qr code c# sample 2: OSI Reference Model in Objective-C

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2: OSI Reference Model
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Multi-segment communications
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PC-A PC-B
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Router-A Switch-A Hub-A
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Physical
the OSI Reference Model Whereas routers make path decisions based on destination layer 3 logical addresses, switches make path decisions based on layer 2 destination MAC addresses found in frames Therefore, the switch s physical layer will have to convert the physical layer signal into bits and pass these bits up to the data link layer, where they are reassembled into a frame The switch examines the destination MAC address and makes a switching decision, finding the port the frame needs to exit It then passes the frame down to the physical layer, where the bits of the frame are converted into physical layer signals Switching is discussed in 4 The next device the physical layers encounter is a router Recall from earlier in the chapter that routers function at layer 3 of the OSI Reference Model The router first converts the physical layer signals into bits at the physical layer The bits are passed up to the data link layer and reassembled into a frame The router then examines the destination MAC address in the frame If the MAC address doesn t match its own MAC address, the router drops the frame If the MAC address matches, the router strips off the data link layer frame and passes the packet up to the network layer At the network layer, one of the functions of the router is to route packets to destinations To accomplish this, the router examines the destination logical address in the packet and extracts a network number from this address The router then compares the network number to entries in its routing table If the router doesn t find a match, it drops the packet; if it does find a match, it forwards the packet out the destination interface
Encapsulation and De-encapsulation
To accomplish the packet forwarding, the router passes the packet down to the data link layer, which encapsulates the packet into the correct data link layer frame format If this were an Ethernet frame, for this example, the source MAC address would be that of the router and the destination would be PC-B At the data link layer, the frame is then passed down to the physical layer, where the bits are converted into physical layer signals
When sending traf c between two devices on different segments, the source device has a layer 2 frame with its own MAC address as the source and the default gateway s (router) MAC address as the destination; however, in the layer 3 packet, the source layer 3 address is the source device and the destination layer 3 address is not the default gateway, but the actual destination the source is trying to
reach Remember that layer 2 addresses are used to communicate with devices on the same physical or logical layer 2 segment/ network, and layer 3 addresses are used to communicate with devices across the network (multiple segments) Another way to remember this is that MAC addresses can change from link to link, but layer 3 logical addresses, by default, cannot
Note that routers separate physical or logical segments, while bridges and switches don t Therefore, if PC-A wants to send traffic to PC-B, PC-A uses the router s MAC (or layer 2) address to get traffic to the exit point of the segment, but it uses PC-B s logical (or layer 3) address to tell the router that this traffic is not for the router but for a machine on a different segment This process is discussed in more depth in s 3 and 10 The next device that receives these physical layer signals is the hub Recall from earlier in the chapter that hubs and repeaters operate at the physical layer Basically, a hub is a multiport repeater: it repeats any physical layer signal it receives Therefore, a signal received on one interface of a hub is repeated on all of its other interfaces These signals are then received by PC-B, which passes this information up the protocol stack as described in the preceding section For a more detailed explanation of the process described in this section, please read 10, which discusses how TCP/IP is used to communicate between devices on different Ethernet segments
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