Mdi Media in the Public Switched Telephone Network (PSTN) Reading 1: Marion Cole, Telecommunications Chapter 5
Communications systems require a medium between transmitter and receiver Metallic Wire was the only choice available in the late 1800 s Provides a complete circuit for voice and signaling
Copper is still the predominant medium in the Local Loop between the subscriber and the local switching exchange More efficient technologies and cabling/media were chosen for routes handling many simultaneous voice calls. Long distance transmission progressed beyond standard copper wire to: Coaxial cable Microwave radio Satellite Fiber optic cable
Voice is traditionally carried across the local loop to the Local Exchange Carrier Switch in analog form. At the LEC switch, it is converted from analog to digital for transmission i across the wide bandwidth d idth portions of the PSTN.
The copper local loop is expensive to maintain, in comparison to long distance transmission facilities. Replacing or upgrading the connection for each individual home or office is often cost prohibitive. In contrast, long distance facilities are shared by many subscribers, so upgrades to more efficient equipment are easier to cost justify.
The copper local loop has limited bandwidth in comparison to the rest of the telephone network. The copper local loop was originally designed for low frequency analog signals, and not for high frequency h digital it communications. Increased demand for transmitting high speed data over the local loop has led to the development of improved DSL services. Fib t th h (FTTH) i b i il bl i li it d Fiber to the home (FTTH) is becoming available in a limited, but growing number of markets.
A device is required to interface the telephone to the fiber optic cable Fiber to the home will require a backup power source at the residence because the fiber optic cable does not conduct electricity. Due to the costs involved in running fiber all the way to the home, LECs have deployed d fiber to a central pedestal in a neighborhood, but normally continue to use the copper pair for the final distribution leg to the home. Multiple Subscriber Line Carrier Systems (SLC 96s) multiplex the voice traffic of up to 96 lines each.
SLC-96
Open p Wire
Metallic Wire was the only choice available in the late 1800 s Iron wire was chosen initially because it was cheaper and stronger Copper wire is a better conductor than Iron wire Copper wire can function at a smaller diameter than iron wire Smaller gauge wire takes up less space, increasing the number of wires that can fit into a cable of a chosen diameter. The first cables were made with shellac coated copper wire wrapped with a pp paper insulator. They were grouped together and coated with lead for protection from damage. Lead covered cable gave way to plastic insulated cable (PIC)
Plastic insulated cable (PIC)provided an efficient color coding scheme to uniquely identify each copper wire in the cable. The colors of the plastic insulation can provide 50 different combinations of hues. These 50 combinations can uniquely identify each individual wire in a 25 pair cable. These 25 pair cables can be further identified by the use of colored plastic strings called binders binders.
Mate M (or primary) colors: White Red Black Yellow Violet Secondary colors: Blue Orange Green Brown Slate
Wire Center Main feeder cables (1800 3600 pairs of wire) Pulled through conduit and manholes Branch feeders (possibly 900 of 3600 pairs) Distribution Cables (25 pair to 400 pair cables) Often aerial Ready Access Terminals Drop wire (or drop cable)
Continued on Friday
Multiplexing p gbetween the CO and subscribers Carrier systems: devices that place many conversations over one cable pair SLC 96 6 converts 96 analog signals from distribution ib ti pairs to a digital signal that will be multiplexed back to the CO. SLC 96 normally uses Time Division Multiplexing (TDM) to send the 96 channels back to a CO. SLC 96 carries 96 voice channels from a field location back to the CO using 10 (or fewer) cable pairs. Smaller versions of SLC are available AML 1 and AML 8 use Amplitude Modulation (AM) and Frequency Division Multiplexing
Drop Cable Digital g Loop Carrier Distribution Cable Pedestal Central Officeand CO switch Images from electronics.howstuffworks.com
Multiplexingp g Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM) T1 T (DS 1) : 24 DS 0 Channels multiplexed li l together The overall system is called a T1 carrier system The signal level is called a DS 1 signal (1.544 Mbps) T3 (DS 3) : 28 T1 systems multiplexed together (672 channels) The overall system is called a T3 carrier system The signal level is called a DS 3 signal (44.736 Mbps) Wavelength Division Multiplexing (WDM) A combination of FDM and TDM over fiber optic cable
Local Loop Trunks Customer Central Office, Class 5 Office The Network Tandem Switch Slide content from Dr. Hans Kruse 19
Local Loop, 2-Wire Trunks 4-Wire Hybrid Slide content from Dr. Hans Kruse 20
Image from http://en.wikipedia.org/wiki/borscht 21
BORSCHT: Battery Overvoltage Protection Ringing Supervision Coding Hybrid Testing Slide content from Dr. Hans Kruse 22
Nominal o a voltage otageis 48 V DC Ringing uses 100 V AC Ear Local Dialer Loop Ringer Switch Hook Hybrid Mouth Slide content from Dr. Hans Kruse 23
Loop Startt Switchhook, Local Loop, and CO Battery form a circuit. Switchhook open = on hook Central Office cannot start a call, only send ring signal Ground Start Start call by grounding either local loop wire Can be started by either side Slide content from Dr. Hans Kruse 24
Column Frequency 1 2 3 4 5 6 7 8 9 Row Frequency * 0 # Slide content from Dr. Hans Kruse 25
Transmission Medium Electromagnetic (including lightwaves) Acoustic Guided Paired metallic wire cable Coax cable Fiber optic cable Unguided Free space medium
Cables and dother media differ: Bandwidth capabilities Susceptibility to electrical interference The ability to handle analog or digital signals Cost