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					Computer
Communications
Sunggu Lee
EE Dept., POSTECH
Sep. 7, 2006
  Analog and Digital Signals



Analog: real-world
       signal

          Digital: “digitized” version of original analog signal
             - represented as sequence of binary bits
             - e.g., 12, 7, 7, 6, 10, 14, 15, 15, …
                     11000111011101101010111011111111…
Digital Signal Representation
   Bit Representation (Logic 0 and 1) in Wireless
    Communication Channel
       Typically based on electromagnetic (EM) waves
            Changes in electrical current flow cause EM waves
       Example methods
            Sine wave frequencies: high frequency = 1, low frequency = 0
            Sine wave phases: 0 degree = 1, 90 degree = 0
   Bit Representation (Logic 0 and 1) in Wired Communication
    Channel
     Optical: no light = 0, light = 1
     Current: no current = 0, positive current = 1
     Voltage
            Positive logic: 0 = low voltage, 1 = high voltage
            Noise margins, voltage ranges used to permit small variations in
             input & output voltage values
                 LSTTL: for inputs, „0‟ = 0.0-0.8V and „1‟ = 2.0-5.0V
                             for outputs, „0‟ = 0.0-0.5V and „1‟ = 2.7-5.0V
Wireless Communication
   Communication “medium” (thing through which the
    data is communicated) is shared
       Each “communication connection” is referred to as a
        “channel”
   Methods for sharing the communication medium
       Time division multiple access (TDMA)
       Frequency division multiple access (FDMA)
       Code division multiple access (CDMA)
            Many different varieties
            Example: use a frequency hopping code
                 Change communication frequencies in a predetermined pattern
TDMA [Agrawal 2006]
FDMA [Agrawal 2006]
CDMA [Agrawal 2006]
Frequency Hopping [Agrawal
2006]
Binary Codes
   Meaning of a sequence of binary bits is
    dependent on the interpretation used
     Example: 01101111
        unsigned integer = 104, character = „o‟

        part of a binary program

        part of a video or audio data stream
        A “service request” command from a client PC

        A “service response” reply from a server PC

        Other

     Example:      01001000 01100101 01101100 01101100
      01101111
          When interpreted as character string  Hello
               „H‟ = 01001000, „e‟ = 01100101, „l‟ = 01101100, …
Packetization of Data
   For transmission of a stream of data
    bits (message), the message is typically
    partitioned into “packets”
    A    packet consist of
       Packet header (destination, routing info, etc.)
       Data payload (the bits of the message)

       Check bits (redundant bits used to check for
        errors in the received packet)
Communication Protocols
   For successful transmission/receipt of a
    packet, the transmitter and receiver must
    agree on a “communication protocol”
     Set of rules on how the packet is interpreted
        How to sample the bits of the packet
               Signaling method
               Synchronization of the transmitter/receiver
          How to determine which parts of the packet are the
           packet header (destination info, etc.), data payload,
           check bits, etc.
          How to interpret the bits of the data payload
               Integer, floating-point, character string, JPEG picture, etc.
Computer Communication Models
and Communication Protocol Suites
   Most commonly used reference base
    communication model is the Open Systems
    Interconnection (OSI) model
     Standardized  by the International Organization for
      Standardization (ISO)
   Most common implementation of the OSI
    model is a set of protocols referred to as the
    TCP/IP protocol suite (or stack)
     TCP  = Transmission Control Protocol
     IP = Internet Protocol
     Communication Protocols
      OSI Model                         TCP/IP Protocol Suite
L7    Application
       Application

                                                  TEL
L6    Presentation
       Presentation               SMTP FTP
                                                  NET
                                                        DNS SNMP NFS HTTP

                                                                      RPC
L5      Session
         Session

L4     Transport
        Transport                             TCP              UDP

L3     Network
       Network        Internet
                       Internet       ICMP   IGMP
                      Protocol
                       Protocol                         IP      ARP     RARP
                      (IP)
                       (IP)


L2     Data Link
        Data Link     Protocol for
                       Protocol for                 Token
                      Underlying
                       Underlying
                                       Ethernet
                                                     Ring
                                                               ATM          ...
L1      Physical      Network
                       Network
         Physical
                                                             [Forouzan 2003]
Computer Communication
Example
    Send picture image and message to friend
                                 sender                   receiver      Hello!




             Microsoft Outlook                                       Netscape Messenger




    Hello!                                                                                Hello!




             system software                                          system software




                                          Communication
                                          Channel
     Layer-by-Layer (OSI Model) View
                 sender                            receiver
                MS Outlook                         Netscape
L7
through            Hello!                             Hello!
L5
            Encoded Data                       Encoded Data


L4   H4 Data1    H4 Data2   H4 Data3    H4 Data1    H4 Data2   H4 Data3



L3   H3 Data1    H3 Other   H3 Data2    H3 Data1   H3 Other    H3 Data2



L2   H2 Data11 H2 Other     H2 Data12   H2 Data11 H2 Other     H2 Data12



L1    001100110000111100001111000        001100110000111100001111000
Activities Required (Sender Side)
   Edit message and enter “send”  MS Outlook Express
   Convert into sequence of bits
       Tags must be inserted so that original message can be
        reconstructed at destination
            E.g., “string” 01001000 … “JPEG” 110011101010 … “end”
                      11001100100010 … 101011111100 … 01111110
   Encrypt message if necessary  for privacy
   Compress if necessary
                                                                 IP address
   Partition into packets of fixed maximum size
       Attach header information (Packet ID, destination, checksum, …)
   Intersperse with packets from messages created by other
    applications
   On first link of path,
       Partition each packet into fixed-size frames (with headers)
       Send each frame out onto the network
Activities Required on Network
   Route each packet to its destination
   During each “hop” of the path
       Send signals back and forth to coordinate the sending and
        receiving of the stream of bits corresponding to a frame
            Handshaking
       Check each frame for errors
            Request retransmission in the case of errors
       Arrange received frames into the proper order
       Wait for all frames of the packet to be received
   Once each packet reaches its destination node,IP address
       Store packet in a memory buffer at destination
       Send signal to destination CPU to inform it of the arrival of
        the new packet

                                             Port Number
Activities at Destination Node
   Receive packets
       Check each packet for errors and request retransmission in
        the case of errors
       Arrange received packets into the proper order
       Once all packets have been received, form a complete
        message
   Decompress if necessary
   Decrypt if necessary
   Check for errors
   Use tags in the bit stream to reconstruct the message
   Show message to user using email tool (e.g., MS
    Outlook Express)
Network Addresses
   IP (Internet Protocol) address
       Address used to identify a computing node on the internet
       Network layer (L3) address
       E.g., 141.223.165.189 (Look up “properties” on “TCP/IP” on
        “Network”)
   MAC (Medium Access Control) address
     Address used to identify a LAN card – cannot be changed
     Data link layer (L2) address
     E.g., abcd1234 (Enter “ipconfig /all” from MS Windows “cmd”
      window)
   Port address
     Address used to identify a network interface point for an application
      prog.
     Corresponds to a memory buffer
            Send a message - write to a memory buffer on a remote computer
            Receive a message – read from a memory buffer on the local computer
       Example: 39 (for FTP), 3000 (for a user-defined port)
Connection-Oriented and
Connectionless Networking
   Connection-oriented networking
       Uses a specific network path that is established for the
        duration of a connection
            Three phases: connection establishment, data transfer,
             connection termination
       Main advantage: reliable communication
       Main implementation method: TCP (transfer control protocol)
   Connectionless networking
       Finds a new path for each packet sent
       Main advantage: fast communication for short messages
       Main implementation method: UDP (user datagram protocol)
Communication Performance
Parameters (1)
   Throughput (데이터 처리량)
     Actual     number of bits transmitted per second
          Note 1: different from latency (지연시간)
          Note 2: different from bandwidth (대역폭)
     Most important communication performance
      parameter
     Typical measurement method
          Send a data file from a source node to a destination node
               Record the time t1 when the first byte of the data is received
               Record the time t2 when the last byte of the data is received
               Divide amount of data received by (t2 – t1)
          Note: Mbps = mega-bits-per-second (not bytes)
Communication Performance
Parameters (2)
   Bandwidth
     Maximum        number of bits that can be transmitted per
      second
          Note 1: different from latency (지연 시간)
          Note 2: different from throughput (데이터 처리량)
     Measures  performance of network only (not the
      computer hardware or software)
     Typical measurement method
          Difficult to measure since effects of small data amounts,
           software and hardware at source and destination nodes
           must be removed
          The “rated” figure stated in the specifications for the
           relevant communication protocol is most commonly used
               E.g., 11 Mbps for IEEE 802.11b
Communication Performance
Parameters (3)
   Latency
     Time  required for the first byte of a message to be
      transferred from the source to the destination node
     Should include software processing time
     Typical measurement method
          At time t1, source node sends a very small message to
           destination node
          Destination node receives message and sends it back to
           the source node
          Source node receives message and records the time t2
          One-way communication latency is (t2 – t1) / 2
          Why can‟t we measure latency directly (record time t3 at
           destination and measure latency as t3 – t1)?
Communication Performance
Parameters (4)
   Other parameters also sometimes
    measured
     Example:   Packet loss rate
       Number of packets dropped by the network
       Most relevant to wireless networks
References
 Behrouz A. Forouzan, TCP/IP Protocol
  Suite, 2nd Ed., McGraw-Hill, Boston,
  2003.
 D. P. Agrawal and Q.-A. Zeng,
    Introduction to Wireless and Mobile
    Systems, 2nd Ed., Nelson, Toronto,
    2006.

				
DOCUMENT INFO
Description: Analog and Digital Signals