Received: by eff.org id AA07520 (5.65c/IDA-1.4.4 for pub-infra-exploder@eff.org); Tue, 10 Dec 1991 19:43:53 -0500 Reply-To: pub-infra Precedence: bulk To: pub-infra Date: Tue, 10 Dec 1991 19:43:29 -0500 Message-Id: <199112110043.AA07515@eff.org> From: mkapor (Mitch Kapor) Subject: Tutorial on Telephone network architecture [Tim Gorman <71336.1270@compuserve.com> 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.