Transmission Links

Transmission Principles

Communication Requires a Sender, Receiver, Message and Medium
Transmission Link -- The Medium
Physical path between terminal devices that carries a coded electronic signal
May be referred to as a circuit (two-way path) or a channel (one-way path)

Transmission Media

Guided Media
In an enclosed path such as wire or cable
Terminals are connected directly to medium
Unguided Media
No enclosed path
Consist of air, water, or empty space
Signals are broadcast (radiated)
Transmission and reception through antennae

Guided Media

Twisted Pair Wire
Most prevalent medium in use today
Most inexpensive; low data rate
Coaxial Cable
Two conductors -- one is a shield
Greater bandwidth
Optical Fiber
Very small fiberglass core strand
extremely high speed and bandwidth

Twisted Pair Wire

Two insulated wires twisted into a spiral pattern
Phone lines are unshielded, data lines are shielded
Single 2X - bandwidth of 250 Khz - will transmit about 60 conversations 2 or 3 miles
Digital mode - data rate of about 20 mbps

Coaxial Cable

Capacity of about 400 mhz analog
Data rate of around 500 mbps digital
Has special shielding features
Limitations include expense and size
Amplifiers or repeaters are required fairly often

Fiber Optic Cable

Consists of a thin glass fiber strand surrounded by cladding
Now being used to connect PCs and other forms of consumer electronics
May (or may not) someday reach every home and business

Unguided Media

Terrestrial Microwave
Uses directional radio broadcast transmission in the 2-40 Ghz range
Satellites
Microwave relay system in the sky -- most are located in geosynchronous orbit
Broadcast Radio Systems
Omnidirectional over-the-air transmission that uses lower frequencies than microwave

Terrestrial Microwave

Requires line of sight transmission -- about 30 miles
Much more cost-effective than laying land lines
Interference from rain and other forms of electro-magnetic radiation
Makes use of high terrain and buildings if possible

Satellites

Satellites -- Geosynchronous or low orbit (LEOs)
C-band -- 4 and 6 gigahertz
Ku-band -- 11 and 14 gigahertz
K-band -- 12 to 18 gigahertz (DBS)
Ka-band --  20 to 30 gigahertz
S-band -- 2 gigahertz (NASA)
X-band -- 7 to 9 gigahertz (military)
Dish sizes -- (large to small) -- C, Ku, VSAT, DBS, Ka 
Footprint -- area covered by the satellite's signal -- can be as large as one third of the earth's service

Broadcast Radio Systems

30 MHz to 1 GHz covers the FM, VHF, and UHF bands
Subject to interference from physical objects
Entertainment radio and TV, cell phones, pagers, mobile communications in a variety of contexts

Price/Performance Trends

Price/Performance Characteristics of Fiber Optics and Satellites Dominate
Transmission Links Are The Largest Part of the Corporate Telecomm Budget
Significant Economies of Scale
Fixed Costs in Transmission Links
Leads to declining cost per unit of information moved ($/bps) as the volume of information increases

Multiplexing

Definition:  Multiplexing
combines low capacity voice and data streams into one larger stream to take advantage of efficient high capacity links
Frequency Division Multiplexing (FDM)
Time Division Multiplexing (TDM)
Synchronous
Statistical or asynchronous
Uses Pulse Code Modulation

Frequency Division Multiplexing

Is An Analog Technology
Divides The Bandwidth Of A Medium Into Slots
Each Separated By A Guardband
Basically Stacks Separate Data Streams
Used Extensively In
Public telephone network
Cable television service

Synchronous Time Division Multiplexing

Is A Digital Technology
Combines multiple data streams when the data rate of the medium is greater than or equal to the sum of the individual streams
Interleaves bits from individual streams into prearranged time slots
Used for high-volume, continuous-load transmissions
Long distance public and private networks that carry both voice and data traffic

Statistical Time Division Multiplexing

Eliminates The Prearranged Time Slots
Input from terminals is buffered while multiplexer assigns data to time slots until the frame is full -- no empty time slots are permitted
Permits more attached devices that there are time slots
Designed for efficient use of transmission links for intermittent and variable transmissions from multiple sources.

Signal Compression

Makes More Efficient Use Of Transmission Link Capacity
Relies on the principle of info redundancy
Redundant elements are temporarily removed and are then regenerated after transmission
Used widely in many business teleconferencing applications and for delivery of distance education

Multidrop Lines & Polling

Multidrop Line
Single transmission circuit to which several terminal devices are directly attached
Is more efficient when transmission is intermittent or “bursty”
Polling
Avoids contention for a single circuit by two terminals attempting to transmit at once
Host computer polls each terminal to determine if it has something to transmit

Transmission Link Systems

Analog Transmission Link Systems
Standard multiples of basic voice circuit units (4 Khz) obtained by FDM
Group: 12 voice channels
Super group: 5 groups (60 channels)
Master group: 10 super groups (600 channels)
4 Khz channel is roughly equivalent to a 64 kbps digital data stream
Digital Transmission Link Systems
Standard multiples of basic voice circuit units (64 kbps) obtained by TDM
DS-0: 1 64 kbps circuit
DS-1: 1.544 mbps (T1 tariff)
DS-2: 6.312 mbps (T2 tariff)
DS-3: 44.736 mbps (T3 tariff)
DS-4: 274.176 mbps (T4 tariff)
T1 Bulk Capacity Pricing Structure
Fractional T1 Service: In Any Increment

Nodes and Switching

Definition Of A Node:
An interface between terminal and network
Or, a hub where several link systems are joined together
Economizes On Direct Links
Concentrates the Intelligence in One Location
May Reduce Speed and Responsiveness
Definition of Switching Device
A microprocessor or computer located at a node -- used to route voice & data traffic
Evolved from the public telephone network, generally arranged in a hierarchy
Medium-to-large Organizations Use Private Branch Exchanges (PBXs) or Private Automatic Branch Exchanges (PABXs)
Server -- Computer that acts as the central routing and control node of the network.  Stores files, software and e-mail systems
Bridge -- Connects Similar LANS Using Identical Protocols
Router -- Connects Dissimilar Networks That Use OSI-compatible Protocols
Gateway -- Connects Totally Dissimilar Subnetworks