Central Office
- Location
- Cabling
- Digital Cross Connect Switch
- Time Division Multiplexors
- Exchanges
- NEBS and Bellcore specifications
- Electrical Power and Backup Power
Location
Early telephone systems started on the roof of the local central office and radiated outward in a spiderweb of connections between the central office, businesses and the residential subscribers. Mother Nature eventually put an end to most of the telephone poles. Today, nearly all telephone cables are underground. Underground, the cables are impervious to most wind and rain conditions, cannot be puled down by ice or snow and are therefore more reliable. Most central offices also put out quite a bit of electromagnetic radiation which interferes with local radio and television signals. Central offices are now built underground to connect to the underground cables and to contain the electromagnetic radiation thrown off by the plant. Concrete, rebar and steel framing. Underground bunkers. Massive cable conduit. These are all components in the new central offices and telephone operations centers.
So just what would you expect to find in a central office?
Cabling
LOTS and LOTS of copper cabling! Cables are everywhere. Thousands of cables will merge at the central office forming bundle sets of 26 pairs of wires. Huge jacketed cables will have multiple bundles inside them. These jacketed cables may have as many as 600 pairs inside them. These bundles are pulled through conduit into the central office, split into individual pairs and then punched down into punchblocks mounted on a distribution frame.Distribution Frame
The purpose of a distribution frame is to split up the pairs and distribute them to other pairs already wired within the central office. All the copper local loops pulled in from outside the central office through huge conduits carring hudreds of bundled copper pairs. These copper pairs are punched down on blocks on the subscriber or loop side of the distribution frame. The opposite side of the distribution frame is wired to digital cross connect switches. This is how local subscriber loops are connected to the digital cross connect switches that actually connect the phone calls. The distribution frames are fusible which means they are designed to melt almost instantly and break the electrical connection thereby provide limited protection for the equipment inside the central office against voltage surges and lightning strikes.
Digital Cross-Connect Switches
InetDaemon has heard these devices referred to variously as:
- DCCS - Digital Cross-Connect Switches
- DACS - Digital Access Cross-Connect
- DXS - Digital Cross Connect
- Cross-Connects (which can mean different things to different repairmen...)
We've heard all of the above acronyms are pronounced dax. The function of these devices is to cross-connect and switch calls between circuits whether they be DS0 local loops or multiplexed trunk lines.
Time Division Multiplexors
Built into most cross-connect switches is the time-division multiplexor which multiplexes multiple channels into a single higher speed circuit. This wasn't always the case. In the early days of the deployment of trunk lines the multiplexor was a separate stand-alone device. As technology advanced the multiplexors shrank until today they are built into most cross connect switches. These multiplexors are classified by the level of multiplexing they can perform.
An M1 multiplexor converts 24 DS0's to a T1. M12 multiplexors once existed as an intermediate step in multiplexing traffic from DS0 to DS3; however, as
Exchanges
An exchange made up of thousands of local lines grouped together into a single switched grouping connected to and switched from a central office. This physical grouping relates to how phone numbers are grouped togther, but this is beginning to change as phone numbers are now portable. Cell phones do not use exchanges because the cell phones move around. Each exchange is a set of one or more cross-connect switches in one or more central offices that answer to a single three digit code. That three digit code is the first three of seven numbers you dial for local calls. Everything in your local exchange will have the same exchange number. For example, all the phone numbers in the 555 exchange will have 555-xxxx as their phone number. Todays central offices often serve more than one exchange. This is why you can often dial numbers that don't have the same exchange number and still only be making a local call.
Local Exchange
The local exchange is the closest central office to your home. These offices handle phone calls within the exchange. From the persective of a phone number, the exchange is the first three digits of your phone number after the area code. One or more local central offices will form a local calling area.
Foreign Exchange
A foreign exchange is any exchange outside your local calling area. It is connected to a local exchange via large, high-speed trunk lines(always a T3 or better). The foreign exchange used to be handled by your local telephone provider and was called the 'extended local calling area'.
National Exchange
The national exchange was the connection from the regional telephone providers to the long-distance telephone providers. This exchange is where your area code comes from.
International Exchange
Your long distance provider will connect to other long distance providers overseas. This is where country codes come in. For the United States, the country code is 01. A phone number with the United States country code included would look like this:
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STANDARDS: NEBS and BELLCORE
Bellcore documents - NEBS
The Network Equipment Building Standards or NEBS (as defined by Telcordia) are the most widely known and used set of guidelines for the design of telecommunications and network data centers. These design criterion include guidelines for power consumption, power provisionoing, radio interferance tolerances and cooling (heat dissipation). Vendors wishing to sell equipment to telecom providers must be 'NEBS' compliant. This set of Bellcore guidelines was developed in the early 70's to assist the old Bell Telephone system designers in desigjning standardized network equipment buildings that are safe, spacious enough to accomodate the equipment and the repairmen, and comply with environmental and federal safety standards. Additional requirements for special cases such as fires, flooding, earthquakes, tornadoes and lightning strikes are also taken into account.
Guidelines for central offices were drawn up in the 1970's. These guidlines called NEBS (Network Equipment Building Systems) govern the design of a central office, assuring that safety considerations. An additional set of recommendations was developed by Bellcore, an organization funded by multiple Regional Bell Operating Carriers. All equipment designed to operate in a telephone central office are generally designed to meet both requirements.
NEBS Criteria
- Bellcore GR-63-CORE covers the physical protection of equipment.
- Bellcore GR-1089-CORE governs electromagnetic interferance and compatibility issues
Electrical Power and Backup power
The telephone system is powered indirectly by the municipal power grid provided by the regional power company. Depending upon the design, two or more connections to the power grid are used. The phone company uses transformers to convert the electricity from the high voltage alternating current (AC) supplied by the power company to a lower voltage direct curent (DC) used by the phone system equipment. The phone system and everything connected to it is powered by direct current (DC) that flows from massive lead-acid battery arrays fed by these redundant connections to the city's power grid. These batteries provide backup power and have the additional side effect of providing a limited amount of surge control and electrical line conditioning. Most central offices are also equipped with diesel generators and huge fuel tanks to keep the equipment running during an extended power outage. Your phone uses the electricity provided by the phone company over the phone line itself. This is why your phone keeps working even when the power is out and why we've all come to expect the dialtone to be there whenever we pick up the phone. Phone systems based on satellite or cable TV networks use radio frequencies and radio frequencies aren't able to power a telephone handset, so consider carefully when choosing IP Telephony solutions. The electrical power is distributed throughout the central office via massive copper bus bars, heavy bolts and thick transfer wiring. Typically, 48v of DC power is supplied to the majority of devices in the central office. The copper bus bars are charged with tens of thousands of volts of electricity and can fry a carelessly placed body part.
Many central offices appear from the outside to be too small to justify the massive HVAC systems and cooling towers attached to the buildings, but once you look underground, it's a whole different story.
- Batteries
- Charging batteries put out explosive hydrogen and/or hydrogen sulfide gas; batteries can leak corrosive sulfuric acid or explode when heated. Provisions are made to vent the gasses from charging and bags of absorbent material and barriers are used to keep the acid from spilling across the floor.
- Transformers
- A telephone central office consumes a large amount of electrical power. Stepping down power from the high external voltage level to the lower internal voltage level generates heat. This involves using transformers for this 'step down' process. Electrical transformers can overheat and occasionally explode; transfer relays produce sparks and other electrical fire hazards. Combine this with th explosive gas from charging batteries and you have a serious fire hazard to consider.
- Diesel Backup Power System
- The diesel fuel which presents it's own special threats of fire and explosions. The deisel motor produces carbon monoxide and other toxic gasses which must be properly vented as well.