Date: Thu, 25 Feb 1999 09:34:53 -0500 From: "Louis A. Mamakos" <louie@TransSys.COM> To: Wes Peters <wes@softweyr.com> Cc: dyson@iquest.net, Warner Losh <imp@village.org>, vincef@penmax.com, dennis@etinc.com, hackers@FreeBSD.ORG Subject: Re: cdrom.com bandwidth limits Message-ID: <199902251434.JAA48676@whizzo.transsys.com> In-Reply-To: Your message of "Tue, 23 Feb 1999 23:55:38 MST." <36D3A26A.922CE5A9@softweyr.com> References: <199902231921.OAA03755@y.dyson.net> <36D3A26A.922CE5A9@softweyr.com>
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> "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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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