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Chapter 11 in Stallings 10Th Edition Local Area Network Overview Chapter 11 in Stallings 10th Edition CS420/520 Axel Krings Page 1 Sequence 11 LAN Applications (1) • Personal computer LANs —Low cost —Limited data rate • Back end networks —Interconnecting large systems (mainframes and large storage devices) • High data rate • High speed interface • Distributed access • Limited distance • Limited number of devices CS420/520 Axel Krings Page 2 Sequence 11 1 LAN Applications (2) • Storage Area Networks — Separate network handling storage needs — Detaches storage tasks from specific servers — Shared storage facility across high-speed network — Hard disks, tape libraries, CD arrays — Improved client-server storage access — Direct storage to storage communication for backup • High speed office networks — Desktop image processing — High capacity local storage • Backbone LANs — Interconnect low speed local LANs — Reliability — Capacity — Cost CS420/520 Axel Krings Page 3 Sequence 11 Storage Area Networks CS420/520 Axel Krings Page 4 Sequence 11 2 LAN Architecture • Topologies • Transmission medium • Layout • Medium access control CS420/520 Axel Krings Page 5 Sequence 11 Bus • Multipoint medium • Transmission propagates throughout medium • Heard by all stations — Need to identify target station • Each station has unique address • Full duplex connection between station and tap — Allows for transmission and reception • Need to regulate transmission — To avoid collisions — To avoid hogging • Data in small blocks - frames • Terminator absorbs frames at end of medium CS420/520 Axel Krings Page 6 Sequence 11 3 A A B C Frame C transmits frame addressed to A Transmission on Bus LAN A Figure 11.1 A B C Frame is not addressed to B; B ignores it A A B C A copies frame as it goes by CS420/520 Axel Krings Page 7 Sequence 11 Figure 11.1 Frame Transmission on a Bus LAN Star Topology • Each station connected directly to common central node —Usually via two point to point links • Central node can broadcast —Physical star, logical bus —Only one station can transmit at a time (hub) • Central node can act as frame switch CS420/520 Axel Krings Page 8 Sequence 11 4 Central Hub, switch, Star or repeater Topology Figure 11.2 CS420/520 Axel Krings Page 9 Sequence 11 Figure 11.2 Star Topology Protocol Architecture • Lower layers of OSI model • IEEE 802 reference model • Physical • Logical link control (LLC) • Media access control (MAC) CS420/520 Axel Krings Page 10 Sequence 11 5 IEEE 802 vs OSI Figure 11.3 OSI Reference Model Application Presentation IEEE 802 Reference Session Model Transport Upper Layer LLC Service Protocols Access Point Network (LSAP) ( ) ( ) ( ) Logical Link Control Data Link Medium Access Control Scope of Physical Physical IEEE 802 Standards Medium Medium CS420/520 Axel Krings Page 11 Sequence 11 Figure 11.3 IEEE 802 Protocol Layers Compared to OSI Model 802 Layers - Physical • Encoding/decoding • Preamble generation/removal • Bit transmission/reception • Transmission medium and topology CS420/520 Axel Krings Page 12 Sequence 11 6 IEEE 802 Layers • Logical Link • Media Access Control Layer (LLC) Control (MAC) —Provide interface to —On transmit assemble higher levels data into frame —Perform flow and error —On reception control disassemble frame, perform address recognition and error detection —Govern access to LAN transmission medium CS420/520 Axel Krings Page 13 Sequence 11 LAN Protocols in Context Figure 11.4 CS420/520 Axel Krings Page 14 Sequence 11 7 Logical Link Control Ø Transmission of link level PDUs between stations Ø Must support multi-access, shared medium Ø Relieved of some details of link access by the MAC layer Ø Addressing involves specifying source and destination LLC users l Referred to as service access points (SAPs) CS420/520 Axel Krings Page 15 Sequence 11 LLC Services Unacknowledged connectionless service • Data-gram style service • Delivery of data is not guaranteed Connection-mode service • Logical connection is set up between two users • Flow and error control are provided Acknowledged connectionless service • Datagrams are to be acknowledged, but no logical connection is set up CS420/520 Axel Krings Page 16 Sequence 11 8 LLC Service Alternatives Unacknowledged connectionless service • Requires minimum logic • Avoids duplication of mechanisms • Preferred option in most cases Connection-mode service • Used in simple devices • Provides flow control and reliability mechanisms Acknowledged connectionless service • Large communication channel needed • Time critical or emergency control signals CS420/520 Axel Krings Page 17 Sequence 11 MAC MAC Destination Source LLC PDU CRC Frame Control MAC Address MAC Address 1 octet 1 1 or 2 variable LLC DSAP PDU SSAP LLC Control Information LLC I/G DSAP value C/R SSAP value Address Fields I/G = Individual/Group C/R = Command/Response Figure 11.5 LLC PDU in a Generic MAC Frame Format CS420/520 Axel Krings Page 18 Sequence 11 9 LLC Protocol Ø Modeled after HDLC Ø Asynchronous balanced mode l Connection mode (type 2) LLC service Ø Unacknowledged connectionless service l Using unnumbered information PDUs (type 1) Ø Acknowledged connectionless service l Using 2 new unnumbered PDUs (type 3) Ø Permits multiplexing using LSAPs CS420/520 Axel Krings Page 19 Sequence 11 Media Access Control • Where —Central • Greater control • Simple access logic at station • Avoids problems of co-ordination • Single point of failure • Potential bottleneck —Distributed • How —Synchronous • Specific capacity dedicated to connection, not optimal —Asynchronous • In response to demand, round robin, reservation, contention CS420/520 Axel Krings Page 20 Sequence 11 10 Asynchronous Systems • Round robin — Good if many stations have data to transmit over extended period • Reservation — Divide medium into slots. — Good for stream traffic • Contention — Good for bursty traffic — All stations contend for time — Distributed — Simple to implement — Efficient under moderate load — Tend to collapse under heavy load CS420/520 Axel Krings Page 21 Sequence 11 MAC Frame Handling Ø MAC layer receives data from LLC layer Ø PDU is referred to as a MAC frame Ø MAC layer detects errors and discards frames Ø LLC optionally retransmits unsuccessful frames CS420/520 Axel Krings Page 22 Sequence 11 11 Bridges Ø Connects similar LANs with identical physical and link layer protocols Ø Minimal processing Ø Can map between MAC formats Ø Reasons for use: l Reliability l Performance l Security l Geography CS420/520 Axel Krings Page 23 Sequence 11 LAN A Frames with addresses 11 through 20 are accepted and repeated on LAN B Bridge Station 1 Station 2 Station 10 Frames with addresses 1 through 10 are accepted and repeated on LAN A LAN B Station 11 Station 12 Station 20 Figure 11.6 Bridge Operation CS420/520 Axel Krings Page 24 Sequence 11 12 Bridge Design Aspects Ø Makes no modification to the content or format of the frames it receives Ø Should contain enough buffer space to meet peak demands Ø Must contain routing and addressing intelligence Ø May connect more than two LANs Ø Bridging is transparent to stations CS420/520 Axel Krings Page 25 Sequence 11 User User t1 t8 LLC LLC t2 t7 MAC MAC MAC t3 t4 t5 t6 Physical LAN Physical Physical LAN Physical (a) Architecture t1, t8 User Data t2, t7 LLC-H User Data t3, t4, t5, t6 MAC-H LLC-H User Data MAC-T (b) Operation Figure 11.7 Connection of Two LANs by a Bridge CS420/520 Axel Krings Page 26 Sequence 11 13 Station 1 Station 2 Station 3 LAN A Bridge Bridge 101 Bridge 102 107 LAN B LAN C Bridge Bridge Bridge Bridge 105 106 103 104 LAN D LAN E LAN F LAN G Station 4 Station 5 Station 6 Station 7 Figure 11.8 Configuration of Bridges and LANs, with Alternate Routes CS420/520 Axel Krings Page 27 Sequence 11 Fixed Routing • Simplest and most common strategy • Suitable for small internets and internets that are relatively stable • A fixed route is selected for each pair of LANs • Usually least hop route Ø Only change when topoloy changes Ø Widely used but limited flexibility CS420/520 Axel Krings Page 28 Sequence 11 14 Spanning Tree • Bridge automatically develops routing table • Automatically update in response to changes • Algorithm consists of 3 mechanisms: —Frame forwarding —Address learning —Loop resolution CS420/520 Axel Krings Page 29 Sequence 11 Frame forwarding • Maintain forwarding database for each port —List station addresses reached through each port • For a frame arriving on port X: —Search forwarding database to see if MAC address is listed for any port (except port X) —If address not found, forward to all ports (except X) —If address listed for some port Y, check port Y for blocking or forwarding state • Blocking prevents port from receiving or transmitting —If Y is not blocked, transmit frame through port Y CS420/520 Axel Krings Page 30 Sequence 11 15 Address Learning • Can preload forwarding database • When frame arrives at port X, it has come from the LAN attached to port X • Use source address to update forwarding database for port X to include that address • Have a timer on each entry in database —If timer expires, entry is removed • Each time frame arrives, source address checked against forwarding database —If present timer is reset and direction recorded —If not present entry is created and timer set CS420/520 Axel Krings Page 31 Sequence 11 Spanning Tree Algorithm • Address learning works for tree layout if there are no alternate routes in the network —Alternate route means there is a closed loop • For any connected graph there is a spanning tree maintaining connectivity with no closed loops • Algorithm must be dynamic
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