Switching and the Local
Loop
• Lecture 2
• Paul Flynn
1
Loop Start
Station
PBX or Central Office
Loop
(Local or Station)
Switch
DC Current
Ringing
+
–
Switch
+
–
Switch
+
–
AC
2
Balanced and Unbalanced
Electrical Circuit
A Complete Electrical Circuit
Source
Source
Replac e thes e two c onduc tors
with one - "Common Signal Retu
rn"
Source
3
Terminology
• AWG - American Wire Gauge
• Bridge Tap - an unused, opencircuited, parallel-connected cable
pair on a loop
Bridge Tap
4
Cable Color Codes
• Pair (2 wires)
• Binder (25 pair)
• Super Unit Binder (24 Binders)
–1-600 White
–601-1200 Red
–1201-1800 Black
–1801-2400 Yellow
–2401-3000 Violet
–3001-3600 Blue
–3601-4200 Orange
Pair No.
Color
Pair No.
Color
1
White-Blue
14
Black-Brown
2
White-Orange
15
Black-Slate
3
White-Green
16
Yellow-Blue
4
White-Brown
17
Yellow-Orange
5
White-Slate
18
Yellow-Green
6
Red-Blue
19
Yellow-Brown
7
Red-Orange
20
Yellow-Slate
8
Red-Green
21
Violet-Blue
9
Red-Brown
22
Violet-Orange
10
Red-Slate
23
Violet-Green
11
Black-Blue
24
Violet-Brown
12
Black-Orange
25
Violet-Slate
13
Black-Green
5
Cable Construction
• Sheath
– Armored (gopher protection) - lead/steel
– Dielectric
• Fill
– Air filled (pressurized?)
– Gel-filled (icky-PIC)
• Insulation
– Paper pulp
– Plastic
6
Loading Coils
• Used to extend useful length of loops
for voice service
• Counteracts the effects of cable
capacitance
• BAD NEWS for high-speed digital
services - causes excessive
attenuation at frequencies above
voice-band
7
Wire Centers and Cable Plant
Political Boundary
Wire Center
18kft
Carrier Serving Area
8
Carrier Systems
Digital
Repeater
• How do we serve customers
outside the loop limits?
–Range extenders
–Carrier systems (Subscriber Loop Carrier)
•Analog (frequency division multiplexing)
•Digital (time division multiplexing)
9
Analogue Amplifier
10
Regenerative Repeater Section
Decision Times
(1)
(3)
Decision
Level
Transmission
Media
Amplifierequalizer
Regenerator
(2)
Timing
Circuit
(4)
Transmission
Media
(5)
Regenerative Repeater
11
Carrier System Block Diagram
1
2
3
4
.
.
21
22
23
24
COR
LR
1
2
3
4
LR
Common
Equipment
Common
Equipment
LR
LR
COR
.
.
21
22
23
24
12
Digital Hierarchies
13
T1 versus E1
•Two different Pulse Code Modulation (PCM)
standards
•T1, or DS1 PCM hierarchy
•North American standard
•E1, or “the European System”
•E1 is used as close as Mexico
•Japan uses a hybrid system
14
Transmission Media
15
Electromagnetic Spectrum
16
Guided Transmission Media
• Twisted Pair
• Coaxial cable
• Optical fiber
17
Twisted Pair
18
Twisted Pair - Applications
• Most common medium
• Telephone network
–Between house and local exchange
(subscriber loop)
• Within buildings
–To private branch exchange (PBX)
• For local area networks (LAN)
–10Mbps or 100Mbps
19
Twisted Pair - Pros and Cons
• Cheap
• Easy to work with
• Low data rate
• Short range
20
Twisted Pair - Transmission
Characteristics
• Analog - Amplifiers every 5km to 6km
• Digital
–Use either analog or digital signals
–repeater every 2km or 3km
• Limited distance
• Limited bandwidth (1MHz)
• Limited data rate (100MHz)
• Susceptible to interference and noise
21
Unshielded
and
Shielded
TP
• Unshielded Twisted Pair (UTP)
–Ordinary telephone wire
–Cheapest
–Easiest to install
–Suffers from external EM interference
• Shielded Twisted Pair (STP)
–Metal braid or sheathing that reduces
interference
–More expensive
–Harder to handle (thick, heavy)
22
UTP Categories
• Cat 3
–up to 16MHz
–Voice grade found in most offices
–Twist length of 7.5 cm to 10 cm
• Cat 4
–up to 20 MHz
• Cat 5
–up to 100MHz
–Commonly pre-installed in new office buildings
–Twist length 0.6 cm to 0.85 cm
23
Near End Crosstalk
• Coupling of signal from one pair to
another
• Coupling takes place when transmit
signal entering the link couples back
to receiving pair
• i.e. near transmitted signal is picked
up by near receiving pair
24
Coaxial Cable
25
Coaxial Cable Applications
• Most versatile medium
• Television distribution
–Ariel to TV
–Cable TV
• Long distance telephone transmission
–Can carry 10,000 voice calls simultaneously
–Being replaced by fiber optic
• Short distance computer systems links
• Local area networks
26
Coaxial Cable - Transmission
Characteristics
• Analog
–Amplifiers every few km
–Closer if higher frequency
–Up to 500MHz
• Digital
–Repeater every 1km
–Closer for higher data rates
27
Optical Fiber
28
Optical Fiber - Benefits
• Greater capacity
–Data rates of hundreds of Gbps
• Smaller size & weight
• Lower attenuation
• Electromagnetic isolation
• Greater repeater spacing
–10s of km at least
29
Optical Fiber - Applications
• Long-haul trunks
• Metropolitan trunks
• Rural exchange trunks
• Subscriber loops
• LANs
30
Optical Fiber - Transmission
Characteristics
• Act as wave guide for 1014 to 1015 Hz
–Portions of infrared and visible spectrum
• Light Emitting Diode (LED)
–Cheaper
–Wider operating temp range
–Last longer
• Injection Laser Diode (ILD)
–More efficient
–Greater data rate
• Wavelength Division Multiplexing
31
Optical Fiber Transmission
Modes
32
Wireless Transmission
• Unguided media
• Transmission and reception via antenna
• Directional
–Focused beam
–Careful alignment required
•
Omnidirectional
–Signal spreads in all directions
–Can be received by many antennae
33
Frequencies
• 2GHz to 40GHz
–Microwave
–Highly directional
–Point to point
–Satellite
• 30MHz to 1GHz
–Omnidirectional
–Broadcast radio
• 3 x 1011 to 2 x 1014
–Infrared
–Local
34
Terrestrial Microwave
• Parabolic dish
• Focused beam
• Line of sight
• Long haul telecommunications
• Higher frequencies give higher data
rates
35
Satellite Microwave
• Satellite is relay station
• Satellite receives on one frequency, amplifies or repeats signal
and transmits on another frequency
• Requires geo-stationary orbit
–Height of 35,784km
• Television
• Long distance telephone
• Private business networks
36
Broadcast Radio
• Omnidirectional
• FM radio
• UHF and VHF television
• Line of sight
• Suffers from multipath interference
–Reflections
37
Telecom Systems
Circuit Switching
–Review Services vs. Operations
–Origins / Background of Circuit Switching
–Switch Stages
–Switch Dimensions
–Matrix Control
–Complexity
38
Circuit Switching
System Components
Access Line
Circuit Switch
Circuit Switch
A
TERMINALS
TRUNK
GROUPS
CrossConnections
B
Circuit Switch
Circuit Switch
39
Circuit Switching
Background: A.G. Bell / Manual Switching
40
Circuit Switching
Background: Almon Strowger / Automatic Switching
41
Manual Switch
Basic Components
Input/Output
Channels
Channel
Terminations
Manually Operated
Cross-Connections
42
Input Lines
Non-Blocking Switch Matrix
1
2
3
4
5
6
Switch i,j and
j,i operate
together
7
8
9
10
1 2 3 4 5 6 7 8 9 10
Output Lines
43
Progressive Control
Calling
Line
Called
Line
Line
Circuit
Line
Finder
Control
Switch
Control
Register
Switch
Control
Translation
Register Progressive Control System
44
Stored Program Control
Other
Offices
Customer
Lines
Switching
Network
Address
Address
Distributor
Scanner
Central
Control
Address
Program
Store
Call
Store
Call Processing Registers
Temp Memory
Maintenance
Center
Stored Programs
Translations
Semi-Permanent Memory
45
Packet Switching
Store and Forward
46
Virtual Circuit vs. Datagram
Service vs. Operation
47
Switching
Services:
- Connectionless (Datagram)
- Connection Oriented (Virtual Circuit)
Operations:
- Circuit Switching
- Store and Forward
- Packet
- Message
48