T1/E1 Line Provisioning for Remote Access
Marcio Saito - Revision 1.0 August 1999

About this document
    This document provides information intended to help in the process of provisioning the T1 or E1 dialup trunk lines for use with the Cyclades-PR4000 Remote Access Server.
    Line provisioning parameters varies widely depending on the switch being used at the central office and the service options offered by the carrier. This document is meant to serve as a reference for the most common configurable parameters.

Some History and Background
   A Remote Access Server is a device that attaches to the Local Area Network at the central site (usually the Ethernet LAN) and provides connection to dialup lines. It allows remote clients to access the LAN (and possibly the Internet) by using analog modems or ISDN terminal adapters.
   RS-232 Terminal Servers were traditionally used to provide remote access. A server-based or stand-alone box would attach to the LAN and provide multiple RS-232 serial interfaces. Those serial ports were connected to a modem bank, which on its turn, connected to the analog phone lines coming into the building.
   With crescent use of dialup remote access triggered by the need for Internet connections, the number of phone lines required to attend the demand started to grow to a point where it was impractical to add more phone lines at the central site.
   Before we continue, let’s look at how the Public Switched Telephone Network (PSTN) is organized.
   Regular phone lines are copper wire pairs carrying information in analog form from the customer’s premise to the phone carrier’s Central Office (CO). At the Central office, the analog lines are connected to a phone switch, which converts analog information into digital form and consolidates multiple connections in higher-density digital trunks (such as T1 and T3).
   In the US and Japan, a digital trunk uses T1 speed of 1.5Mbps (equivalent to 24 analog channels). In Europe and most of the other countries, a digital trunk runs at E1 speeds of 2Mbps (equivalent to 32 analog channels). A digital trunk is Time Division Multiplexed (TDM) line that carries digitized information from several channels in digital form.
   At the Central Office closest to the destination of the call, it is converted back to analog form and carried to the customer premises by a copper wire pair.
   Notice that phone connections are analog only in "the last mile", while it travels between the Central Office and the customer premises. Nowadays, most phone switches are digital and the information travels between Central Offices in digital form as time slots within T1/E1 or T3/E3 TDM trunks.
   That said, we can look at the solution for the problem of explosive demand for phone lines at Internet Service Provider sites or points-of-presence.
   The solution was not to convert the information from digital form back to analog at the CO. The phone carrier brings the digital T1 or E1 trunk directly to the customer premises.
   That way, only 2 pairs of copper wire can carry up the equivalent to 24 or 32 analog channels (because one or 2 channels are reserved for control purposes, the actual number of phone lines is usually 23 for T1 and 30 for E1).
   Since the ISP is not getting individual analog phone lines, it cannot use the traditional analog modem banks to provide connectivity. It needs equipment that can interface directly to the T1/E1 trunk, separate the multiple channels in the TDM bus and process the modem connection in digital form.
   This box is the Integrated Digital Remote Access Server and the Cyclades-PR4000 is one example. Because it is a digital box, it can terminate both analog (modem, V.34, V.90) and digital (terminal adapter, ISDN-BRI) calls.
   Another advantage of processing calls digitally is that it allows (the explanation is out of the scope of this document) support of new modem technologies (k56 flex, V.90) that can provide throughput higher than the V.34 limit of 33.6kbps.
   The Remote Access Server communicates to the phone switch at the Central Office through the T1 or E1 line. For that to be possible both sides have to agree on a common "language". This is set by configuration at both sides and, when you order the T1 or E1 line, the service provider will ask you about some of the parameters.
   This document is intended to help you in this process.

Signaling: ISDN-PRI or CAS?
    Besides carrying the voice/data connection, phone lines have to carry signaling information.
   Signaling is the information that allows a connection to be established and maintained. In a regular phone set this information translates into things such as dialing information, ring, busy signal, dial tones, caller ID information, dialing tones, etc. In a regular analog phone line, the signaling information shares the same logic channel used to carry voice.
   In a T1 or E1 trunk, the signaling information for the trunk can be carried by a separated logic channel or can share the same channels used to carry data.
   Newer systems use ISDN-PRI signaling protocol. In this case, channels dedicated for control (called D-channels) carry the signaling information, separated from the voice/data channels (called B-channels). Because of signaling, a T1 line with ISDN PRI carries 23 phone connections and an E1 line with ISDN PRI carries 30 phone connections.
   Older systems use CAS (Channel Associated Signaling) signaling protocols. In T1 lines, these protocols "steal" some of the bandwidth from the data channels using a scheme called "bit robbed" (BR) and allow a T1 line to carry 24 phone connections. In E1, CAS signaling uses two dedicated channels, so that the E1 trunk carries 30 phone connections. Lines with CAS signaling are some times referred as "Channelized T1/E1" or "DS-1".
   Summarizing, there are two basic signaling protocols: ISDN-PRI or CAS. There are two different speeds for digital trunks (T1 and E1). So, there are 4 possible combinations:

    The choice between T1 and E1 is not an option and depends on which country you are in. In the US and Japan, you will get T1 lines. In almost every other country, you get an E1 lines.
   ISDN-PRI is a modern set of protocols and provides a lot more control over the connections than older CAS schemes.
Given the choice between ISDN-PRI and CAS, select ISDN-PRI. Depending on where you are located and on your telephone carrier, you might not be offered that choice.

Provisioning the T1/E1 dialup lines
    The configuration options of an ISDN-PRI or T1/E1 CAS line can vary greatly depending on the switch being used at the central office and on your carrier service options. This chapter comments some of the choices you may be asked to make.
   First of all, tell the person configuring the line that you are an ISP and will be using the line to provide dialup Internet access to your users. Use of T1/E1 lines for remote access has become very common and the person configuring the line is probably familiar with the typical configuration.
    Number of phone lines
   As said before, T1 lines can support up to 23 (ISDN-PRI signaling) or 24 (CAS-BR signaling) voice or digital channels, while E1 lines can support up to 30 voice or digital channels. In most cases, you will be getting the maximum number of phone lines, but it is also possible to get fewer lines.
   ISDN Switch Type (ISDN-PRI only)
   This refers to variations on implementation of the signaling protocols by different switch vendors. The Cyclades-PR4000 supports almost any ISDN switch type in use today (for a complete list, check product documentation or contact technical support).
   If you are in the US and is given the choice of ISDN switch type, select "National ISDN 2", which is intended to be the US standard switch type. Other common and acceptable options are "AT&T 5ESS" and "Northern Telecom DMS-100".
   In Europe, "Euro ISDN (ETSI)" is the standard ISDN switch type, but there are still some variations in use, depending in which country you are. Examples are TR6 (Germany) and VN6 (France).
   Australia, Japan and Korea have their own "standard" switch type. Other countries usually adopt the European standard.
   Data/Voice Support
   From the phone system standpoint, analog modem connections (V.34, V.90, K56 flex) are "voice" while "data" refers to digital connections using ISDN terminal adapters in the client side.
   Most ISDN-PRI lines support both data and voice channels. But we have seen lines configured to support only voice or only data. If asked, you probably would want both voice and data support.
   If you must choose only one, voice is the choice to support modem (V.34, V.90) clients and data is the choice to support clients using ISDN-BRI.
   The Cyclades-PR4000 supports both digital (ISDN-BRI) and analog calls (V.90/V.34 modem).
   Phone Numbers/Hunting groups/Hunting sequence
   When you get a T1/E1 line, you get the equivalent to 23 or 24 (T1) or 30 (E1) phone lines. Those lines can all have different phone numbers or be organized in "hunting groups" with the same phone number. When a client dials-in to a phone number, it gets the first available line within the hunting group.
   The line allocation can be done in linear (the first available line gets a new call) or round robin fashion. If it is linear, you may be asked whether you want it to be from the first line to the last or vice-versa (it doesn’t matter to the PR4000).
   Those parameters should be configured according to your needs. From the Cyclades-PR4000 standpoint, any configuration is fine.
   ISPs usually group all the lines in one hunting group so that your customers can all call the same phone number and get the first available line. Breaking the trunk in more than one hunting group can be useful if you want to reserve a certain number of lines to customers who pay for a higher service-level.
   One-way or Two-way service
   This refers to either being able to only receive calls (dial-in) or to be able to receive or generate calls (dial-out).
   An ISP usually needs only to receive calls and One-way service is the recommended configuration, unless you plan to support services that require dial-out (fax servers, call back, etc).
   The Cyclades-PR4000 supports both dial-in and dial-out operation.
   Signaling dialing method (T1 CAS-BR only)
   If you are using T1 with CAS signaling, you may be asked to select a few additional parameters.
   For Signaling method, you may have to select between MFR1, DTMF or no signaling. The PR4000 supports any of them.
   For Dialing method, you may have to select between wink-start and loop-start. The PR4000 supports both and the suggested choice is wink-start.
   Line Coding
   This refers to the way the digital data is encoded in the line.
   For T1 lines, the options are usually "Bipolar with 8 zeroes substitution" (B8ZS) or "Alternate Mark Inversion" (AMI). B8ZS is the most modern and better suited for digital transmissions, so it should be the choice if available.
   For E1 lines, the options are usually "High Density Bipolar of Order 3" (HDB3) and "Alternate Mark Inversion" (AMI). HDB3 is the most modern and better suited for digital transmissions, so it should be the choice if available.
   The Cyclades-PR4000 supports all of the above options.
   Framing
   This refers to how the data bits are "framed" in the TDM bus.
   For T1 lines, usually the choices are "D4 Super frame" (D4) or "Extended Super Frame" (ESF). ESF provides better error correction and should be the choice if available.
   For E1 lines, usually you can choose Frame Alignment Signal with or without CRC (Cyclic Redundancy Check). If given the choice, select line with CRC, which will provide better error correction.
   The Cyclades-PR4000 supports all of the above options.
   Termination at the customer premise
   The Cyclades-PR4000 supports both T1 and E1 on a standard 120 Ohms RJ-48C connector and that should be the choice if available.
   In some countries, especially those using E1 lines, the termination may be provided on a Coax G703 connector (75 Ohms). In that case, you will need an external interface converter.
   Comments or suggestions to this document can be sent to Marcio Saito marcio@cyclades.com.