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Connecting LANs, Backbone Networks,
and Virtual LANs
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Connecting devices
Backbone networks
Virtual LANs
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Five Categories of Connecting Devices
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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
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Repeater
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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
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Function of a Repeater
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A repeater is a regenerator, not an amplifier
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Active Hubs
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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)
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Repeaters/Hubs
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Bridges
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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
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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
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Transparent Bridges: Learning
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Loop Problem in a Learning Bridge
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Transparent Bridges: Spanning Tree
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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
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The one with the smallest built-in ID
2. Select the root port of each bridge
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The port with the least-cost path from the bridge to the root bridge
3. Choose a designated bridge for each LAN
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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
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Spanning Tree: Graph Representation
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Spanning Tree:Finding the Shortest Path
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Spanning Tree: Forwarding and
Blocking Ports
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Bridges Connecting Different LANs
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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
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Two-Layer (Layer 2) Switch
• Layer 2 switch is an N-port bridge
• Ethernet switch or LAN switch
• Switched Ethernet (←)
• Full-duplex switched Ethernet (↓)
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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
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Broadcast and Collision Domains
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LAN Segmentation
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Backbone Network: Bus Backbone
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The topology of the backbone is a bus
To connect different buildings in an organization
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Star Backbone
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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
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Connecting Remote LANs
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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
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Virtual LANs
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LAN configured by software, not by physical wiring
VLANs create broadcast domains
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Example
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Membership is characterized by port numbers, MAC addresses, IP addresses,
multicast IP addresses or a combination of the above
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VLAN
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Membership
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Configuration
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VLAN can be configured in one of three ways: manual, semiautomatic, and
automatic
Communication between switches
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Membership is characterized by port numbers, MAC addresses, IP addresses,
Multicast IP addresses, or a combination of the above
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
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Cost and time reduction
Creating virtual workgroups
Security
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Virtual-Circuit Networks:
Frame Relay and ATM
• Frame Relay
• ATM
• ATM LANs
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Wide area network and switching methods
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Circuit switching
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Create a real circuit (dedicated line) between source and destination
Physical layer technology
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Packet Switching: Datagram Approach
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Mostly used in the network layer
Routing (selecting the best route for a packet) is performed at each router
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Packet Switching: Virtual Circuit Approach
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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
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VCI has switch scope; it is used between two switches
Each switch can use its own unique set of VCIs
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VCI Phases
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Two approaches for the VC setup
– Permanent virtual circuit (PVC):
– Switched virtual circuit (SVC): setup, data transfer, teardown
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Data Transfer Phase
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All switches need to have a table entry for the virtual circuit
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Data Transfer using VCI
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SVC Setup: Request and Acknowledgment
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Frame Relay
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Frame Relay is a virtual circuit wide area network
VCIs in Frame Relay are called DLCIs(Data Link Connection Identifier)
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Frame Relay Features
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Frame relay operates at a higher speed. It can easily be used instead of a mesh of T1 or T-3 lines (1.544 Mbps or 44.376 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
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Frame Relay vs. T-line Network
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Frame Relay vs. X.25 Network
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Frame Relay Layers
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Frame relay operates only at the physical and data link layers
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Comparing Layers: X.25 & Frame Relay
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Frame Relay Frame
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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
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BECN
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FECN
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Four Cases of Congestion
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Extended Address: Three Address Formats
•FRAD
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ATM
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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
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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
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Multiplexing using Cells
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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
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Synchronous vs. Asynchronous TDM
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ATM Architecture
• UNI: user-to-network interface
• NNI: network-to-network interface
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Virtual Connection
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Connection between two endpoints is accomplished through
– Transmission path (TP)
– Virtual path (VP)
– Virtual circuit (VC)
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A virtual connection is defined by a pair of numbers: VPI and VCI
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VPI and VCI: Hierarchical Switching
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Identifiers and Cells
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VP Switch and VPC Switch
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ATM Layers
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ATM Layer and Headers
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Application Adaptation Layer (AAL)
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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
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AAL1
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AAL2
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AAL3/4
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AAL5
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ATM LAN
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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.
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Pure and Legacy ATM LAN
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Mixed Architecture ATM LAN
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LAN Emulation (LANE)
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Connectionless versus connection-oriented
Physical addresses versus virtual-circuit identifiers
Multicasting and broadcasting delivery
Interoperability
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Client/Server model in a LANE
– LANE Configuration Server (LECS), LANE Server (LES), LANE Client (LEC)
– Broadcast/Unknown Server (BUS)
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Mixed Architecture Using LANE
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