Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!ucsd!ucsdhub!esosun!seismo!uunet!cos!howard From: howard@cos.com (Howard C. Berkowitz) Newsgroups: comp.dcom.lans Subject: Re: Ethernet bridges above 56Kb Summary: T1 is not transparent Message-ID: <4399@cos.com> Date: 29 Jul 88 12:54:36 GMT References: <9392@dartvax.Dartmouth.EDU> <116@ernie.NECAM.COM> Organization: Corporation for Open Systems, McLean, VA Lines: 100 In article , ron@topaz.rutgers.edu (Ron Natalie) writes: > > A T1 (sometimes called DS1) facility is composed of 24 multiplexed 56 > > Kb (called T0 or DS0) circuits. If your application will use less > > than half of the T1 facility, you might consider a T1 mux and use as > > many T0 circuits out of the 24 that you feel you require for the > > particular application. Actually, the original derivation was 24 64KBPS channels, into each of which could be inserted one digital voice channel or approximately 56 KBPS of data. T1 systems replaced two N systems, an analog carrier system which carried twelve voice channels. The 1.544 MBPS was derived as follows: A normal telephone voice channel has 4kHz of analog bandwidth. An analog channel can be digitized by sampling it at twice its maximum frequency, thus 8000 BPS. At each sample, the amplitude is converted to a digital value. Originally, this was a 7 bit (i.e., 128 level) code with the 8th bit used for telephone signaling; the quality was sufficiently low that the code was changed to 8 bits and other methods used for signaling. 8000 * 8 = 64000; 64000 * 24 = 1544000 Again for purists, the encoding process is done by a D-type channel bank, the OUTPUT of which is a DS1 signal. > > Bogus. A T1 circuit is a serial line of 1,544,000 bits per second. > It is not "composed of" anything. The frequent telephone use is to > use a channel bank that decomposes it into 24 56K data channels or > even more voice or telegraph channels. It is quite true that a T1 circuit provides 1,544,000 bits per second, but this can be misleading because all those bits are not normally available. There are several constraints on bit patterns which can run through the line. The original constraints are due to the way that clocking information is carried in T1 signals. DS1 (really more proper than T1; T1 is specifically a Bell System code for DS1 signals carried on repeatered twisted pair) is an _isochronous_ signal, different from synchronous (i.e., separate clock) or asynchronous (i.e., carrying in-band explicit timing bits). Isochronous signals carry in-band implicit timing information. The reason for the clock constraints are that a certain average number of pulses per unit time are needed to assure that clock can be recovered with the signal. A common analogy is that the average pulse density keeps a "flywheel" spinning in the receiver, and this flywheel (actually a tank circuit or phase lock loop) generates local timing. The original T1 system used repeaters every 6000 wire feet on 24 AWG or larger cables; these repeaters replaced the loading coils previously used every 6000 feet, so no new splices were needed on existing cables. These repeaters needed the following rules to be followed to assure adequate zero-crossing signals: 1. At least 3 one bits in every 24 time slots 2. No more than 15 consecutive zeroes 3. One bits sent with "alternate mark inversion:" zero bits are of zero voltage, but ones are sent with alternating polarity above or below zero. Two consecutive bits of the same nonzero polarity are a "bipolar violation," usually considered an error but in fact used for local loop control in DDS customer-premises-to-central-office links. Additional constraints have been added for network management and signal quality monitoring. I don't remember all of the details, but, from memory, the Extended Superframe Format calls for something like every 2047th bit to be a one. Someone with the ESF references handy can correct this. The point of all this is that a user who expects to pump 1.544 MBPS of live data into a DS1 facility will be sorely disappointed, because, unless significant signal scrambling is done, the transmission network will stuff bits into the data in order to guarantee the network stays synchronized. In applications where 56KBPS data is being sent, it's quite easy to guarantee appropriate signals by forcing every 8th bit to be a one, shifting the speed from 56 to 64 KBPS. Where higher speeds are needed, more processing is required. There are rumors, for example, that NSA crypto gear operating into DS1 facilities have their encryption algorithms designed to meet DS1 pulse density criteria. Normally, however, there are no more than 1.344 to 1.536 MBPS available for transparent data use. Incidentally, the higher speeds of the DS hierarchy have even more constraints. DS2 signals are not an integral multiple of the DS1 rate, nor is DS3, because bit stuffing is used for speed matching and synchronization. -- howard@cos.com OR {uunet, decuac, sun!sundc, hadron, hqda-ai}!cos!howard (703) 883-2812 [W] (703) 998-5017 [H] DISCLAIMER: Opinions expressed are not necessarily those of the Corporation for Open Systems, its members, or any standards body.