# Computer Communications

Document Sample

```					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
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
   How to determine which parts of the packet are the
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)

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

Microsoft Outlook                                       Netscape Messenger

Hello!                                                                                Hello!

system software                                          system software

Communication
Channel
Layer-by-Layer (OSI Model) View
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
   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
   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)
   Address used to identify a computing node on the internet
   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
 E.g., abcd1234 (Enter “ipconfig /all” from MS Windows “cmd”
window)
 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 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.

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Description: Analog and Digital Signals