Chapter 11 in Stallings 10 th Edition

Local Area Network Overview Chapter 11 in Stallings 10 th Edition CS420/520 Axel Krings Page 1 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 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 Storage Area Networks CS420/520 Axel Krings Page 4 2

LAN Architecture Topologies Transmission medium Layout Medium access control CS420/520 Axel Krings Page 5 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 3

A Frame Transmission on Bus LAN A B C transmits frame addressed to A A C 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 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 4

Star Topology Central Hub, switch, or repeater Figure 11.2 CS420/520 Axel Krings Page 9 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 5

IEEE 802 vs OSI Figure 11.3 OSI Reference Model Application Presentation Session IEEE 802 Reference Model Transport Network Data Link Physical Medium Upper Layer Protocols ( ) ( ) ( ) Logical Link Control Medium Access Control Physical Medium LLC Service Access Point (LSAP) Scope of IEEE 802 Standards CS420/520 Axel Krings Page 11 802 Layers - Physical Encoding/decoding Preamble generation/removal Bit transmission/reception Transmission medium and topology CS420/520 Axel Krings Page 12 6

IEEE 802 Layers Logical Link Control Layer (LLC) Provide interface to higher levels Perform flow and error control Media Access Control (MAC) On transmit assemble data into frame On reception disassemble frame, perform address recognition and error detection Govern access to LAN transmission medium CS420/520 Axel Krings Page 13 LAN Protocols in Context Figure 11.4 CS420/520 Axel Krings Page 14 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 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 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 MAC Frame MAC Control Destination MAC Address Source MAC Address LLC PDU CRC LLC PDU 1 octet 1 1 or 2 variable DSAP SSAP LLC Control Information I/G DSAP value C/R SSAP value LLC 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 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 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 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 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 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 LAN A Frames with addresses 11 through 20 are accepted and repeated on LAN B Station 1 Station 2 Station 10 Bridge LAN B Frames with addresses 1 through 10 are accepted and repeated on LAN A Station 11 Station 12 Station 20 Figure 11.6 Bridge Operation CS420/520 Axel Krings Page 24 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 User LLC MAC Physical t 1 t 8 t 2 t 7 MAC t 3 t 4 t 5 t 6 LAN Physical Physical LAN User LLC MAC Physical (a) Architecture t 1, t 8 User Data t 2, t 7 LLC-H User Data t 3, t 4, t 5, t 6 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 13

Station 1 Station 2 Station 3 LAN A Bridge 101 Bridge 107 Bridge 102 LAN B LAN C Bridge 103 Bridge 104 Bridge 105 Bridge 106 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 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 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 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 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 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 IEEE 802.1 Spanning Tree Algorithm: Each bridge assigned unique identifier Cost assigned to each bridge port Exchange information between bridges to find spanning tree Automatically updated whenever topology changes CS420/520 Axel Krings Page 32 16

Station B LAN Y t 1 t 2 Bridge Bridge LAN X t 0 t 0 Station A Figure 11.9 Loop of Bridges CS420/520 Axel Krings Page 33 Hubs Ø Active central element of star layout Ø Each station connected to hub by two lines Ø Hub acts as a repeater Ø Length of a line is limited to about 100m Ø Optical fiber may be used to about 500m Ø Physically a star, logically a bus Ø Transmission from any one station is received by all other stations Ø If two stations transmit at the same time there will be a collision CS420/520 Axel Krings Page 34 17

Two cables (twisted pair or optical fiber) HHUB Transmit IHUB IHUB Station Receive Station Station Station Station Figure 11.10 Two-Level Star Topology CS420/520 Axel Krings Page 35 Shared Bus - 10 Mbps 10 Mbps 10 Mbps 10 Mbps 10 Mbps A B C D (a) Shared medium bus Total capacity up to 10 Mbps 10 Mbps 10 Mbps 10 Mbps 10 Mbps A B C D (b) Shared medium hub Total capacity N 10 Mbps 10 Mbps 10 Mbps 10 Mbps 10 Mbps A B C D (c) Layer 2 switch Figure 15.11 LAN Hubs and Switches CS420/520 Axel Krings Page 36 18

Layer 2 Switch Benefits Ø No change is required to the software or hardware of the attached devices to convert a bus LAN or a hub LAN to a switched LAN Ø Have dedicated capacity equal to original LAN l Assuming switch has sufficient capacity to keep up with all devices Ø Scales easily l Additional devices attached to switch by increasing capacity of layer 2 CS420/520 Axel Krings Page 37 Types of Layer 2 Switch Store-and-forward switch Accepts frame on input line Buffers it briefly, Then routes it to appropriate output line Delay between sender and receiver Boosts integrity of network Cut-through switch Takes advantage of destination address appearing at beginning of frame Switch begins repeating frame onto output line as soon as it recognizes destination address Highest possible throughput Risk of propagating bad frames Switch unable to check CRC prior to retransmission CS420/520 Axel Krings Page 38 19

Layer 2 Switch vs. Bridge Differences between switches and bridges: Bridge Frame handling done in software Analyzes and forwards one frame at a time Uses store-andforward operation Switch Performs frame forwarding in hardware Can handle multiple frames at a time Can have cutthrough operation Layer 2 switch can be viewed as fullduplex hub Incorporates logic to function as multiport bridge New installations typically include layer 2 switches with bridge functionality rather than bridges CS420/520 Axel Krings Page 39 Inaho for takeout. Love to All Tricia Internet Z Server One big LAN Router Ethernet switch Workstation Printer No separation for broadcasts W X Y Figure 11.12 A LAN Configuration CS420/520 Axel Krings Page 40 20

Internet Z Server Router Ethernet switch Workstation Router V: Now 4 separate LANS V Printer W X Y Figure 11.13 A Partitioned LAN CS420/520 Axel Krings Page 41 Problems with Routers Routers do all IP-level processing in software High-speed LANs and high-performance layer 2 switches pump huge # of packets per second Software-based router are slower Solution: layer 3 switches Implement packet-forwarding logic of router in hardware Two categories Packet by packet Flow based CS420/520 Axel Krings Page 42 21

Internet VLAN A VLAN B Server Z VLAN: (Virtual LAN) VLAN D VLAN A Workstation Printer Logical subgroup within LAN W VLAN A Ethernet switch with VLAN and IP routing capability VLAN E VLAN A X Y VLAN C VLAN B Figure 11.14 A VLAN Configuration CS420/520 Axel Krings Page 43 Defining VLANs Ø Broadcast domain consisting of a group of end stations not limited by physical location and communicate as if they were on a common LAN Ø Membership by: l Port group l MAC address l Protocol information CS420/520 Axel Krings Page 44 22

Communicating VLAN Membership Switches need to know VLAN membership Ø Configure information manually Ø Network management signaling protocol Ø Frame tagging (IEEE802.1Q) CS420/520 Axel Krings Page 45 Summary Bus and tree topologies and transmission media Topologies Choice of topology Choice of transmission medium LAN protocol architecture IEEE 802 reference model Logical link control Medium access control Hubs and switches Hubs Layer 2 switches Bridges Functions of a bridge Bridge protocol architecture Fixed routing The spanning tree approach Virtual LANs The use of virtual LANs Defining VLANs Communicating VLAN membership CS420/520 Axel Krings Page 46 23