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Telcos and their Connectivity

What started off with the invention of the telegraph is now a wide spread network of copper and fiber across the world. We are so used with the fact that we pick-up the phone, dial a number and get a connection with the person we want to speak to that most of us stopped thinking about what is making this happen and that is what this article is about.

Premises to Telco cloud

The customer premises can be a house, a factory or a mobile telephone. But what is important is that it contains the terminal. The terminal can be an ordinary telephone, a fax or a modem. Before the terminal (seen from the telco network) there might be a PBX (Private Branche Exchange).

The local loop is the line that connects the customer premises to the local telco office (also called central office or CO). Another name that you hear often is the last mile, although it is normally about 2.5 miles or 4 km. These lines were mostly analog, but are rapidly becoming digital.

The local telco office is the interface between the customers connection and the entire world of PTTs. Here channel banks convert analog signals to digital and multiplex those signals to a single line that goes into a digital switch. This digital switch is the interface to other PTTs. The output line is mostly called the trunk, which enters the PTT cloud.

Every part of this network has its own devices and techniques, although sometimes techniques from the trunk are used on the local loop and devices from the local telco office can be found at the customer premisis.

To give a rough indication:

Customer Premisis   Fax, Telephone, Modem, PBX, PABX
Local loop Analog lines, ISDN, xDSL  
Local Telco Office   Channel Bank, Switches
Trunk E1, T1, SONET, SDH  

This seemes easy, but it isn't. The different telcos thought differently and so we ended up with different standards, like T1 for the USA and E1 for Europe. This has mainly to do with the way signals are multiplexed and the overhead that is placed between the signals to seperate them, also the knowledge growth and smarter technics added some new features. To give you a brief overview:

Non-Synchronous Hierarchies
North America Europe
Carrier Signal Speed Channels Signal Speed Channels
  DS-0 64 kbps 1x DS-0 E-0 64 kbps 1x E-0
T-1 DS-1 1.54 Mbps 24x DS-0 E-1 2.048 Mbps 32x E-0
T-1c DS-1c 3.152 Mbps 48x DS-0      
T-2 DS-2 6.3 Mbps 96x DS-0 E-2 8.45 Mbps 128x E-0
T3 DS-3 44.8 Mbps 28x DS-1 E-3 34 Mbps 16x E-1
T-4 DS-4 274.760 Mbps 168x DS-1 E-4 144 Mbps 64x E-1

Synchronous Hierarchies
STS-1, OC-1 51.84 Mbps  
STS-3, OC-3 155.52 Mbps STM-1
STS-9, OC-9 466.56 Mbps STM-3
STS-12, OC-12 622.08 Mbps STM-4
STS-18, OC-18 933.12 Mbps STM-6
STS-24, OC-24 1244.16 Mbps STM-8
STS-36, OC-36 1866.24 Mbps STM-12
STS-48, OC-48 2488.32 Mbps STM-16
STS-192, OC-192 9953.28 Mbps STM-64

Welcome to the world of high bandwidth
The Telecommunications Corner
Glossary Terms: SDH, VC, STM-1, STM-4, STM-16, STM-64
Series G Recommendations : G.700 to G.799