Title: CIS 267 Transmission Efficiency Lecture Chapter 10
1CIS 267 Transmission EfficiencyLecture Chapter
10
2Networked Environments
- Major source of expense is transmission cost
- Goal is to
- Maximize amount of information carried, or
- Minimize transmission capacity to satisfy a given
business communications requirement - Two approaches to do the above is
- Multiplexing
- Data Compression
3Chapter Overview
- Multiplexing
- FDM (can be used with analog signals)
- TDM
- Synchronous
- Asynchronous (Statistical)
- Data Compression
- Lossless
- Lossy
4Multiplexing
- Several data sources share a common transmission
medium simultaneously - Line sharing saves transmission costs
- Higher data rates mean more cost-effective
transmissions - Takes advantage of the fact that most individual
data sources require relatively low data rates
5Multiplexing Diagram
6Data Compression
- Process of eliminating redundancies in data
- Reduces the size of data files to move more
information with fewer bits - Used for transmission and for storage
- jpeg
- ZIP
- Often combined with multiplexing to increase
efficiency
7Data Compression
8Approaches to Terminal Support
- Direct point-to-point links
- I/O port and line for each terminal
- Multidrop line
- One terminal transmitting at a time at same rate
- Polled by host for communication
- Multiplexer
- Integrated MUX function in host
9Direct Point-to-Point
10Multidrop Line
11Multiplexer
12Integrated MUX in Host
13Frequency Division Multiplexing
- Simple example - use in cable TV systems
- FDM for voice transmission is declining rapidly
- FDM still used almost exclusively for television
distribution systems - Figure 10.3a on p.236 shows a general case
14Frequency Division Multiplexing
15Frequency Division Multiplexing
16Frequency Division Multiplexing
17Frequency Division Multiplexing
- Requires analog signaling transmission
- Bandwidth sum of inputs guardbands
- Modulates signals so that each occupies a
different frequency band - Standard for radio broadcasting, analog telephone
network, and television (broadcast, cable,
satellite)
18FDM Example ADSL
- ADSL uses frequency-division modulation (FDM) to
exploit the 1-MHz capacity of twisted pair. - There are three elements of the ADSL strategy
- Reserve lowest 25 kHz for voice, known as POTS
- Use echo cancellation 1 or FDM to allocate a
small upstream band and a larger downstream band - Use FDM within the upstream and downstream bands,
using discrete multitone
19Discrete Multitone (DMT)
- Uses multiple carrier signals at different
frequencies, sending some of the bits on each
channel. - Transmission band (upstream or downstream) is
divided into a number of 4-kHz subchannels. - Modem sends out test signals on each subchannel
to determine the signal to noise ratio it then
assigns more bits to better quality channels and
fewer bits to poorer quality channels.
20Time Division Multiplexing
- Two variants in common use
- Synchronous TDM
- Statistical TDM
- Figure 10.3b on p. 236 shows a general case
21Time Division Multiplexing
22Synchronous Time-Division Multiplexing (TDM)
- Used in digital transmission
- Requires data rate of the medium to exceed data
rate of signals to be transmitted - Signals take turns over medium
- Slices of data are organized into frames
- Used in the modern digital telephone system
- US, Canada, Japan DS-0, DS-1 (T-1), DS-3 (T-3),
... - Europe, elsewhere E-1, E3,
23Synchronous TDM
24SONET/SDH
- SONET (Synchronous Optical Network) is an optical
transmission interface proposed by BellCore and
standardized by ANSI. - Synchronous Digital Hierarchy (SDH), a compatible
version, has been published by ITU-T - Specifications for taking advantage of the
high-speed digital transmission capability of
optical fiber.
25SONET/SDH Signal Hierarchy
26STS-1 and STM-N Frames
27Statistical Time Division Multiplexing
- Intelligent TDM
- Data rate capacity required is well below the sum
of connected capacity - Digital only, because it requires more complex
framing of data - Widely used for remote communications with
multiple terminals
28STDM Cable Modems
- Cable TV provider dedicates two channels, one for
each direction. - Channels are shared by subscribers, so some
method for allocating capacity is
needed\--typically statistical TDM
29Cable Modem Scheme
30Data Compression
- Reduces the size of data files to move more
information with fewer bits - Used for transmission and for storage
- Combines w/ multiplexing to increase efficiency
- Works on the principle of eliminating redundancy
- Codes are substituted for compressed portions of
data - Lossless reconstituted data is identical to
original (ZIP, GIF) - Lossy reconstituted data is only perceptually
equivalent (JPEG, MPEG)
31Run Length Encoding
- Replace long string of anything with flag,
character, and count - Used in GIF to compress long stretches of
unchanged color, in fax transmissions to transmit
blocks of white space
32Run-Length Encoding Example
33Huffman Encoding
- Length of each character code based on
statistical frequency in text - Tree-based dictionary of characters
- Encoding is the string of symbols on each branch
followed. String Encoding TEA 10 00 010
SEA 011 00 010 TEN 10 00 110
34Lempel-Ziv Encoding
- Used in V.42 bis, ZIP
- buffer strings at transmitter and receiver
- replace strings with pointer to location of
previous occurrence - algorithm creates a tree-based dictionary of
character strings
35Lempel-Ziv Example
36Video Compression
- Requires high compression levels
- Three common standards used
- M-JPEG
- ITU-T H.261
- MPEG
37MPEG Processing Steps
- Preliminary scaling and color conversion
- Color subsampling
- Discrete cosine transformation (DCT)
- Quantization
- Run-length encoding
- Huffman coding
- Interframe compression