西安交通大学计算机网络与通讯课件第五章 The Data Link Layer_B.pptVIP

Summary of MAC protocols What do you do with a shared media? Channel Partitioning, by time, frequency or code Time Division,Code Division, Frequency Division Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technoligies (wire), hard in others (wireless) CSMA/CD used in Ethernet Taking Turns polling from a central cite, token passing LAN technologies Data link layer so far: services, error detection/correction, multiple access Next: LAN technologies addressing Ethernet hubs, bridges, switches 802.11 PPP ATM LAN Addresses and ARP 32-bit IP address: network-layer address used to get datagram to destination network (recall IP network definition) LAN (or MAC or physical) address: used to get datagram from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM LAN Addresses and ARP LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to assure uniqueness) Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address = portability can move LAN card from one LAN to another IP hierarchical address NOT portable depends on network to which one attaches Recall earlier routing discussion ARP: Address Resolution Protocol Each IP node (Host, Router) on LAN has ARP module, table ARP Table: IP/MAC address mappings for some LAN nodes IP address; MAC address; TTL ………………………….. TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) ARP protocol A knows Bs IP address, wants to learn physical address of B A broadcasts ARP query pkt, containing Bs IP address all machines on LAN receive ARP query B receives ARP packet, replies to A with its (Bs) physical layer address A caches (saves) IP-to-physical address pairs until information becomes old (times out) soft state: information that t