Date: Thu, 25 Feb 1999 10:03:32 -0500 From: Dennis <dennis@etinc.com> To: "Louis A. Mamakos" <louie@TransSys.COM> Cc: isp@freebsd.org Subject: Re: cdrom.com bandwidth limits Message-ID: <199902251455.JAA14731@etinc.com> In-Reply-To: <199902251434.JAA48676@whizzo.transsys.com> References: <Your message of "Tue, 23 Feb 1999 23:55:38 MST." <36D3A26A.922CE5A9@softweyr.com> <199902231921.OAA03755@y.dyson.net> <36D3A26A.922CE5A9@softweyr.com>
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At 09:34 AM 2/25/99 -0500, you wrote: >> "John S. Dyson" wrote: >> > >> > It is not likely to get that much due to protocol overheads, but I >> > have seen >160KBytes/sec on a good T1. Don't T1's do bit stealing >> > for signalling (I forget?) >> >> Unless you are on a "clear channel." If you are, the throughput is >> 24 x 64 Kbits/sec, if not, 24 x 64 Kbits/sec - 8 Kbits/sec. The bit >> stealing totals 8 Kbits/sec for the entire channel, regardless of how >> "big" the channel is. > >Some CSU/DSU's can be pretty stupid when framing the synchronous data coming >in for >transmission on the T1 span. The problem they face is that there is a >specific one's density requirement when pushing bits over the wire; if you >have too many zero bits in a row, then the T1 span will blow it's clocking. > >So, some really stupid CSU/DSU's will format the data where they "force" every >8th bit to a one. This is how you end up with 1344 kb/s of bandwidth. > >The other more common way to do this is to observe that most modern uses of >T1 spans for data transmission use HDLC bit-synchronous framing these days. >If the DSU inverts the data coming in, then the HDCL framing will ensure >adequate one's density on the T1 span. In this instance, you get to use >all 1536 kb/s of capacity (64kb/s*24 channels) of the T1 span. > >The 1544 kb/s number you see including the T1 frame overhead, and isn't >normally >available if you expect to push your bits though a transmission system with >other multiplexing equipment (M31 muxes, digital cross connect systems, etc.) > >louie > They are not "stupid"..it depends on the encoding in place. With ESF you get the extra bandwidth, which is what most lines use. Dennis To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-isp" in the body of the message
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