15-1 Connecting LANs, Backbone Networks, and Virtual LANs Connecting devices Backbone networks Virtual LANs

15-2 Five Categories of Connecting Devices Below the physical layer: passive hub At the physical layer: repeater or active hub At the physical and data link layers: bridge or two-layer switch At the physical, data link, network layers: router or three-layer switch At all five layers: gateway

15-3 Repeater A repeater operates only in the physical layers A repeater connects segments of a LAN A repeater forwards every frame; it has no filtering capability The repeater is a two-port device that extends the LANs’ physical length

15-4 Function of a Repeater A repeater is a regenerator, not an amplifier

15-5 Active Hubs An active hub is actually a multiport repeater It is normally used to create connections between stations in a star topology Hubs can also be used to create multiple levels of hierarchy; removing the length limitation of 10Base-T (100m)

15-6 Repeaters/Hubs

15-7 Bridges A bridge operates in both physical and data link layers A bridge has filtering capability: Having a table used in filtering decisions A bridge can check, does not change the physical (MAC) addresses in a frame

15-8 Bridges Transparent bridges –A bridge in which the stations are completely unaware of the bridge’s existence –Three criteria for a transparent bridge Frames must forward from one station to another The forwarding table is automatically made by learning frame movements in the network Loops in the system must be prevented Source routing bridges –A sending station defines the bridges that the frame must visit –Not very common today –It can prevent loops in a system with redundant bridges

15-9 Transparent Bridges: Learning

15-10 Loop Problem in a Learning Bridge

15-11 Transparent Bridges: Spanning Tree Spanning tree is a graph in which there is no loop To solve the looping problem, IEEE spec requires that bridges use the spanning tree algorithm 1.Select the root bridge The one with the smallest built-in ID 2.Select the root port of each bridge The port with the least-cost path from the bridge to the root bridge 3.Choose a designated bridge for each LAN The bridge with the least-cost path from the LAN to the root bridge The corresponding port is the designated port 4. Mark the root port and designated port as forwarding port, others as blocking port

15-12 Spanning Tree: Graph Representation

15-13 Spanning Tree:Finding the Shortest Path

15-14 Spanning Tree: Forwarding and Blocking Ports

15-15 Bridges Connecting Different LANs Many technical issues to connect LANs using different protocols at the data link layer Frame format Maximum data size Data rate Bit order Security Multimedia support and QoS

15-16 Two-Layer (Layer 2) Switch Layer 2 switch is an N-port bridge Ethernet switch or LAN switch Switched Ethernet (←) Full-duplex switched Ethernet (↓)

15-17 Two-Layer and Three-Layer Switch Two-Layer Switch : bridge with many ports –Filtering based on the MAC address of the frame it received –Builds switching table by “learning” host addresses from source addresses of incoming packets –Unknown destination addresses are flooded out other ports –Broadcast frames are flooded out other ports Router –Three-layer device that routes packets based on their logical (network layer) address. –Builds routing table by neighbor routers using routing protocols –Unknown IP packets are discarded –Broadcast frames are discarded Three-Layer Switch : a router, but a faster and more sophisticated –Router and three-layer switch interchangeably

15-18 Broadcast and Collision Domains

15-19 LAN Segmentation

15-20 Backbone Network: Bus Backbone The topology of the backbone is a bus To connect different buildings in an organization

15-21 Star Backbone Collapsed or switched backbone The topology of the backbone is a star; the backbone is just one switch Mostly used as a distribution backbone inside a building

15-22 Connecting Remote LANs When a company has several offices with LANs Remote bridges A point-to-point link acts as a LAN in a remote backbone connected by remote bridges

15-23 Virtual LANs LAN configured by software, not by physical wiring VLANs create broadcast domains

15-24 Example Membership is characterized by port numbers, MAC addresses, IP addresses, multicast IP addresses or a combination of the above

15-25 VLAN Membership –Membership is characterized by port numbers, MAC addresses, IP addresses, Multicast IP addresses, or a combination of the above Configuration –VLAN can be configured in one of three ways: manual, semiautomatic, and automatic Communication between switches –Each switch must know not only which station belongs to which VLAN, but also the membership of stations connected to other switches –Three methods are devised: table maintenance, frame tagging, and TDM Advantages of VLAN –Cost and time reduction –Creating virtual workgroups –Security

18-26 Virtual-Circuit Networks: Frame Relay and ATM Frame Relay ATM ATM LANs

18-27 Wide area network and switching methods

18-28 Circuit switching Create a real circuit (dedicated line) between source and destination Physical layer technology

18-29 Packet Switching: Datagram Approach Mostly used in the network layer Routing (selecting the best route for a packet) is performed at each router

18-30 Packet Switching: Virtual Circuit Approach Packets (frames) are switched along a pre-determined path from source to destination Virtual circuit network has two addresses –Global address which is unique in the WAN –Virtual circuit identifier which is actually used for data transfer VCI has switch scope; it is used between two switches Each switch can use its own unique set of VCIs

18-31 VCI Phases Two approaches for the VC setup –Permanent virtual circuit (PVC): –Switched virtual circuit (SVC): setup, data transfer, teardown

18-32 Data Transfer Phase All switches need to have a table entry for the virtual circuit

18-33 Data Transfer using VCI

18-34 SVC Setup: Request and Acknowledgment

18-35 Frame Relay Frame Relay is a virtual circuit wide area network VCIs in Frame Relay are called DLCIs(Data Link Connection Identifier)

18-36 Frame Relay Features Frame relay operates at a higher speed. It can easily be used instead of a mesh of T- 1 or T-3 lines (1.544 Mbps or Mbps) Frame relay operates just the physical and data link layers. It is good as a backbone to provide services to protocols that already have a network layer protocol, such as Internet It allows bursty data It allows a frame size of 9000 bytes accommodating all LAN frame sizes It is less expensive than other traditional WANs It has error detection at the data link layer only. There is no flow control pr error control X.25  Leased Lines  Frame Relay

18-37 Frame Relay vs. T-line Network

18-38 Frame Relay vs. X.25 Network

18-39 Frame Relay Layers Frame relay operates only at the physical and data link layers

18-40 Comparing Layers: X.25 & Frame Relay

18-41 Frame Relay Frame

18-42 Congestion Control Frame relay requires congestion control, because –Frame Relay does not have a network layer –No flow control at the data link layer –Frame Relay allows the user to transmit bursty data Congestion avoidance –Two bits in the frame are used –BECN(Backward Explicit Congestion Notification) –FECN(Forward Explicit Congestion Notification) Discard eligibility(DE): –Priority level of the frame for traffic control –Discarding frame to avoid the congestion or collapsing

18-43 BECN

18-44 FECN

18-45 Four Cases of Congestion

18-46 Extended Address: Three Address Formats FRAD

18-47 ATM Asynchronous Transfer Mode ATM is the cell relay protocol designed by ATM forum and adopted by ITU-T ATM uses asynchronous TDM Cells are transmitted along virtual circuits Design Goals –Large bandwidth and less susceptible to noise degradation –Interface with existing systems without lowering their effectiveness –Inexpensive implementation –Support the existing telecommunications hierarchies –Connection-oriented to ensure accurate and predictable delivery –Many functions are hardware implementable

18-48 Multiplexing using Cells The variety of packet sizes makes traffic unpredictable A cell network uses the cell as the basic unit of data exchange –A cell is defined as a small, fixed sized block of information –Cells are interleaved so that non suffers a long delay –A cell network can handle real-time transmissions –Network operation is more efficient and cheaper

18-49 Synchronous vs. Asynchronous TDM

18-50 ATM Architecture UNI: user-to-network interface NNI: network-to-network interface

18-51 Virtual Connection Connection between two endpoints is accomplished through –Transmission path (TP) –Virtual path (VP) –Virtual circuit (VC) A virtual connection is defined by a pair of numbers: VPI and VCI

18-52 VPI and VCI: Hierarchical Switching

18-53 Identifiers and Cells

18-54 VP Switch and VPC Switch

18-55 ATM Layers

18-56 ATM Layer and Headers

18-57 Application Adaptation Layer (AAL) Convert data from upper-layer into 48-byte data units for the ATM cells AAL1 – constant bit rate (CBR) video and voice AAL2 – variable bit rate (VBR) stream  low-bit-rate traffic an short-frame traffic such as audio (ex: mobile phone) AAL3/4 – connection-oriented/connectionless data AAL5 – SEAL (Simple and Efficient Adaptation Layer) No sequencing and error control mechanisms

18-58 AAL1

18-59 AAL2

18-60 AAL3/4

18-61 AAL5

18-62 ATM LAN ATM is mainly a wide-area network (WAN ATM); however, the technology can be adapted to local-area networks (ATM LANs). The high data rate of the technology has attracted the attention of designers who are looking for greater and greater speeds in LANs.ATM is mainly a wide-area network (WAN ATM); however, the technology can be adapted to local-area networks (ATM LANs). The high data rate of the technology has attracted the attention of designers who are looking for greater and greater speeds in LANs.

18-63 Pure and Legacy ATM LAN

18-64 Mixed Architecture ATM LAN

18-65 LAN Emulation (LANE) Connectionless versus connection-orientedConnectionless versus connection-oriented Physical addresses versus virtual-circuit identifiersPhysical addresses versus virtual-circuit identifiers Multicasting and broadcasting deliveryMulticasting and broadcasting delivery InteroperabilityInteroperability Client/Server model in a LANEClient/Server model in a LANE –LANE Configuration Server (LECS), LANE Server (LES), LANE Client (LEC) –Broadcast/Unknown Server (BUS)

18-66 Mixed Architecture Using LANE