Subject: Tutorial on Telephone network architecture

From: mkapor (Mitch Kapor)
Date: Tue, 10 Dec 1991 19:43:29 -0500

[Tim Gorman <[email-masked]> provides the following useful
tutorial on ISDN in the telephone network. -MK]
 
Probably the first comment that needs to be made about ISDN availability is
the capability of the network to provide the service. There are several
pieces of the network involved in providing the service and all must be
available or none of the rest are useful. Basically these consist of:
    1. local subscriber outside plant facilities
        a. copper pairs
        b. Subscriber carrier (either copper or fiber based)
    2. Central Office line card modules (and corresponding line cards)
    3. Central office line concentrating modules (primarily software)
    4. Central office interoffice trunk modules
    5. Central office switch generics

Central office
---------------
 
First, some generic background on central office switch architecture is
needed. For the most part, the following descriptions apply to all
digital central offices regardless of vendor.
 
  |      |     |     |     |             |     |         |
  |dist. |_____|line |_____|concentrating|_____|switch   |__________
  |frame |     |cards|     |module       |     |fabric   |         |
  |      |     |     |     |             |     |interface|         |
  |______|     |_____|     |_____________|     |_________|         |
     |                                                             |
     |                                                             |
     |                                        |          |         |
  Subscriber                                  |trunk     |         |
  Outside                interoffice  ________|interface |_________|
  Plant                  facilities           |module    |
                                              |__________|

                     |          |   (controls the interoperation of
                     |Central   |    the modules and some of the
                     |Processing|    complex internal operations)
                     |Unit (CPU)|
                     |__________|

 
The line card module, concentrating module, and switch fabric module are
all
smart, highly sophisticated pieces of equipment. Although some call them
"multiplexing" equipment, in essence they are all dedicated purpose
computers. As such, they all have software (or firmware or whatever is
appropriate to the vendors architecture and jargon) which dictate their
operation. In many cases, this software is arranged as various packages or
"features" that are licensed (via a right-to-use fee) from the vendor.
These
software features include software for the various module operations as
well
as for the CPU "generic" software. ISDN functionality is usually included
as
one or more of these features (e.g. basic ISDN, D-channel packet, D-channel
backup could all be different feature packages).
 
There may be multiple physical variants of each module for use in providing
different serving arrangements in different switches. These may also many
times be distinguished by different pricing levels. At least one vendor has
available a standard, analog line card/line drawer/line frame module and a
different ISDN, 2B1Q compatible line card/line drawer/line frame module.
The
2B1Q ISDN line frame module is to be upgraded in 1992 to be physically and
software compatible with all standard analog line cards as well as the ISDN
2B1Q line cards.

Network
-----------
 
Making ISDN a viable service will require providing the right physical
equipment and software in the network. Individual physical pieces that
will need to be considered are:
 
    a. line terminating equipment
    b. interoffice trunking equipment (for 64kb clear channel signaling)
    c. interoffice facilities (for 64kb clear channel signaling)
    d. subscriber loop carrier equipment

It is probable that most existing line terminating equipment is not ISDN
capable. The primary reason in most cases will probably have to do with the
need for splitting off the D-channel signaling and routing it through the
switch to the necessary processing modules. Most existing line module
equipment just doesn't provide for this. This does, of course, depend on
the switch vendors equipment design.
 
Most trunk interface module equipment now in use is not 64kb clear channel
signaling capable. For at least two major switch vendors, making the
equipment capable will not require provisioning new equipment modules but
will require adding "common equipment" to the existing modules. The common
equipment consists of additional plug-in boards providing the necessary
functionality. The major problem with this upgrade is the complexity of
adding these plug-ins. If you have a trunk interface module that handles
twenty DS-1 lines and they are all in service only two approaches are
available. You can obtain "turn-down releases" on all of the circuits,
busy them out, perform appropriate maintenance activity on the interface
module, add the new circuit packs, download any needed software to the
trunk interface peripheral, and then reverse the previous steps to put the
circuits back in service. The alternative is to provide a "conversion"
trunk interface module (properly equipped of course), transition all DS-1
lines from the next module to be converted to the conversion module,
convert
the just vacated one, and then continue the process with the next module in
line and the one that was just converted. Both methods are VERY manpower
intensive and therefore expensive to perform. It is also not a quick
process
in terms of overall interval. Manpower must be allocated to this task when
other, higher priority service-impacting activities are not pending.
 
Providing full ISDN capability in the interoffice facility network is much
like providing it in the central office trunk interface modules.
Multiplexing
equipment, fiber terminating bays, line repeaters, etc. must be changed out
or upgraded. The major complicating factor here is the number of circuits
that can be involved and the resulting service impacts.
 
Subscriber Loop Carrier
--------------------------
 
Subscriber Loop Carrier (SLC) was developed to take advantage of DS1
signaling to save outside plant investment. By using two pairs to provide
a DS1 span with a capability of 24 circuits, a savings of 22 pairs could be
realized. Most Subscriber Loop Carrier (SLC) equipment being provided today
operates much in the same fashion as standard T-1 equipment. This means
each
individual module provides some kind of grouping of 24 56kb lines.
Different
vendors provide different line capacities such as AT&T's SLC-96 (tm?)
equipment. In order to provide full ISDN capability the equipment must
first
be upgraded to provide 64kb channel service instead of 56kb. Then the
appropiate plug-in cards must be installed to interface with the actual
pair
going toward the subscribers location. The major concern in this serving
arrangement will be the impact on equipment quantities. To serve 2B+D at a
160kb rate will probably require three channels (64kb * 3 = 192kb) on the
SLC module. A large penetration of ISDN could result in having to increase
equipment investment to a level of about three times what it is now. Some
investigation also is needed to determine if ISDN and standard analog lines
can co-exist on the same SLC module. If not, even a small penetration of
ISDN could result in a large requirement for additional equipment.
 
Generic SLC architecture looks as follows:
 
                                                                        24
              24                                                
subscribers
 |         | Lines |     |       |SLC     |     DS1      |        |    __
 |Line     |       |Dist |       |Central |              |SLC     |  o/  \o
 |Interface|_______|Frame|_______|Office  |_____//_______|Remote  |____/\
 |Module   |       |     |       |Terminal|    //        |Terminal|   /  \
 |_________|       |_____|       |________|              |________|   ----
                                              2 lines
                                           (1-transmit)
                                           (1-receive )

The arrangement shown here is what is generically termed "dedicated" mode.
Each subscriber has a channel to the switch available at all times. Most
vendors offer enough intelligence in their terminals (usually via
additional
plug-ins) to also provide concentration ratio's ranging from 48 subscribers
vying for 24 channels (2:1) to as high as 5:1. This would mean that of the
120 customers connected to the remote terminal, only 24 could have calls in
progress at once.
 
One primary reason for for SLC equipment not being exactly like standard T1
systems is the need to provide for additional signaling modes required in
the
loop plant. Interoffice trunk signaling generally uses what is termed
2-State
signaling. The far end can signal you as either off-hook or on-hook
(busy/idle, seizure/idle, etc.) Similarly, you can signal the far end an
off-hook or on-hook. Physically, this signaling can take many forms: loop
closed/loop open, battery reversal/normal battery, E lead ground/E lead
open,
etc. Basic station service typically uses 3-State signaling from the
Central
Office (CO) toward the station and 2-State from the station to the CO. The
CO
needs to signal battery applied to the line, battery not applied to the
line,
and alerting (usually ringing voltage). Special stations such as ground
start
lines or coin lines can use 3-State signaling or even higher (the CO needs
to signal a coin station for coin collect, coin return, dialing pad enable,
dialing pad disable, and ringing among others). This requires the SLC
equipment to encode the signaling information in the PCM bitstream
differently than standard T1.
 
As an enhancement to this service, some switch vendors make what is termed
"integrated" SLC modules for their switches. All this really does is take
the
central office terminal and integrate it directly into the switch
architecture. The DS1 lines from the remote terminal can then be terminated
directly on the central office switch. This architecture has never "caught
on" in this area because of the difficulty in maintaining load balance
objectives (i.e. you can't move a high usage line to a low usage line
interface module to keep switch usage in balance without moving at least
23 other customers).

 
Interoffice Facilities
----------------------
 
In general, interoffice facilities are arranged as follows:

  |         |________|           |        |                 |
  |CO       | DS1    |DS1        |        |Light Fiber      |
  |Trunk    |________|to         |        |Terminating Bay  |
  |Interface|________|DSx        |________|     or          |______\ WORLD
  |Module   |________|Multiplexer|        |Digital Radio Bay|      /
  |_________|        |___________|        |_________________|
 
           LFTB  - Light Fiber Terminating Bay
           DRB   - Digital Radio Bay
           MUX   - DS1-to-DSx Multiplex/Demultiplex Bay
           DSX   - Digital Signal X-conn (cross-connect) Bay
           COTIM - Central Office Trunk Interface Module
           DCS   - Digital Crossconnect System
 
A more detailed breakdown of a CO would look as follows:
 
                                                               (fiber)
             |   |_____|    |                           |    |
             |  /|     |    |             _|   |________|LFTB|____\  WORLD
  |     |____|/  |_____|DACS|    _|   |   _|   |        |____|    /
  |COTIM|    |  /|     |____|    _|   |   _|DS3|
  |     |    |  \|________________|MUX|____|DSX|________|    |
  |     |____|DSX|________________|   |   _|   |        |LFTB|_____\ WORLD
  |_____|    |___|                |___|   _|___|        |____|     /

Instead of the fiber shown in this drawing, digital radio or cable based
T1,
T1C, and T2 may be used in the interoffice plant.
 
Each piece of equipment in this arrangement must be capable of 64kb clear
channel signaling or it cannot be made available on an end-to-end basis.
For
service protection, the trunk paths between two end points are usually
spread over as wide a range of equipment as possible. Thus, providing 64kb
clear
channel capability for certain interoffice connections may involve updating
a significant amount of equipment in the central office.