Part 1, DS-1 (1.544 Mb/s) Private Line All brand names and product names are trademarks or registered trademarks of their respective companies.

1. Introduction This document provides guidelines and helpful tips aimed at facilitating the purchase of high speed digital communication services. It is often the case that ordering and receiving a successful installation of DS-1 is painless and quick. The author recently purchased DS-1 from a provider that has modern digital switches and knows how to provide V.90 speeds. The process of ordering took about twenty minutes and the installation was on-time and provided the expected services. In this case, the service provider required only minimal information; for readers already familiar with DS-1, the service parameters were the following:

Basic service requested: Channelized Trunk-Side DS-1 with all channels configured for data only Customer Premise Equipment: Equinox Digital Modem Pool (includes integrated CSU/DSU) operating at V.90 speeds. Line Coding: Bipolar with 8 Zeroes Substitution (B8ZS) Framing: Extended Super Frame (ESF) Trunk Direction: Combo on all 24 trunks Signaling: E&M Wink for both Instart and Outstart Hunt Sequence: Ascending Termination: Smartjack

It can be that simple; however, there are potential potholes that can be avoided with a little knowledge of DS-1 and V.90. There may be many options to choose from when purchasing DS-1 service. The list of options varies among providers and even among branches of the same provider. The options combine with a list of terms that often are misused or not standardized, and with differences in equipment at providers' facilities, form a dizzying number of possible combinations that arise in practice. Generally speaking, purchasing DS-1 cannot be boiled down to a black-and-white recipe due to the large number of variables involved in the design and provisioning of solutions. While this paper offers useful advice based on extensive experience with purchasing and using DS-1, the most important ingredient for success in buying DS-1 is careful communication with the provider about how the service will be used and what the service expectations are.

2. Some basic concepts and terminology This section describes some basics we need in order to get started. Consider the case of an Internet Service Provider (ISP) using a modem pool to provide access and value-added services (see Fig. 1). Central Offices (CO) provide local gateways into the telephone network infrastructure. This infrastructure, of course, allows access for analog lines from customers' homes. These lines are called "analog" because they use continuous wave forms of electrical energy to represent voice and data. This method of transmission is in stark contrast to digital lines, which use a small number of energy levels, usually, two or three, to represent voice and data. The telephone connection via modem from the home to the CO is an example of analog transmission while the link between the PC and an external modem uses digital transmissions. Another example of digital transmission is the link between the CO and the ISP, which in the figure uses a digital transmission method called DS-1. The definition of DS-1 has many facets - this entire paper can be considered only a partial description. However, we can start the definition by noting that it is but one member in a whole family of related digital technologies. The family is collectively known by the name of "Digital Hierarchy."  Table 1 brings up several other interesting terms. The carrier system for DS-1 is called "T-1", a term that is often used in practice interchangeably with "DS-1" but really refers only to the cables and digital repeaters used in the infrastructure.

Signal Level Digital Bit Rate Equivalent Voice Circuits Carrier System DS-0 64 Kb/s 1 DDS, 56Kb DS-1 1.544 Mb/s 24 T-1 DS-2 6.312 Mb/s 96 T-2 (rarely used) DS-3 44.736 Mb/s 672 T-3 DS-4 274.176 Mb/s 4,032 T-4 (rarely used) Table 1: North American Digital Hierarchy

For each member in the hierarchy, column 3 in Table 1 tells how many standard 64 Kb/s voice circuits it will carry; these numbers reflect the history of DS-1 which originated as a way of carrying voice calls in digital form in order to transmit many calls over just a few copper wires. DS-1 will carry up to 24 voice circuits, each known as a DS-0 circuit, using only two pairs of wire. The bit rate for DS-0 directly reflects its origin as a carrier for voice: to enable retrieval of analog voice signals after conversion to digital form, it is necessary to sample the analog signal at least 8000 times each second and each sample is represented by an 8 bit value, resulting in the 64 Kb/s transmission rate. The history of DS-1 goes back more than 20 years. The age of the technology is important because at least a few CO facilities are still using very old equipment which can cause huge problems in providing high speed services to ISP customers, as explained below. This is why it is important to describe to a prospective provider of DS-1 services how the service will be used and what transmission speeds (e.g., V.90) are expected. Transmitting data between a home computer and the CO involves conversions of digital representations to analog and vice-versa. It's crucial to understand the basics of these conversions because they deeply affect the transmission speeds (and therefore the quality of service) that the ISP can supply to its customer. There are two basic things to remember: Converting from analog to digital anywhere in the telephone infrastructure generally causes transmission speeds to be limited to V.34 levels (28.8 or 33.6); Converting from digital to analog does not slow transmission - these conversions can happen without hampering true V.90 speeds ( 40 - 54kbits/s ). These two facts, in the best circumstances, lead to a situation where the data transmitted from the customer to the ISP is handled at V.34 speeds while the data returned to the customer from the ISP proceeds at higher V.90 speeds. This asymmetry in speeds is expected even in the best of circumstances and generally is not a problem since much more data typically moves from the ISP to the customer. It's important, therefore, for the ISP to ask the prospective DS-1 service provider if his CO uses equipment that causes Analog to Digital conversions on the path from the ISP through the DS-1 service provider's CO. The prospective DS-1 service provider cannot, of course, guarantee that the entire path through the PSTN will be free of such conversions; however, if the service provider's CO does not use such conversions then there's a good chance that the conditions for V.90 speeds will be satisfied.  One more area of terminology needs to be introduced before going further: Customer Premise Equipment (CPE). From a DS-1 service provider's perspective, CPE refers to the equipment that connects directly to the DS-1 service (e.g., the equipment that the ISP connects to the T-1 line). To satisfy all of the FCC and ILEC requirements for direct attachment to a DS-1 service, a Channel Service Unit (CSU) is installed that provides the CO with a means of testing the DS-1 service, using various loopback methods, and retrieving information about how the DS-1 service is performing. Another important component is the Digital Service Unit (DSU). The DSU is responsible for converting the digital signals produced by a PC or other computer to an alternate digital form capable of longer distance transmission. The digital signals produced by a PC use a code of electrical pulses known as "unipolar coding" which is designed for short distances. The digital signals on a DS-1 line use "bipolar coding" capable of much longer distance transmissions. Unipolar coding uses two distinct voltage levels, positive and zero, to represent 1 and 0 respectively. Bipolar coding uses three energy levels: negative, positive, and zero. Under bipolar coding, binary 1's are represented by alternate occurrences of negative and positive voltage while binary zero is represented by zero voltage.

3. How the DS-1 service will be used and Quality of Service expectations From the information given in the previous section, the reader can begin to appreciate how important it is to be careful to help the prospective DS-1 service provider understand how the DS-1 service will be used.  The DS-1 service provider also needs to be told about the modem speeds that are expected to be supported by the DS-1. In particular, if V.90 speeds (40 - 54 Kb/s) are expected, it is important to let the service provider know before agreeing to buy the service. In this regard, the buyer should ask questions about the age of the switching equipment used in the provider's CO; any mention of channel banks or "special assemblies" should be cause for concern if V.90 speeds are expected. If the switches are not modern digital devices capable of providing a digital path through the CO, then the buyer may want to explore other possible providers. Also be sure to ask about how much lead time is required before installation can start and how long installation will take. It would be wise to negotiate an acceptance test before agreeing that the installation is acceptable. The acceptance test should exercise the DS-1 circuit in a realistic application as close as possible to the way it will be used when fully operational.

4. DS-1 Configuration Parameters This section describes some of the DS-1 service parameters that might be discussed during the DS-1 ordering process. This paper provides some guidance regarding choices of parameter values; however, each parameter should be discussed thoroughly with the perspective DS-1 service provider since local equipment and standards can vary greatly. A. Channelized DS-1 vs. Unchannelized DS-1: You should order Channelized DS-1 service. If all 24 channels of the DS-1 are to be used by the CPE, then be sure to let the service provider know that all channels are for data. B. "Trunk-Side" T1 vs. "Line-Side" T1: Generally, you should order Trunk-Side T1 since many CO facilities are organized such that the conditions for V.90 are more likely to be satisfied by Trunk-Side circuits. This is not universally true so be sure to discuss the topic with the prospective DS-1 provider.  If you need special features normally provided by the Class 5 switch of the telephone company (e.g. ANI delivery, voicemail, CLASS functions) then a line side connection is needed. C. Tarriffed Service vs. "Special Assemblies": Generally, you want to be sure that the service provider offers a Tarriffed Service since that means, among other things, that the service is a normal offering of the company and therefore they probably have experience and equipment to make it work. If the offered service involves the use of special assemblies or is not a normally offered service, then beware. D. Line Coding options: Generally there are two choices for DS-1 (bipolar) digital coding. Bipolar with 8 zeroes substitution (B8ZS) Alternate Mark Inversion (AMI) While both methods are usually supported by most CPE, B8ZS is usually preferred because it is designed specifically for the needs of digital transmission of digital data while AMI was designed primarily for digital transmission of analog (i.e., voice). Again, discuss the options with the service provider. E. DS-1 Framing: There are also two choices for how DS-1 data is "framed." The framing convention allows the DMP and the CO to clearly understand which of the 24 channels owns each bit in the transmission stream. D4 Super Frame Extended Super Frame (ESF) ESF is the more modern method and provides improved error detection and correction when compared to D4. D4 was designed with voice applications in mind while ESF was designed for digital transmission of digital data. F. Trunk Direction: DS-1 channels are sometimes referred to as "trunks" and may be purchased as inbound only, outbound only, or comb, which handles traffic in both directions. The choice depends on the application, but generally combo trunks are needed. G. Line Supervision and Signaling: These parameters refer to the use of specific bits to indicate when a caller has gone "off-hook" and when the called end responds. In voice communications, this is handled by use of dial tones; for digital devices, tones aren't necessary - it is sufficient to change the value of a few bits that are understood, by virtue of the framing convention, to mean "off-hook" and "off-hook acknowledged." For example, in a scheme called "E&M Wink" four bits in each Extended Super Frame (2 bits in each D4 Super Frame) are used to signal off-hook and the called end responds by setting corresponding bits to specific values for a few hundred milliseconds (a "wink"). There are four common types: E&M Wink Start; Ground Start; Loop Start; and Immediate Start. H. Termination: When the DS-1 service provider installs the T1 line at the customer's premise, the termination interface needed is either a simple RJ-48C modular plug or a DSX-1 jack connection. You should ask the service provider if they offer the option of also installing a "Smart Jack" which will allow the provider to test the T1 thoroughly; sometimes providers charge extra for the Smart Jack. I. Hunt Sequence: DS-1 service providers typically offer the option of having a single phone number that can be used in rotary fashion to channel a new call to the next available DS-0 circuit. If this option is needed, the provider will typically offer several ways of choosing which DS-0 gets the next call. Many times it will be fine to simply choose ascending or descending order. In ascending order, trunk 1 gets the first call, trunk 2 gets the second (if trunk 1 is still busied), and so on until all 24 trunks are busied.

5. Providers of DS-1 Service Your local Regional Bell Operating Company is always a candidate.  Depending on whether you think of these in the pre- or post-consolidation names, you know these companies as Southwestern Bell, Ameritech, Nynex, Pacific Bell, Bell Atlantic, etc. or, today, these are Qwest, SBC, Verizon, Bell South.  Depending on your location, there may be others new competitive access carriers such as AT&T Local, Worldcom, McLeod, XO, etc. that will usually provide the same or better service at a lower cost and deliver the service much faster if they have their own facilities (rather than just reselling others).

Part 2, DS-3 (44.736 Mb/s)

Part 3, SONET/Optical Services (OC-x)

Part 4, Ethernet and Gigabit Ethernet Services

Part 5, Asynchronous Transfer Mode (ATM) & Frame Relay (FR)