BROCADE IP PRIMER

The Data Link Layer (Layer 2)By far, the most well-known Network Layer protocol is the Internet Protocol (IP).This is the IP portion of the TCP/IP protocol suite. Well-known routing protocolsinclude Routing Information Protocol (RIP), Open Shortest Path First (OSPF),and Border Gateway Protocol (BGP), all of which we'll be talking more about infuture chapters.The Data Link Layer (Layer 2)We're getting closer now. We have the data (Application Layer). It's pretty (PresentationLayer). We've set up our conversation (Session Layer). We'll makesure the conversation will be heard (Transport Layer). And we know how we'regoing to get there (Network Layer). What's next?The Data Link Layer is the layer where the data gets packed up and ready togo. We've received the packet from the Network Layer, so we know how it'sgoing to get there. One of the Data Link Layer's jobs is to make sure the datagets to the physical devices it needs to, in order to follow the Network Layer'sinstructions.In a way, the Data Link Layer is similar to the Network Layer. It, too, has todetermine how its data (frames) will get to where they need to go. But the DataLink Layer's communication is local. Think of the telephone system. Let's say aperson in Los Angeles is calling someone in New York. This call will go from theperson's phone to a switch, to another switch, to another switch, and so on,and so forth, until it reaches the phone of the person in New York. The NetworkLayer's job would be to get the call from Los Angeles to New York. The DataLink Layer's job would be to get the call from the phone to the first switch.Then, there would be a new Data Link Layer job to get the call from the firstswitch to the next switch, and so on. The Network Layer will take you anywhereyou need to go. The Data Link Layer just takes you to the next step. It is local.To do its job, the Data Link Layer is divided into two sublayers: the Logical LinkControl (LLC) Layer and the Media Access Control (MAC) Layer.Logical Link Control (LLC)This sublayer has two jobs:• Detection and retransmission of dropped frames• Protocol multiplexingThe first job, detection and retransmission, is just as it sounds. The LLC mustdetect when frames are dropped, and then retransmit them. Protocol multiplexingsounds fancy, but it's actually very simple. This is where the frameidentifies what Network Layer protocol it received information from. For example,if the Network Layer protocol were IPX, the Logical Link Control sublayerwould identify that it received the packet from IPX (on the Network Layer). Onthe receiving side, the Logical Link Control sublayer will demultiplex the frame,and realize that it should be handed to the IPX protocol at the Network Layerfor further processing. Think of it as a name-stamp to make sure that the Net-Brocade IP Primer 17