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1、1Chapter NineLocal Area Networks2Chapter Nine Local Area NetworksLAN 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 topolog
2、y3 Bus Contention Token Ring StarChapter Nine Local Area NetworksLAN Topologies4Chapter Nine Local Area NetworksSub-layers at DLL5Chapter Nine Local Area NetworksData Link Layer Standards Data link layer can be divided into 2 parts Logical Link Control(LLC)Frame formats,error control,flow control Me
3、dium 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.6Chapter Nine Local Area NetworksE
4、thernet: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 protocol7Chapter Nine Local Ar
5、ea NetworksEthernet: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 card8Chapter Nine Local Area NetworksEthernet:IEEE 802.3 Transceiver cable(AUI cable)5 twisted pair
6、2 pair for sending data and control 2 pair for receiving data and control 1 pair for power9Chapter Nine Local Area NetworksEthernet:IEEE 802.3 Network Interface Card(NIC)Receives packets from PC software Formats frames Executes binary exponential backoff algorithm if required Detects incoming frames
7、 for itself Performs CRC error checking Has a hardware address收？收？发？发？10Chapter Nine Local Area NetworksEthernet: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 Data length fiel
8、d Number of octets in the combined data and pad fields11Chapter Nine Local Area NetworksEthernet: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 Data length field Number of octe
9、ts 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=281,474,976,710,65612Chapter Nine Local Area NetworksEthernet:IEEE 802.3 Preamble 7-octet pattern of alternating 0s and 1s Start
10、 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 fieldsEthernet hardware address Every Ethernet host has a unique address,burned into ROM by manufacturer Each manufacturer is given
11、 a unique prefix Unicast addresses:one-to-one comunication Broadcast 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 address13Chapter Nine Local Area NetworksEthernet:IEEE 802.3 Data field-Maximum is 1500
12、octets Pad field:decided by the minimum size of a frame.(512bits)=64 octets Frame check sequence-32-bit CRCWHY?14Packet Size Max packet size stops a host from monopolising the medium Min packet size set for reliable collision detection15Min packet size Max segment length 500m Max of four repeaters j
13、oining segments 2500m/(200)m/s=12.5s,25s round-trip Allow 25s for(worst-case)repeater delay 50 s at 10 Mb/s=500 b,plus a few more 512 b=64 B16Chapter Nine Local Area NetworksPhysical Implementations of 10Mbps Ethernet 17Chapter Nine Local Area NetworksPhysical Implementations of 10Mbps Ethernet 18Chapter Nine Local Area NetworksPhysical Implementations of 10Mbps Ethernet 19Chapter Nine Local Area NetworksPhysical Implementations of 10Mbps Ethernet
