Central Office Operations
                         Western Electric 1ESS,1AESS,
                      The end office network environment
Topics covered in this article will be:
        Call tracing
        Input/output messages
        SCC and SCCS
        COSMOS and LMOS
        BLV, (REMOB) and "No test trunks"
        Recent change messages
        Equal Access
Did I get your attention? Good, everyone should read this. With the time, effort, and balls it has taken me compile this knowledge it is certainly worth your time. I hope you appreciate me taking the time to write this.

I should point out that the information in this article is correct to the best of my knowledge. I'm sure there are going to be people that disagree with me on some of it, particularly the references to tracing. However, I have been involved in telecommunications and computers for 12+ years.

I'm basing this article around the 1AESS since it is the most common switch in use today.

This is the wiring between your telephone and the central office. That is another topic in itself. If you are interested read Phucked Agent 04's article on The Outside Loop Distribution Plant (OLDP) in the LOD/H Technical Journal, Issue #1. The article explains those green boxes you see on street corners, aerial cables, manholes etc. So where that article stops, this one starts. All of the cables from other offices and from subscribers enter the central office underground. They enter into a room called the cable vault. This is a room generally in the basement located at one end or another of the building. The width of the room varies but runs the entire length of the building. Outside cables appear through holes in the wall. The cables then run up through holes in the ceiling to the frame room.

Understand that each of these cables consist of an average of 3600 pairs of wires. That's 3600 telephone lines. The amount of cables obviously depends on the size of the office. All cables (e.g. interoffice, local lines, fiber optic, coaxial) enter through the cable vault.

The frame is where the cable separates into individual pairs and attach to connectors. The frame runs the length of the building, from floor to ceiling. There are two sides to the frame, the horizontal side and the vertical side. The vertical side is where the outside wiring attaches and the protector fuses reside. The horizontal side is where the connectors to the switching system reside. Multi-conductor cables run from the connectors to actual switching equipment. So what we have is a large frame called the Main Distribution Frame (MDF) running the entire length of the building. From floor to ceiling it is 5 feet thick. The MDF consists of two sides, the VDF and the HDF. Cables from outside connect on one side and cables from the switching equipment connect to the other side and jumper wires connect the two. This way any piece of equipment can be connected to any incoming "cable pair". These jumper wires are simply 2 conductor twisted pair, running between the VDF and the HDF.

What does all this mean? Well if you had access to COSMOS you would see information regarding cable and pair and "OE" (Office Equipment). With this information you could find your line on the frame and on the switch. The VDF side is clearly marked by cable and pair at the top of the frame, however the HDF side is a little more complicated and varies in format from frame to frame and from switch to switch. Since I am writing this article around the 1AESS, I will describe the OE format used for that switch.



      A = Control Group (when more than one switch exists in that C.O.)
      B = LN  Line Link Network
      C = LS  Line Switching Frame
      D = CONC or CONCentrator
      E = Switch (individual, not the big one)
      F = Level
There is one more frame designation called LOC or LOCation. This gives the location of the connector block on the HDF side. Very simply, looking at the frame:

H ---------------------------------------------------------------------

G ---------------------------------------------------------------------

F ---------------------------------------------------------------------

E ---------------------------------------------------------------------

D ---------------------------------------------------------------------

C ---------------------------------------------------------------------

B ---------------------------------------------------------------------

A ---------------------------------------------------------------------

123456789 etc.

Please note that what you are looking at here represents the HDF side of the MDF, being up to 100 feet long, and 20 feet high. Each "-" represents a connector block containing connections for 4 x 24 (which is 96) pairs.

So far I've covered how the wires get from you to the switching equipment. Now we get to the switching system itself.

Writing an article that covers them all would be lengthy indeed. So I am only going to list the major ones and a brief description of each. _ No.3ESS
      Western Electric
      Analog switching under digital control
      Even smaller version of No.1AESS
      Rural applications for up to 4500 lines
How much does a switch cost? A fully equipped 5ESS for a 40,000 subscriber end office can cost well over 3 million dollars. Now you know why your phone bill is so much. Well...maybe you parents bill. This was the first switch of it's type put into widespread use by Bell. Primarily an analog switch under digital control, the switch is no longer being manufactured. The 1ESS has been replaced by the 5ESS and other full scale digital switches, however, it is still by far the most common switch used in today's Class 5 end offices.

The #1 and 1A use a crosspoint matrix similar to the X-bar. The primary switch used in the matrix is the ferreed (remreed in the 1A). It is a two state magnetic alloy switch. It is basically a magnetic switch that does not require voltage to stay in it's present position. A voltage is only required to change the state of the switch.

The No. 1 utilized a computer style, common control and memory. Memory used by the #1 changed with technology, but most have been upgraded to RAM. Line scanners monitor the status of customer lines, crosspoint switches, and all internal, outgoing, and incoming trunks, reporting their status to the central control. The central control then either calls upon program or call store memories to chose which crosspoints to activate for processing the call. The crosspoint matrices are controlled via central pulse distributors which in turn are controlled by the central control via data buses. All of the scanner's AMA tape controllers, pulse distro, x-point matrix, etc., listen to data buses for their address and command or report their information on the buses. The buses are merely cables connecting the different units to the central control.

The 1E was quickly replaced by the 1A due to advances in technology. So 1A's are more common, also many of the 1E's have been upgraded to a 1A. This meant changing the ferreed to the remreed relay, adding additional peripheral component controllers (to free up central controller load) and implementation of the 1A processor. The 1A processor replaced older style electronics with integrated circuits. Both switches operate similarly. The primary differences were speed and capacity. The #1ESS could process 110,000 calls per hour and serve 128,000 lines.

Most of the major common control elements are either fully or partially duplicated to ensure reliability. Systems run simultaneously and are checked against each other for errors. When a problem occurs the system will double check, reroute, or switch over to auxiliary to continue system operation. Alarms are also reported to the maintenance console and are in turn printed out on a printer near the control console.

Operation of the switch is done through the Master Control Center (MCC) panel and/or a terminal. Remote operation is also done through input/output channels. These channels have different functions and therefore receive different types of output messages and have different abilities as for what type of commands they are allowed to issue. Here is a list of the commonly used TTY channels.

Maintenance - Primary channel for testing, enable, disable etc. Recent Change - Changes in class of service, calling features etc. Administrative - Traffic information and control Supplementary - Traffic information supplied to automatic network control SCC Maint. - Switching Control Center interface Plant Serv.Cent.- Reports testing information to test facilities

At the end of this article you will find a list of the most frequently seen Maintenance channel output messages and a brief description of their meaning. You will also find a list of frequently used input messages.

There are other channels as well as back ups but the only ones to be concerned with are Recent Change and SCC maint. These are the two channels you will most likely want to get access to. The Maintenance channel doesn't leave the C.O. and is used by switch engineers as the primary way of controlling the switch. During off hours and weekends the control of the switch is transferred to the SCC.

The SCC is a centrally located bureau that has up to 16 switches reporting to it via their SCC maint. channel. The SCC has a mini computer running SCCS that watches the output of all these switches for trouble conditions that require immediate attention. The SCC personnel then have the ability to input messages to that particular switch to try and correct the problem. If necessary, someone will be dispatched to the C.O. to correct the problem. I should also mention that the SCC mini, SCCS has dialups and access to SCCS means access to all the switches connected to it. The level of access however, may be dependent upon the privileges of the account you are using.

The Recent Change channels also connect to a centrally located bureau referred to as the RCMAC. These bureaus are responsible for activating lines, changing class of service etc. RCMAC has been automated to a large degree by computer systems that log into COSMOS and look for pending orders. COSMOS is basically an order placement and record keeping system for central office equipment, but you should know that already, right? So this system, called Work Manager running MIZAR logs into COSMOS, pulls orders requiring recent change work, then in one batch several times a day, transmits the orders to the appropriate switch via it's Recent Change Channel.

Testing of the switch is done by many different methods. Bell Labs has developed a number of systems, many accomplishing the same functions. I will only attempt to cover the ones I know fairly well.

The primary testing system is the trunk test panels located at the switch itself. There are three and they all pretty much do the same thing, which is to test trunk and line paths through the switch.

         Trunk and Line Test Panel
         Supplementary Trunk Test Panel
         Manual Trunk Test Panel

     MLT (Mechanized Loop Testing) is another popular one. This system is
often available through the LMOS data base and can give very specific measurements of line levels and losses. The "TV Mask" is also popular giving the user the ability to monitor lines via a call back number.

DAMT (Direct Access Mechanized Testing) is used by line repairmen to put tone on numbers to help them find lines. This was previously done by Frame personnel, so DAMT automated that task. DAMT can also monitor lines, but unfortunately, the audio is scrambled in a manor that allows one only to tell what type of signal is present on the line, or whether it is busy or not.

All of these testing systems have one thing in common: they access the line through a "No Test Trunk". This is a switch which can drop in on a specific path or line and connect it to the testing device. It depends on the device connected to the trunk, but there is usually a noticeable "click" heard on the tested line when the No Test Trunk drops in. Also the testing devices I have mentioned here will seize the line, busying it out. This will present problems when trying to monitor calls, as you would need to drop in during the call. The No Test Trunk is also the method in which operator consoles perform verifications and interrupts.

Calls coming into and leaving the switch are routed via trunks. The switches select which trunk will route the call most effectively and then retransmits the dialed number to the distant switch. There are several different ways this is done. The two most common are Loop Signaling and CCIS, Common Channel Interoffice Signaling. The predecessor to both of these is the famous and almost extinct "SF Signaling". This utilized the presence of 2600hz to indicate trunks in use. If one winks 2600Hz down one of these trunks, the distant switch would think you hung up. Remove the 2600, and you have control of the trunk and you could then MF a number. This worked great for years. Assuming you had dialed a toll free number to begin with, there was no billing generated at all. The 1AESS does have a program called SIGI that looks for any 2600 winks after the original connection of a toll call. It then proceeds to record on AMA and output any MF digits received. For more information on AMA see Phantom Phreaker's article entitled, Understanding Automatic Message Accounting in the LOD/H TJ Issue #3. However due to many long distant carriers using signaling that can generate these messages it is often overlooked and "SIG IRR" output messages are quite common.

Loop signaling still uses MF to transmit the called number to distant switches, however, the polarity of the voltage on the trunk is reversed to indicate trunk use.

CCIS sometimes referred to CCS#6 uses a separate data link sending packets of data containing information regarding outgoing calls. The distant switch monitors the information and connects the correct trunk to the correct path. This is a faster and more efficient way of call processing and is being implemented everywhere. The protocol that AT&T uses is CCS7 and is currently being accepted as the industry standard. CCS6 and CCS7 are somewhat similar.

Interoffice trunks are multiplexed together onto one pair. The standard is 24 channels per pair. This is called T-1 in it's analog format and D-1 in its digital format. This is often referred to as carrier or CXR. The terms frame error and phase jitter are part of this technology which is often a world in itself. This type of transmission is effective for only a few miles on twisted pair. It is often common to see interoffice repeaters in manholes or special huts. Repeaters can also be found within C.O.s, amplifying trunks between offices. This equipment is usually handled by the "carrier" room, often located on another floor. Carrier also handles special circuits, private lines, and foreign exchange circuits.

After a call reaches a Toll Switch, the transmit and receive paths of the calling and called party are separated and transmitted on separate channels. This allows better transmission results and allows more calls to be placed on any given trunk. This is referred to as 4 wire switching. This also explains why during a call, one person can hear crosstalk and the other cannot. Crosstalk will bleed over from other channels onto the multiplexed T-Carrier transmission lines used between switches.

So with the Loop Signaling standard format there is no information being transmitted regarding the calling number between switches. This therefore causes the call tracing routine to be at least a two step process. This is assuming that you are trying to trace an anticipated call, not one in progress. When call trace "CLID" is placed on a number, a message is output every time someone calls that number. The message shows up on most of the ESS output channels and gives information regarding the time and the number of the incoming trunk group. If the call came from within that office, then the calling number is printed in the message. Once the trunk group is known, it can usually be determined what C.O. the calls are coming from. This is also assuming that the calls are coming from within that Bell company and not through a long distance carrier (IEC). So if Bell knows what C.O. the calls are coming from, they simply put the called number on the C.I. list of that C.O. Anytime anyone in that C.O. calls the number in question another message is generated showing all the pertinent information.

Now if this were a real time trace it would only require the assistance of the SCC and a few commands sent to the appropriate switches (i.e. NET-LINE). This would give them the path and trunk group numbers of the call in progress. Naturally the more things the call is going through, the more people that will need to be involved in the trace. There seems to be a common misconception about the ability to trace a call through some of the larger packet networks i.e. Telenet and TYMNET. Well I can assure you, they can track a call through their network in seconds (assuming multiple systems and/or network gateways are not used) and then all that is needed is the cooperation of the Bell companies. Call tracing in itself it not that difficult these days. What is difficult is getting the different organizations together to cooperate. You have to be doing something relatively serious to warrant tracing in most cases, however, not always. So if tracing is a concern, I would recommend using as many different companies at one time as you think is necessary, especially US Sprint, since they can't even bill people on time much less trace a call. But...it is not recommended to call Sprint direct, more on that in the Equal Access section.

The first thing you need to understand is that every IEC Inter Exchange Carrier (long distance company) needs to have an agreement with every LEC Local Exchange Carrier (your local phone company) that they want to have access to and from. They have to pay the LEC for the type of service they receive and the amount of trunks, and trunk use. The cost is high and the market is a zoo. The LECs have the following options: This was the first access form offered to the IECs by the LECs. Basically whenever you access an IEC by dialing a regular 7 digit number (POTS line) this is FGA. The IECs' equipment would answer the line and interpret your digits and route your call over their own network. Then they would pick up an outgoing telephone line in the city you were calling and dial your number locally. Basically a dial in, dial out situation similar to Telenet's PC pursuit service. FGB is 950-xxxx. This is a very different setup from FGA. When you dial 950, your local switch routes the call to the closest Access Tandem (AT) (Toll Switch) in your area. There the IECs have direct trunks connected between the AT and their equipment. These trunks usually use a form of multiplexing like T-1 carrier with wink start (2600Hz). On the incoming side, calls coming in from the IEC are basically connected the same way. The IEC MFs into the AT and the AT then connects the calls. There are many different ways FGB is technically setup, but this is the most common.

Tracing on 950 calls has been an area of controversy and I would like to clear it up. The answer is yes, it is possible. But like I mentioned earlier, it would take considerable manpower which equals expensive to do this. It also really depends on how the IEC interface is set up. Many IECs have trunks going directly to Class 5 end offices. So, if you are using a small IEC, and they figure out what C.O. you are calling from, it wouldn't be out of the question to put CLID on the 950 number. This is highly unlikely and I have not heard from reliable sources of it ever being done. Remember, CLID generates a message every time a call is placed to that number. Excessive call trace messages can crash a switch. However, I should mention that brute force hacking of 950s is easily detected and relatively easy to trace. If the IEC is really having a problem in a particular area they will pursue it.

FGC is reserved for and used exclusively by AT&T. The following is a compiled list of common switch messages. The list was compiled from various reference materials that I have at my disposal.
                     1AESS COMMON OUTPUT MESSAGES
AR01 Office alarm
AR02 Alarm retired or transferred
AR03 Fuse blown
AR04 Unknown alarm scan point activated AR05 Commercial power failure
AR06 Switchroom alarm via alarm grid
AR07 Power plant alarm
AR08 Alarm circuit battery loss
AR09 AMA bus fuse blown
AR10 Alarm configuration has been changed (retired,inhibited) AR11 Power converter trouble
AR13 Carrier group alarm
AR15 Hourly report on building and power alarms
                    1AESS COMMON INPUT MESSAGES

 Messages always terminate with ". ctrl d "      x=number or trunk network #
NET-LINE-xxxxxxx0000 Trace of path through switch
NET-TNN-xxxxxx         Same as above for trunk trace
T-DN-MBxxxxxxx         Makes a # busy
TR-DEACTT-26xxxxxxx    Deactivates call forwarding
VFY-DNxxxxxxx          Displays class of service, calling features etc.
VFY-LENxxxxxxxx        Same as above for OE
VFY-LIST-09 xxxxxxx Displays speed calling 8 list