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this level.These protocols include (and I'm reciting this from memory!)....FTP,TFTP,DNS,POP3,SMTP,HTTP and TELNET.Layer 6 Presentation-exactly as it says on the tin!!Data is manipulated in this layer,compression,encoding and translation of character sets happen in this layer and of course on the recieving end decompression,re-encoding and translation.Layer 5 Session-The key phrase here is 'END TO END COMMUNICATION',this is where the communication between two or more hosts is maintained and where the communication (session) can be 'torn down',it is arguably the singlemost important layer of all,in my opinion anyway.NN SAP resides here.Now for the lower layers (yes,the top 3 are upper layers)Layer 4 Transport-Now things really start to happen with our data.This layer manipulates our data into 'SEGMENTS' and here is where we find the TCP/UDP protocols residing and I'm open to correction here but I believe IPX/SPX might be round here somewhere..!Layer 3 Network-All of our ROUTING is handled here,with routers and layer 3 switches being part of the network layer.Here we see the SEGMENTS passed down from the transport layer turned into PACKETS.Protocols here include IP,ICMP,IGMP,ARP AND RARP (IP to MAC and MAC to IP resolution)Layer 2 Data Link-an oddball layer really,since it is actually two sub layers rolled into one.The LLC (logical link control)layer talks with our newtork layer and deals with the IP address and flow control while our second sub layer-MAC-deals with physical addressing (the MAC address no less!) and talks to the lower 'PHYSICAL' layer.Error checking (NOT ERROR CORRECTION) takes place at this layer using CRC (cyclic redundancy check).We see Bridges,switches and WAP's here.Our packets passed down from the network layer are now turned into FRAMES and made ready for the last layer...Layer 1 Physical-exactly that,all the physical aspects of the communication are residing here including hubs,MsAU's,transexuals-no makes that transcievers,NIC's and repeaters,cables and here we have our frames broken down into the now familiar 1's & 0's,or BITS(signal encoding).The data is transmitted across our physical link and ends up at layer 1 on the recieving end,where this whole mullarkey repeats itself in the exact reversre order..not bad for a welshy huh?OK,lets break things down and makes this lot a little easier to digest..Use some sort of nmemonic All Poeple Seem To Need Data Processing,that floats my boat.Remember S P F B,segments into packets into frames into bits.Look at the application layer protocols,all directly linked to applications we can use,web browsers,email,remote services,file transfers..kapiche?Presentation has no real protocols that we are concerned with,but presenting a decent picture might include .mpeg .jpeg .gif....all found here...kapiche?Session...we all like a good session down the pub don't we?well to keep the session going the pub needs to remain open!...kapiche?Transport...lets box this stuff up ready for delivery and we can either phone the recipient to say it's on its way (TCP) or just send the bugger regardless (UDP-connectionless)Network..choose the quickest route and lets make sure we have the right address....kapiche?Data Link..ok,well lets check for damage and make sure we use a postcode...kapiche?Physical...posties on the case...package delivered...kapiche?
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