tailieunhanh - Scalable voip mobility intedration and deployment- P12

Scalable voip mobility intedration and deployment- P12: The term voice mobility can mean a number of different things to different people. Two words that can be quite trendy by themselves, but stuck together as if forgotten at a bus station long past the last ride of the night, the phrase rings a number of different, and at times discordant, bells. | 110 Chapter 5 share the same pool of addresses. This is because an access point functions as a bridge between the wireless and wireline networking relaying frames and so the addresses must be unique across both technologies. However there are differences between Ethernet and Wi-Fi frames the latter taking into account the wireless nature of the protocol. Unlike Ethernet frames which generally come in only one type frames come in three types control frames management frames and data frames. The first two frames are intimately involved in the underlying protocol keeping the connection up and running. These are needed because the connection cannot be defined by a cable. Multiple devices share the same air and the access point and clients need to keep what data is destined to them separate from the other transmissions. The data frames not surprisingly are the closest to the Ethernet frames and carry data payloads. Because of the three types of frames and the number of subtypes mentioned shortly all frames have to have additional header fields. In contrast Ethernet headers contain three fields only. The first field is the 48-bit destination MAC address which either names a specific device on the network or a multicast group address including the standard broadcast address of FF FF FF FF FF FF . The second field is the 48-bit address of the sender. The final field is the two-byte protocol field or EtherType designating whether the payload is IP 08 00 ARP 08 08 IPv6 86 DD or another type. After the header comes the payload padded if the overall frame size is shorter than 64 bytes. At the very end is a four-byte CRC32 checksum as the FCS see Table . Table Ethernet frame format Destination Source EtherType Frame Body FCS 6 bytes 6 bytes 2 bytes n bytes 4 bytes To this base adds a third address known as the basic service set identifier BSSID . This address is the MAC address of the wireless network itself and is needed because unlike with .