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Date Created: 09/19/15
CEN 4500 Network Fundamentals Chapter 4b The Medium Access Sublayer IEEE Standard 802 IEEE Standard 802x for LANs Set standards for CSMACD token bus and token ring LANs The standards differ at the physical level and MAC sublayer but are compatible at the data link layer Standards are comprised of parts we ll concentrate on 8021 to 8025 8021 defines the network interface primitives 8022 describes the upper part of the data link layer which uses the Logical Link Control LLC protocol 8023 8025 describe the three LAN standards CSMACD token bus and token ring Each standard addresses both the physical layer and MAC sublayer IEEE Standard 8023 Standard for 1persistent CSMACD LAN Originally developed by Xerox PARC for the ALOHA system in mid 70 s introduced carriersensing and came to be known as Ethernet So successful became the basis for the 8023 standard Ethernet has become the generic name for 8023based 1 persistent CSMACD LANs 8023 actually addresses a broader array of CSMACD LANs than just the Ethernet implementation Cabling 4 types of cabling used 10Base5 thick coax 10Base2 thin coax 1OBaseTtwisted pair phone line 10BaseF fiber optic cable 10Base5 can be identi ed by it s thickness about same diameter as a garden hose and the use of vampire tap connectors Vampire Tap F Transceiver Maximum segment length 500 meters Still used but losing favor to smaller easiertoinstall cabling and connectors ie 1OBase2 Thin Coax Cable TVlike coaxial cabling using BNC connectors Very common often used to connect terminals directly to each other Maximum segment length is 200 meters Cable om previous terminal or hub Connector to network card Cable to next terminal or hub loBaseT Like phone cable but with 8 wires instead of4 Very popular due to ease of installation Uses modular RJ45 connector Requires the use ofa hub Maximum segment length of100 meters Cabling 1OBaseF RJ45 Connector Fiber Optic very expensive but costs dropping as use increases Flexible cable about 14 thick Maximum segment length of 2000 meters 10BaseF Connector Some more Hardware Terminology Hub A common connection point for devices in a network Hubs are commonly used to connect segments ofa LAN A hub contains multiple ports When a frame arrives at one port it is copied to the other ports so that all segments of the LAN can see all packets A passive hub serves simply as a conduit for the data enabling it to go from one device or segment to another Socalled intelligenthubs include additional features that enables an administrator to monitor the traf c passing through the hub and to configure each port in the hub Intelligent hubs are also called manageable hubs A third type of hub called a switching hub actually reads the destination address of each packet and then fonNards the packet to the correct port 3Com SuperStack 11 Baseline 12P01 t Hub e Repeater A netwurk gevrce useg tn regenerate ur rephcate a srgnat Repeaters are used rn transrnrssrun systems tn regenerate anatug ur grgrtat srgnats grsturteg bytransmtsstun tuss Anatug repeaters frequermy can umy arnptrry tn can rerunstruct a srgnat tn near rts ungrnat guat an retay messages between subnetwurks tnat e srgnat wnrte grgrtat repeaters W m a data naqu a repeater use grrrerent prutucuts ur cab A repeater annut get tne rnte Tuday repeaters are often rnugutes wrtnrn a nun M GTquot 471 Jeri56 1W A a te urgent ruutrng penunneg by pngges ang ruuters u n g am a mm LED Clscu Catatyst 282D Hun lEIEIEaseTX Repeater Mudule men n nemmks a dancemamhevsandmmms amesbelween LAN SEumems Swnches upeva e a me data hnk ayev and mevemve suppun any packet pmlucm LANs m use Mtches mm SEEmems ave caHed swmeauws uv m me case an Ememe nelwuvks switched Enema uws 2mg swuhonm Atypical LAN con guration using switches and hubs Manchester Encodin None ofthe versions of8023 use straight binary encoding ie 0 volts for a 0bit and 5 volts for a 1bit because stations could not tel he difference between an idle sender and a 0bit transmission To overcome this dilemma Manchester Encoding and Differential Manchester Encoding was developed Manchester Encoding Each bit s timeperiod is divided into two equal timeintervals A 1bit is sent by having the voltage set high during the rst interval a o E a S 5 Lo 5 m m m o o a Ensures every bit period has a transition in its middle making sender receiver synchronization easy Disadvantage of Manchester Encoding is that it requires twice the bandwidth of simple binary encoding Bit stream 1 O 0 0 0 1 O Binary encoding Manchester encoding i i Transition neie indicates Transition neie indicates a 3 mm Interval Lb me Penod Penod Differential Manchester Encoding Differs from basic ME in that a 0bit is indicated by the presence of a transition at the start of the time period Bil stream 1 0 0 0 U 1 0 1 1 1 1 Binary encoding IIJII UUUFL gs g renwding mmquot 939 W No tiaiisitior hale Wales a Hui 8023 MAC Sublayer Protocol MAC Frame Structure Each frame starts with a 7byte preamble field with all 7 bytes containing the bit string 10101010 This creates a 10MHZ wave for 56 microseconds and allows the sender and receiver to synchronize Next comes the 1byte StartofFrame delimiter which always has the value of 10101011 The highorder bit of the destination address dictates whether the receiver is a single station a group of stations or all stations on the network lf 0 then a single receiver is intended unicast if 1 then the destination is a group multicast If the address is comprised of all 1 bits then the message is sent to all stations on the network broadcast Further bit 46 next one down from the highorder bit is used to distinguish local from global addresses As 8023 states that valid frames must be at least 64 bytes long if the data field contains less than 46 bytes then the pad field is used to fill out the frame to the minimum size Bytes 7 1 2 or 6 2 or 6 2 071500 0746 4 l u Preamble Desuna on source Data Pad Checksum add ess address f n l I T Start of Length of frame delimiter data field Required Reading Tanenbaum chapter 43 431