T1、fractional T1和DS0

参考资料：http://www.techfest.com/networking/wan/t1_primer.htm

首先解释一个单词fraction：

1 a : a numerical representation (as  3/4,  5/8, 3.234) indicating the quotient of two numbers  b (1) : a piece broken off : FRAGMENT (2) : a discrete unit : PORTION
2 : one of several portions (as of a distillate) separable by fractionation
3 : BIT, LITTLE

T1 Overview

T1 is a digital service made available to the public in 1983, with the break up of AT&T. Prior to 1983 T1 was only used internally by the phone companies to inter-connect Central Offices (CO's).
T1 operates at a baseband bit rate of 1.544 Mbps with 1.536 Mbps being available for payload data and the remaining 8 Kbps reserved for framing. Typically the T1 is comprised of 24 DS0's, 64 Kbps each. The T1 is transmitted as a Time Domain Multiplexed group of DS0's, sometimes referred to as channels or timeslots.
T1 operates over special low capacitance shielded twisted pair cabling, although in some instances standard twisted pair cabling can be used if precautions are taken to avoid crosstalk. The phone company will normally terminate the T1 with an 8 pin RJ type of jack, called an RJ-48C, it looks like a large phone jack. Due to the high bandwidth (frequency) of T1, repeaters are placed along the line at about every 6000 feet.
These repeaters compensate for line loss and signal distortion. The signal itself is a "return-to-zero", bipolar pulse stream. Bipolar means that the pulses deviate from the "zero" line both positively and negatively. Typically the pulses alternate in polarity, this is done for a couple of reasons; to maintain a zero Volt DC offset and to serve as a form of error detection. Should a piece of equipment receive two consecutive pulses of the same polarity (a bipolar violation), it is an indication of one or more bit errors. This alternating pulse standard is known as Alternate Mark Inversion (AMI).
The 8 Kbps framing format has two common standards that are available today, the newest standard is called Extended Super Frame (ESF) and the preceding one is D4, sometimes referred to as Super Frame (SF). Both of these standards still are commonly used today. While both D4 and ESF offer ESF offers a few advantages over D4 in the form of additional error information (CRC6) and it provides a 4 Kbps channel, called a Facility Data Link (FDL), outside the 1.536 Mbps payload capacity. The FDL gives the phone company the ability to non-intrusively interrogate the customers terminating equipment for error statistics and line performance information to aid in predicting and preventing line outages and maintain the highest possible quality of service. D4 on the other hand provides for only framing synchronization. ESF has become the norm for new installation throughout the majority of North America.
Due to the synchronous operation of T1 and its utilization of a return to zero, bipolar format, T1 has a "ones density" requirement. This requirement stipulates a 12 1/2 % minimum ones to zeros ratio. Another words, on average, there must be at least a single "1" for every eight "0"s. Since this is not always practical, a couple of solutions have been adopted for T1. The first is called "bit stuffing", every eighth (the LSB of each DS0) is stuffed with a "1", this of course makes the overall bandwidth a multiple of 56Kbps. The second method is called "Bipolar Eight Zero Substitution" or B8ZS, here every series of eight consecutive zeros is substituted with four "1"s and four "0"s and two intentional bipolar violations. This technique requires the T1 line to be configured for B8ZS so as not to correct the intentional bipolar violations.
A typical T1 line between two locations is comprised of two twisted pair lines, one for transmit and one for receive, running from the customer site to the central office. These pairs are usually carefully selected pairs within standard multi-pair cables already installed, repeaters are placed along the cable in required locations and powered by a simplex current (60 ma) between the two pairs. When the T1 enters the central office it is usually multiplexed up into a T3 or higher for transport to the distant location. At the other end, the serving central office demultiplexes the T1 out of the higher bandwidth service and routes it to a twisted pair cable to the customer site.

 

Fractional T1 Overview

A Fractional T1 is nothing more than a T1 as previously described, but with only some of the DS0s being used. The cost savings comes from the phone company's ability to allow different customers to use separate portions of the same T1. Another words as an example, customer "A" would use the first half of a T1 (DS0 #1 - #12) and customer "B" would use the second half (DS0 #13 - #24). Fractional T1 lines "look" the same as full T1 lines in the sense of termination and overall bit rate, still 1.544 Mbps.
The "path" from the customer site to the point-of-presence (POP) of the carrier is a full T1. However, the customer is restricted in how many DS0s can be used to carry the data, the remaining DS0s are stuffed with all "1"s. At the POP the DS0s carrying the customer's data are multiplexed with other customers DS0s onto a full T1 and then multiplexed up to T3s and up for transport to the other location.
At the other end all the required demultiplexing is done to transport the customer's DS0s to the remote location via a full T1 with the unused DS0s once again stuffed with all "1"s. At each end the terminating equipment is configured to ignore the unused DS0s and "drop" the all "1"s and only pass on the DS0s carrying the customer's data.

DS0：Digital Signalling Level Zero

参考资料：http://en.wikipedia.org/wiki/Digital_Signal_0

Digital Signal 0 (DS0) is a basic digital signalling rate of 64 kbit/s, corresponding to the capacity of one voice-frequency-equivalent channel.[1] The DS0 rate, and its equivalents E0 and J0, form the basis for the digital multiplex transmission hierarchy in telecommunications systems used in North America, Europe, Japan, and the rest of the world, for both the early plesiochronous systems such as T-carrier and for modern synchronous systems such as SDH/SONET.

The DS0 rate was introduced to carry a single digitized voice call. For a typical phone call, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse-code modulation for each of the 8000 samples per second. This resulted in a data rate of 64 kbit/s.

Because of its fundamental role in carrying a single phone call, the DS0 rate forms the basis for the digital multiplex transmission hierarchy in telecommunications systems used in North America. To limit the number of wires required between two involved in exchanging voice calls, a system was built in which multiple DS0s are multiplexed together on higher capacity circuits. In this system, twenty-four (24) DS0s are multiplexed into a DS1 signal. Twenty-eight (28) DS1s are multiplexed into a DS3. When carried over copper wire, this is the well-known T-carrier system, with T1 and T3 corresponding to DS1 and DS3, respectively.

Besides its use for voice communications, the DS0 rate may support twenty 2.4 kbit/s channels, ten 4.8 kbit/s channels, five 9.67 kbit/s channels, one 56 kbit/s channel, or one 64 kbit/s clear channel.

E0 (standardized as ITU G.703) is the European equivalent of the North American DS0 for carrying a single voice call. However, there are some subtle differences in implementation. Voice signals are encoded for carriage over E0 according to ITU G.711. Note that when a T-carrier system is used as in North America, robbed bit signaling can mean that a DS0 channel carried over that system is not an error-free bit-stream. The out-of-band signaling used in the European E-carrier system avoids this.