Download Chapter 11 Extending LANs: Fiber Modems, Repeaters

Chapter 11 Extending LANs:
Fiber Modems, Repeaters, Bridges, & Switches
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Repeater
 Hub
 Bridge
 Switch
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Each LAN technology is designed for a specific
combination of speed, distance, and cost.
 A maximum length specification is a fundamental
part of LAN technology. LAN hardware is
engineered for a fixed maximum length cable and
the hardware will not work correctly over wires
that exceed the bound
 Fiber Optic Extensions (fig 11.1)
– a pair of fiber transceivers and optical fibers can be
used to provide a connection between the AUI on a
hub and a remote AUI on a router on a remote LAN.
Repeater (Hub)
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a hardware device used to extend a LAN.
 Provides connectivity between two cable segments
 amplifies and sends all electrical signals that occur
on one segment to the other segment (fig 11.2)
(ANIM09_1.MOV)
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A hub is essentially a multiport repeater
 Speed of signals in copper cable = 2 x 108 m/s
 any two stations cannot be separated by more
than 4 repeaters (fig 11.3)
 Drawback of repeaters
– all signals are repeated, including those overlapping
signals that correspond to collisions and those due to
electrical noise. Therefore repeaters cause the same
problem to occur on all other segments.
Bridge
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a hardware device that extends a LAN by forwarding
complete, correct frames from one segment to another (fig
11.4) (ANIM09_2.MOV)
Reads each frame in promiscuous mode and verifies data
integrity
bridge will not forward collisions or interference from
one segment to another
Computers do not know whether a bridge separates them
Bridges operate at data-link (layer 2) layer.
A bridge filters, forwards, or floods an incoming frame
based on the MAC address of that frame
Frame filtering function of bridge: frames are not
forwarded across bridge unless necessary.
Using bridges and fiber modem(transceiver) (fig 11.6)
Adaptive (learning) bridge
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looks at the physical addresses in the header of
each frame it receives.
source address used to determine location of
sender
destination address used to determine whether to
forward a frame
Since bridge does not know location of computers
at bootup, frames are forwarded until location of
computer can be determined (fig 11.4) (fig 11.5)
Broadcast and multicast frames are always
forwarded
Broadcast storms can cripple network
Switch (Layer 2)
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Multiport bridge
 Segment
– a section of a network that is bounded by bridges,
switches, or routers

switching simulates a bridged LAN with one
computer per segment while a hub simulates a
single shared medium (fig 11.10)
 Flooding
– forwarding packets out all ports(segments) except
source segment
– Occurs at power up and for unknown destinations
– occurs for all broadcast packets
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Maximum bandwidth of a switch = RN/2
R is data transmission rate on a given port
N is the total number of switch ports.
Memory buffering in switches
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port-based (separate I/O queues per port)
 shared memory buffering
Switching methods
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store & forward
 cut-through switching
– fast forward [wait 8-bytes]
– collision fragment-free [wait 64-bytes]
Collision domain
 the network area within which frames that have
collided are propagated.
 Repeaters and hubs propagate collisions.
 LAN bridges, switches, and routers do not.
Broadcast domain
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the set of all devices that will broadcast
frames originating from any device within
the set
 crosses layer 2 switches
 bounded by layer 3 devices (router)
Advantages of bridged(switched)
network over repeaters and hubs
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Larger aggregate bandwidth
– allows simultaneous communication between more than
one pair of computers
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Traffic isolation
– Packets forwarded only when needed
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Fewer collisions
 Bridging across longer distances
– point-to-point and special bridge hardware at each end.
Both sites filters packets due to bandwidth constraints.
Buffering is done on bridge to accommodate speed
differences between bridge ports.
– leased serial line to connect sites
– leased satellite channel
Cycle of Bridges
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Occurs when multiple paths between two
machines exist
 may cause infinite loops
 To prevent infinite loops in a bridge
network that contains a cycle of bridge
segments, some bridges must agree not to
forward frames (fig 11.9)
Spanning-Tree protocol
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used to insure loop-free path between any two
nodes on network.
 protocol detects and breaks loops by placing some
connections in standby mode
 a bridge does not forward frames if the bridge
finds that each segment to which it attaches
already contains a bridge that has agreed to
forward frames
 Spanning-tree protocol is used in fault-tolerant
networks
VLAN
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A VLAN is a broadcast domain.
VLANS group users by logical association instead
of physical location.
broadcast domain of VLAN A is separate from
that of VLAN B
VLAN is a logical grouping of network
devices/users not restricted to a physical switch
segment
Frame tagging (802.1Q) in trunk connections
Types of VLAN
– port-based (layer1)
– mac-based (layer2)
– IP-based VLANS (layer3)