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1、1,Chapter Nine,Local Area Networks,2,Chapter Nine Local Area Networks,LAN Standards,LAN standards for different topologies IEEE Standard 802.3 Ethernet Bus/star topology IEEE Standard 802.5 Token Ring Ring topology IEEE Standard 802.11 IEEE Standard 802.4(obsolete)Token Bus,logical token Ring Bus to
2、pology,3,Bus Contention Token Ring Star,Chapter Nine Local Area Networks,LAN Topologies,4,Chapter Nine Local Area Networks,Sub-layers at DLL,5,Chapter Nine Local Area Networks,Data Link Layer Standards,Data link layer can be divided into 2 parts Logical Link Control(LLC)Frame formats,error control,f
3、low control Medium Access Control(MAC)Access to medium,contention protocols Standard for LAN at LLC sublayer Standard:IEEE 802.2 IEEE 802.3,802.4,and 802.5 are Medium Access Control(MAC)sub-layer protocols Note:octet(8 bits)is used instead of byte in standards to avoid confusion.,6,Chapter Nine Loca
4、l Area Networks,Ethernet:IEEE 802.3,The standard for Ethernet Originated from ALOHA system First Ethernet was developed by Xerox PARC.Ethernet named after ether,which was thought to propagate radio.Bus/star topology Originally used coax Can be extended by repeaters 1-persistent CSMA/CD protocol,7,Ch
5、apter Nine Local Area Networks,Ethernet:IEEE 802.3,Transceiver Interface between the PC and the cable Detects idle backbone Listens for collisions Puts bits on cable from interface card Passes received bits to interface card,8,Chapter Nine Local Area Networks,Ethernet:IEEE 802.3,Transceiver cable(AU
6、I cable)5 twisted pair 2 pair for sending data and control 2 pair for receiving data and control 1 pair for power,9,Chapter Nine Local Area Networks,Ethernet:IEEE 802.3,Network Interface Card(NIC)Receives packets from PC software Formats frames Executes binary exponential backoff algorithm if requir
7、ed Detects incoming frames for itself Performs CRC error checking Has a hardware address,收？发？,10,Chapter Nine Local Area Networks,Ethernet:IEEE 802.3,Preamble 7-octet pattern of alternating 0s and 1s Start of frame delimiter 1010 1011 pattern indicating the start of a frame Destination address Sourc
8、e address Data length field Number of octets in the combined data and pad fields,11,Chapter Nine Local Area Networks,Ethernet:IEEE 802.3,Preamble 7-octet pattern of alternating 0s and 1s Start of frame delimiter 1010 1011 pattern indicating the start of a frame Destination address Source address Dat
9、a length field Number of octets in the combined data and pad fields,Ethernet hardware address Six octets:XX XX XX XX XX XX,where X is a hexadecimal number Example:00 05 02 74 8D 0D Number of addresses=281,474,976,710,656,12,Chapter Nine Local Area Networks,Ethernet:IEEE 802.3,Preamble 7-octet patter
10、n of alternating 0s and 1s Start of frame delimiter 1010 1011 pattern indicating the start of a frame Destination address Source address Data length field Number of octets in the combined data and pad fields,Ethernet hardware addressEvery Ethernet host has a unique address,burned into ROM by manufac
11、turerEach manufacturer is given a unique prefixUnicast addresses:one-to-one comunicationBroadcast address for one-to-all communication:all 1s Multicast address for one-to-N communication:first bit is one,followed by any not-all-1s multicast address,13,Chapter Nine Local Area Networks,Ethernet:IEEE 8
12、02.3,Data field-Maximum is 1500 octets Pad field:decided by the minimum size of a frame.(512bits)=64 octets Frame check sequence-32-bit CRC,WHY?,14,Packet Size,Max packet size stops a host from monopolising the mediumMin packet size set for reliable collision detection,15,Min packet size,Max segment
13、 length 500mMax of four repeaters joining segments2500m/(200)m/s=12.5s,25s round-tripAllow 25s for(worst-case)repeater delay50 s at 10 Mb/s=500 b,plus a few more512 b=64 B,16,Chapter Nine Local Area Networks,Physical Implementations of 10Mbps Ethernet,17,Chapter Nine Local Area Networks,Physical Implementations of 10Mbps Ethernet,18,Chapter Nine Local Area Networks,Physical Implementations of 10Mbps Ethernet,19,Chapter Nine Local Area Networks,Physical Implementations of 10Mbps Ethernet,
