Business Data Communications. Packetizing and Framing. Agenda. Why we break things up for transmission Limits to the process Packetization Framing. Why Break Things Up?. If you had a big file (an illegal MP3 perhaps) and wanted to transmit it, why not just send the whole thing at once?
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Business Data Communications Packetizing and Framing
Agenda • Why we break things up for transmission • Limits to the process • Packetization • Framing
Why Break Things Up? • If you had a big file (an illegal MP3 perhaps) and wanted to transmit it, why not just send the whole thing at once? • Implications for • Resource sharing • Error occurrence • Error detection • Error correction
Breaking Things Up • Each packet/frame contains additional information to support addressing, error detection, etc. • These bits are called overhead and place a limit on the extent to which we should break things up. • Implications • Processing requirements at source/destination • Efficiency of transmission • So, on what conditions do you think the best packet size depends?
Packetization and Framing • Same concept – different uses. • Packetization occurs at the Transport Layer • Packetization is an internetworking concept. • Sometimes a message will fit inside a single packet. • Framing occurs and the Data Link Layer. Sometimes frames are called cells. • Framing is a node to node concept. • Sometimes a packet will fit inside a single frame/cell.
Packetization • At the source node, the transport layer breaks the application data up into packets. Each packet is handed separately to the Network layer for addressing and routing. • At the destination node, the transport layer strips off the overhead bits added by the sending TL, rejoins the packets to reproduce the original application data and, when it’s complete and error free, hands the data to the layers above it.
Packetization • Packets exist only for the Transport and Network layers. • Applications don’t deal with packets. They deal only with AL data. • The DLL just thinks of the packets as data to be transmitted. It doesn’t even know they’re all part of the same AL data. • Only devices that operate at the Transport or Network layers work with (e.g. address, route, filter, switch) packets
Framing • At the source node, the DLL takes whatever it’s given, breaks it up, and adds overhead bits for framing. Each frame is then passed to the physical layer for transmission. • At the destination node, the DLL strips off the overhead bits added by the sending DLL, rejoins the frames to reproduce whatever data was given to the DLL at the sending end, and when the data is complete and error free, passes it to the layers above it.
Framing • Frames exist only for the Data Link Layer. • The TL and NL don’t deal with frames. They only deal with packets. • The PL doesn’t think. It doesn’t know about “data”. It just deals with signals on a medium. • Only devices that operate at the Data Link Layer work with (e.g. filter, switch) frames.
Packetization and Framing • AL data is broken into packets, each of which may be broken into frames. Data to be transmitted Packetized by the transport layer. P1 Data P2 Data P3 Data The data link layer considers each packet separately. F1 P1 Data F1 P2 Data F1 P3 Data F2 Data F2 Data F2 Data
Packetization and Framing • Sometimes a packet will fit into a single frame. Data to be transmitted Packetized by the transport layer. P1 Data P2 Data P3 Data The data link layer still considers each packet separately. F1 P1 Data F1 P2 Data F1 P3 Data
Designs of Packets and Frames • Overhead bits associated with each packet/frame are grouped into fields, each of which has a specific purpose. • What factors do you think influence • which fields are in each packet? • which fields are in each frame? • how large each field is?