Computer
Communications
Sunggu Lee
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, …
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
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
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 01101
111
When interpreted as character string Hello
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
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
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)
Communication Protocols
L1
L2
L3
L4
L5
L7
L6
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
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,
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
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 pro
g.
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
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
Communication Performance
Parameters (1)
Throughput ( 데데데 데데데 )
Actual number of bits transmitted per second
Note 1: different from latency ( 데데데데 )
Note 2: different from bandwidth ( 데데데 )
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)
Communication Performance
Parameters (2)
Bandwidth
Maximum number of bits that can be transmitted
per second
Note 1: different from latency ( 데데 데데 )
Note 2: different from throughput ( 데데데 데데데 )
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
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
Communication Performance
Parameters (4)
Other parameters also sometimes
measured
Example: Packet loss rate
Number of packets dropped by the network
References
Behrouz A. Forouzan,
TCP/IP Protocol
Suite, 2nd Ed.
, McGraw-Hill, Boston,
2003.
D. P. Agrawal and Q.-A. Zeng,
Introdu
