting with physical media interconnections at
    Layer 1, through applications at Layer 7.
  • OSI model defines only the functions of each of
    the seven layers and the interfaces between them.
  • Implementation
  • details are not part
  • of the model.

7
ISO/OSI Reference Model Physical Layer
  • The Physical layer receives a stream of bits from
    the Data Link layer above it, encodes them and
    places them on the communications medium.
  • The Physical layer conveys transmission frames,
    called Physical Protocol Data Units, or Physical
    PDUs. Each physical PDU carries an address and
    has delimiter signal patterns that surround the
    payload, or contents, of the PDU.
  • Issues
  • mechanical and electrical interfaces
  • time per bit
  • distances

8
Modulation
  • Process of varying a carrier signal in order to
    use that signal to convey information
  • Carrier signal can transmit far away, but
    information cannot
  • Modem amplitude, phase, and frequency
  • Analog AM, amplitude, FM, frequency, Vestigial
    sideband modulation, TV
  • Digital mapping digital information to different
    constellation Frequency-shift key (FSK)

9
ISO/OSI Reference Model Data Link
  • The Data Link layer negotiates frame sizes and
    the speed at which they are sent with the Data
    Link layer at the other end.
  • The timing of frame transmission is called flow
    control.
  • Data Link layers at both ends acknowledge packets
    as they are exchanged. The sender retransmits the
    packet if no acknowledgement is received within a
    given time interval. ARQ
  • Medium Access Control - needed by mutiaccess
    networks.
  • Issues
  • framing (dividing data into chunks)
  • header trailer bits
  • addressing

01100010011
10110000001
10
Automatic Repeat-reQuest (ARQ)
  • Alice and Bob on their cell phones
  • Both Alice and Bob are talking
  • What if Alice couldnt understand Bob?
  • Bob asks Alice to repeat what she said
  • What if Bob hasnt heard Alice for a while?
  • Is Alice just being quiet?
  • Or, have Bob and Alice lost reception?
  • How long should Bob just keep on talking?
  • Maybe Alice should periodically say uh huh
  • or Bob should ask Can you hear me now? ?

11
Time-Division Multiplexing
Figure Block diagram of TDM system.
12
ISO/OSI Reference Model Network
  • At the originating computers, the Network layer
    adds addressing information to the Transport
    layer PDUs.
  • The Network layer establishes the route and
    ensures that the PDU size is compatible with all
    of the equipment between the source and the
    destination.
  • Its most important job is in moving PDUs across
    intermediate nodes.
  • Issues
  • packet headers
  • virtual circuits

13
London Metro Map
14
ISO/OSI Reference Model Transport
  • the OSI Transport layer provides end-to-end
    acknowledgement and error correction through its
    handshaking with the Transport layer at the other
    end of the conversation.
  • The Transport layer is the lowest layer of the
    OSI model at which there is any awareness of the
    network or its protocols.
  • Transport layer assures the Session layer that
    there are no network-induced errors in the PDU.
  • Issues
  • headers
  • error detection CRC
  • reliable communication

15
Parity Check
  • Add one bit so that xor of all bit is zero
  • Send, correction, miss
  • Add vertically or horizontally
  • Applications ASCII, Serial port transmission

16
ISO/OSI Reference Model Session
  • The Session layer arbitrates the dialogue between
    two communicating nodes, opening and closing that
    dialogue as necessary.
  • It controls the direction and mode (half -duplex
    or full-duplex).
  • It also supplies recovery checkpoints during file
    transfers.
  • Checkpoints are issued each time a block of data
    is acknowledged as being received in good
    condition.
  • Responsibilities
  • establishes, manages, and terminates sessions
    between applications.
  • service location lookup

17
ISO/OSI Reference Model Presetation
  • The Presentation layer provides high-level data
    interpretation services for the Application layer
    above it, such as EBCDIC-to-ASCII translation.
  • Presentation layer services are also called into
    play if we use encryption or certain types of
    data compression.
  • Responsibilities
  • data encryption
  • data compression
  • data conversion

18
Substitution Method
  • Shift Cipher (Caesars Cipher)
  • I CAME I SAW I CONQUERED
  • H BZLD H TZV H BNMPTDSDC
  • Julius Caesar to communicate with his army

Language, wind talker
19
ISO/OSI Reference Model
  • The Application layer supplies meaningful
    information and services to users at one end of
    the communication and interfaces with system
    resources (programs and data files) at the other
    end of the communication.
  • All that applications need to do is to send
    messages to the Presentation layer, and the lower
    layers take care of the hard part.
  • Issues
  • application level protocols
  • appropriate selection of type of service
  • Responsibilities
  • anything not provided by any of the other layers

20
TCP/IP Architecture
  • TCP/IP is the de facto global data communications
    standard.
  • It has a lean 3-layer protocol stack that can be
    mapped to five of the seven in the OSI model.
  • TCP/IP can be used with any type of network, even
    different types of networks within a single
    session.

21
TCP/IP Architecture
  • The concept of the datagram was fundamental to
    the robustness of ARPAnet, and now, the Internet.
  • Datagrams can take any route available to them
    without human intervention.

22
Layering Headers
  • Each layer needs to add some control information
    to the data to do its job.
  • This information is typically pre-pended to the
    data before being given to the lower layer.
  • Once the lower layers deliver the data and
    control information - the peer layer uses the
    control information.

23
Protocols and networks in the TCP/IP model
  • How a call is made?

24
Summary
  • Physical Language between two machines
  • Data-Link communication between machines on the
    same network.
  • Network communication between machines on
    possibly different networks.
  • Transport communication between processes
    (running on machines on possibly different
    networks).
  • Connecting Networks
  • Repeater physical layer
  • Bridge data link layer
  • Router network layer
  • Gateway network layer and above.

25
IEEE 802 Standards
  • The 802 working groups. The important ones are
    marked with . The ones marked with ? are
    hibernating. The one marked with gave up.

26
Multiple Access
  • How can we share a wireless channel
  • Results in Wireless Media Access Control
    Protocols
  • How we can change base stations Results in
    Handoff algorithms and protocols
  • How can we seamlessly support mobile applications
    over wireless links
  • Results in mobility protocols like Mobile IP,
    Cellular IP, etc.
  • How can we design efficient transport protocols
    over wireless links
  • Results in solutions like SNOOP, I-TCP, M-TCP,
    etc.
  • How different wireless networks/systems are
    designed?
  • Bluetooth, IEEE 802.11, GSM, etc.

27
Duplexing
  • It is sharing the media between two parties.
  • If the communication between two parties is one
    way, the it is called simplex communication.
  • If the communication between two parties is two-
    way, then it is called duplex communication.
  • Simplex communication is achieved by default by
    using a single wireless channel (frequency band)
    to transmit from sender to receiver.
  • Duplex communication achieved by
  • Time Division (TDD)
  • Frequency Division (FDD)
  • Some other method like a random access method

28
Duplexing
  • Usually the two parties that want to
    communication in a duplex manner (both send and
    receive) are
  • A mobile station
  • A base station
  • Two famous methods for duplexing in cellular
    systems are
  • TDD Time Division Duplex
  • FDD Frequency Division Duplex

29
Scheduling and Spectrum Allocation
  • For TDMA system whose information to transmit,
    Scheduling Problem
  • Round Robin
  • Opportunistic scheduling channel good? transmits
  • Fairness Max-min fair and proportional fair
  • Cross-layer design delay issue
  • For FDMA system where to load the bits
  • Bit loading problem
  • For OFDMA system
  • Time and frequency slots are assigned
  • Complicated assignment problem
  • Single Cell without interference case, or
    multicell interference case
  • Channel Allocation problem
  • Cognitive radio
  • Wireless ad hoc/sensor networks
  • Admission control
  • Reject the users if there is no more resources
  • Handoff has higher priority

30
Random Access
  • Packet Radio Protocols
  • Multihop radio network that carries packets
  • Not circuit oriented like GSM, CDMA, etc.
  • Example Protocols
  • Pure Aloha
  • Slotted Aloha
  • CSMA Protocols
  • 1-persistent CSMA
  • non-persistent CSMA
  • p-persistent CSMA
  • CSMA/CD
  • Reservation Protocols
  • Reservation Aloha
  • PRMA
  • Others
  • MACA, MACAW
  • IEEE 802.11 MAC

31
Pure Aloha
Algorithm A mobile station transmits immediately
whenever is has data. It then waits for ACK or
NACK. If ACK is not received, it waits a random
amount of time and retransmits.
Ignoring the propagation delay between
mobiles and base station
B
The time difference between the time a mobile
send the first bit of packet and the time the
base station receives the last bit of the packet
is given by 2T. T C/P T packet
time. C channel data rate (bps)
P packet length (bits)
Ack/Nack
Data
M3
M1
M2
During this 2T period of time, the packet may
collide with someone else packet.
32
Contention for Aloha
33
Throughput of Aloha
Normalized Throughput
0.185
0.5
Normalized Channel Occupancy
34
Slotted Aloha
35
Reservation Protocols
  • Reservation Aloha
  • Packet Reservation Multiple Access

36
CSMA Carrier Sense Multiple Access
  • Aloha does not listen to the carrier before
    transmission.
  • CSMA listen to the carrier before transmission
    and transmits if channel is idle.
  • Detection delay and propagation delay are two
    important parameters for CSMA
  • Detection delay time required to sense the
    carrier and decide if it is idle or busy
  • Propagation delay distance/speed_of_ligth. The
    time required for bit to travel from transmitter
    to the receiver.

37
CSMA Variations
  • 1-persistent CSMA
  • A station waits until a channel is idle. When it
    detects that the channel is idle, it immediately
    starts transmission
  • Non-persistent CSMA
  • When a station receives a negative
    acknowledgement, it waits a random amount of time
    before retransmission of the packet altough the
    carrier is idle.
  • P-persistent CSMA
  • P-persistent CSMA is applied to slotted channels.
    When a station detects that a channel is idle, it
    starts transmission with probability p in the
    first available timeslot.
  • CSMA/CD
  • Same with CSMA, however a station also listen to
    the carrier while transmitting to see if the
    transmission collides with someone else
    transmission.
  • Can be used in listen-while-talk capable channels
    (full duplex)
  • In single radio channels, the transmission need
    to be interrupted in order to sense the channel.

38
MACA Medium Access with Collision Avoidance
  • CSMA protocols sense the carrier, but sensing the
    carrier does not always releases true information
    about the status of the wireless channel
  • There are two problems that are unique to
    wireless channels (different than wireline
    channels), that makes CSMA useless in some cases.
    These problems are
  • Hidden terminal problem
  • Exposed terminal problem.

39
Hidden Terminal Problem
Cs cell
As cell
B
C
A
Hidden terminal
  • A is transmitting to B.
  • C is sensing the carrier and detects that it is
    idle (It can not hear As transmission).
  • C also transmits and collision occurs at B.
  • A is hidden from C.

40
Exposed Terminal Problem
Bs cell
Cs cell
B
C
A
D
Exposed terminal
  • B is transmitting to A. C is hearing this
    transmission.
  • C now wants to transmit to D. It senses the
    existence of carrier signal and
  • defers transmission to D.
  • However, C can actually start transmitting to D
    while B is transmitting to A,
  • Since A is out of range of C and Cs signals can
    not be heard at A.
  • C is exposed to Bs transmission.

41
MACA Solution Concept
Ali, lets talk! I am available.
Can
Can, I want to talk to you!
Can, I want to talk to you!
Biltepe Mountain
Ali
Veli
42
MACA Protocol
  • When a station wants to transmit data
  • It sends an RTS (Ready-to-Send) packet to the
    intended receiver
  • The RTS packet contains the length of the data
    that needs to be transmitted
  • Any station other than the intended recipient
    hearing RTS defers transmission for a time
    duration equal to the end of the corresponding
    CTS reception
  • The receiver sends back CTS (Clear-to-Send)
    packet back to sender if it is available to
    receive.
  • The CTS packet contains the length of the data
    that original sender wants to transmit
  • Any station other than the original RTS sender,
    hearing CTS defers transmission until the data is
    sent.
  • The original sender upon reception of the CTS,
    starts transmitting.

43
Solution for Hidden Terminal Problem
A is transmitting to B.
Cs cell
As cell
CTS(n)
RTS(n)
RTS(n)
B
C
A
X
CTS(n)
C defers transmission for duration of n bytes of
data transmission. Node A is no longer hidden
from C effectively.
X defers transmission until expected CTS
reception time by RTS sender.
Data(n)
Waiting time of node X is much smaller than
waiting time of node C.
44
Solution for Exposed Terminal Problem
B is transmitting to A
Bs cell
Cs cell
RTS(n)
RTS(n)
B
C
A
D
RTS(m)
CTS(n)
CTS(m)
Data(n)
Data(m)
  • C defers transmission upon hearing Bs RTS until
    B could get CTS from A.
  • After that C can start transmission to D. For
    that it first sends an RTS.
  • C is not longer exposed to the data transmission
    of B.

45
CSMA/CA Collision Avoidance
RTS/CTS is used to reserve channel forthe
duration of the packet transmission. This
prevents hidden and exposed terminalproblems ACK
is required to understand if the packet is
correctly received (without any collisions ) at
the receiver. Ethernet does not require ACK to
be sent, since the transmitter can detect the
collision on the channel (cable) without
requiring an explicit feedback from the receiver.
A wireless transmitter can not detect collision,
because1) Transmit power is much larger than
the received power received signal is regarded
as noise (not collision). 2) There could be a
hidden terminal
Access Point
Mobile
RTS
CTS
DATA
ACK
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