Computer Communications Sunggu Lee EE Dept., POSTECH Sep. 7, 2006.

ComputerCommunications

Sunggu LeeEE Dept., POSTECHSep. 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

ApplicationApplication

PresentationPresentation

SessionSession

TransportT ransport

NetworkNetwork

Data LinkData Link

PhysicalPhysical

Protocol forUnderlyingNetwork

Protocol forUnderlyingNetwork

OSI Model TCP/IP Protocol Suite

TCP UDP

SMTP FTPTELNET

DNS SNMP NFS HTTP

RPC

InternetProtocol(IP)

InternetProtocol(IP)

IPICMP IGMP

RARPARP

Ethernet ATMTokenRing . . .

L1

L2

L3

L4

L5

L7

L6

[Forouzan 2003]

Computer Communication Example Send picture image and message to friend

Microsoft Outlook

system software

Hello!

Netscape Messenger

system software

Hello!

Hello!

Netscape Messenger

sender receiver

CommunicationChannel

001100110000111100001111000

sender

Hello!Hello!

Encoded Data

MS Outlook

Data1H4 Data2H4 Data3H4

Data1H3 OtherH3 Data2H3

Data11H2 OtherH2 Data12H2

receiver

L7throughL5

L4

L3

L2

L1 001100110000111100001111000

Hello!Hello!

Encoded Data

Netscape

Data1H4 Data2H4 Data3H4

Data1H3 OtherH3 Data2H3

Data11H2 OtherH2 Data12H2

Layer-by-Layer (OSI Model) View

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

IP address

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, 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

IP address

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

measuredExample: 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.