Date: Sun, 28 Mar 1999 05:09:27 +0200 From: Gerald Heinig <heinig@hdz-ima.rwth-aachen.de> To: danny@alpha.net.au Cc: net@FreeBSD.ORG, Tony Finch <dot@dotat.at> Subject: Re: OSI layering Query.. Please Help ME Message-ID: <36FD9D67.4BFE39D@hdz-ima.rwth-aachen.de> References: <199903262358.JAA27392@sydney.alpha.net.au>
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Danny Ho wrote: > > Hi everyone I am sort of new( 1 year experience in this ISP job) > > Anyway, I have one query about OSI layering. > > What are some of the advantages of OSI layer as seen in the OSI model?? Advantages? Hmmm. I suppose perhaps somewhat finer granularity than the IP model. It also defines layers above 4 (transport layer) which AFAIK IP doesn't. It's perhaps somewhat more complete. I certainly make no claims to any great expertise on the subject. Comments, anyone? > > And what exactly are PDU? Protocol Data Units. The information packages/packets that get sent up/down the protocol stacks during protocol operation/data transmission/reception. For example, a TPDU (transport protocol data unit) contains the TSAP address (transport service access point address - corresponds loosely to the "port number" in IP) and the "payload" ie. data you're trying to transmit. This gets sent down (if you're transmitting) to the NSAP (network service access point - equivalent to a network interface in IP) which adds its NSAP address (which corresponds to the IP address in IP). This then gets sent down to the link layer (layer 2) to the LSAP (link layer service access point, ie. the network card) which adds its LSAP address (ie. MAC/hardware/ethernet address) plus header/trailer/FCS (frame check sequence) and sends the caboodle off onto the wire. > > What exactly is SAP?? Service Access Point. The place where you can access a service. A telephone socket is the SAP for a connection-oriented voice data transfer service. IP mail exchange has an (IP) transport service access point address of 25 ie.it uses IP port 25 for mail exchange. Your machine's network interface would be the NSAP, the interface's IP address would correspond to its NSAP address. Note: *CORRESPOND*!!! There is such a thing as an NSAP address and it is *NOT* the same as an IP address!!! we're talking analogies here! > > I am really confused help me. I hope the above is correct. It's certainly what I was taught, and what it says in the book about networking by Fred Halsall "Data Communications, computer networks and OSI". I believe there's a second edition out (I've got the first one, and it's somewhat dated...). I admit freely that I'm a bit shaky on the subject: the book gives no concrete examples and is pretty abstract for the most part. Corrections/amendments are *most* appreciated!! Anyway, if it's right (I'm fairly sure it is!), I hope that helps Cheers, Gerald -- "Would you like to buy an encyclopaedia to help your child get to college?" "He doesn't need it. He takes the bus!" To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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