Download Ch. 11

Ch. 11 LAN Overview
Definition of a LAN
• A communication network that provides
interconnection of a variety of data
communicating devices within a small area.
11.1 Bus and Star Topologies
• Key Elements of a LAN
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Topology
Transmission Media
Layout
Medium access control
11.1 Bus and Star Topologies
• Bus and Tree Topologies
– Bus
• All stations are attached directly to the media.
– Tree
• The media is a branching cable with no closed loops.
• The tree starts at the “headend” and branches out from
there.
– Each station must have an address and access is
controlled (multipoint configuration.)—Fig.11.1
11.1 Bus and Star Topologies
• Star Topology(Fig. 11.2)
– Each station is connected to a common central
node using two point-to-point links.
– Received frames can either be "broadcast" or
"switched" to a particular link.
11.2 LAN Protocol Architecture
• Fig. 11.3 IEEE 802 vs. OSI Reference Model.
• Physical Layer
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Encoding/decoding of signals.
Preamble generation/removal (for synchronization).
Bit transmission/reception.
IEEE 802 also specifies the transmission medium
and topology.
11.2 LAN Protocol Architecture (p.2)
• Medium Access Control (MAC) Layer
– Assemble data into a frame with address and
error-detection fields.
– Disassemble frames, perform address
recognition and error detection
– Govern access to the LAN transmission
medium.
11.2 LAN Protocol Architecture (p.3)
• Logical Link Control (LLC) Layer
– Provide an interface to higher layers and
perform flow and error control.
• Fig. 11.4 LAN protocols in context.
11.2 LAN Protocol Architecture (p.4)
• Logical Link Control
– Specifies the mechanisms for addressing and
the control of the data exchange.
– Operation and format are based on HDLC.
– Three Services
• Unacknowledged connectionless service.
• Connection-mode service.
• Acknowledged connectionless service.
11.2 LAN Protocol Architecture (p.5)
• Logical Link Control (cont.)
– LLC PDU (Fig. 11.5)
• Destination Service Access Point (1 octet)
– 7 bits for the address.
– One bit to indicate if it is a group address or not.
• Source Service Access Point (1 octet)
– 7 bits for the address.
– One bit is used to indicate if it is a command or response.
• LLC Control Field (1 or 2 octets)
– Similar to HDLC control field.
• Information Field (variable length)
11.2 LAN Protocol Architecture (p.6)
• Differences between LLC and HDLC
– LLC uses asynchronous balanced mode to
support connection-mode service (type 2
operation).
– LLC supports and unacknowledged
connectionless service using the unnumbered
information PDU (type 1 service).
– LLC supports an acknowledged connectionless
service by using two new unnumbered PDUs
(type 3 operation.)
– LLC permits multiplexing (using LSAPs).
11.2 LAN Protocol Architecture (p.7)
• Medium Access Control
– MAC protocols control access to the transmission
medium in some type of orderly and efficient
manner.
– Access control could be centralized or
distributed.
– Centralized schemes tend to be simpler and avoid
various "distributed control" problems, but
performance and reliability can be a concern.
11.2 LAN Protocol Architecture (p.8)
• Medium Access Control (cont.)
– Synchronous Techniques
• Specific capacity is dedicated to a connection, such
as with circuit-switching, FDM, and TDM.
• Generally do not work well in LANs.
11.2 LAN Protocol Architecture (p.9)
• Medium Access Control (cont.)
– Asynchronous techniques--capacity is allocated
in a dynamic fashion.
• Round Robin--each station is given a turn to transmit.
• Reservation--a station wishing to transmit "reserves"
slots of "time".
• Contention--all stations "contend" for the medium.
11.2 LAN Protocol Architecture (p.10)
• Medium Access Control (cont.)
– Generic MAC Frame Format--Fig. 15.6
• MAC Control Field
• Destination MAC Address
• Source MAC Address
• LLC PDU
• CRC
Problem 11.3
• Consider the transfer of a file containing
one million 8-bit characters from one
station to another. What is the total elapsed
time and effective throughput for the
following cases?
• a. Circuit-Switched LAN
– TtotalSwitch=S + L/B+tprop
– ThroughputSwitch= L/TtotalSwitch
Problem 11.3 (p.2)
• b. Bus Topology
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D--distance between stations.
B--data rate (use R bps if you wish.)
P--packet size.
Header is 80 bits.
Information field is P-80.
Acknowledgement is 88bits.
v=200 m/microsecond.
Problem 11.3 (p.3)
• b. Bus Topology (cont.)
– Assume that each packet is acknowledge before
the next is sent (stop-and-wait.)
– Let NoPa= the number of packets.
– NoPa= L/(P-80), rounded up (assuming fixed
length packets and L is the number of
inoformation bits in the message.)
– There will be NoPa cycles needed to transfer
the entire message.
Problem 11.3 (p.4)
• b. Bus Topology (cont.)
– Ignore additional overhead--then tframe=P/B.
– Also let tprop= D/v and tack=88/B.
– Then TcycleBus=tframe +tprop+tack+tprop
(ignoring processing delays.)
– Thus, TtotalBus=NoPa (TcycleBus)
– ThroughputBus=L/TtotalBus
11.3 Bridges
• Bridges were originally used to interconnect
LANs using the same physical and MAC
protocols.
• Eventually, bridges were developed that
interconnected LANs with different MAC
protocols.
• In general, bridges are simpler than routers.
Bridge Operation
• Why use a bridge, instead of simply operating
as one large LAN?
– Reliability--bridges can be used to partition a large
LAN environment.
– Performance--in general, as stations are added to a
LAN, the performance decreases.
– Security--different types of traffic with different
security needs can be kept on physically separate
media.
– Geography--two LANs in different locations can be
bridged using point-to-point communications.
Functions of a Bridge
• See Fig. 11.6
• The bridge reads all frames transmitted on
network A, accepting those addressed to B.
• Frames accepted are transmitted on B.
• The same is done for B-to-A traffic.
Design Considerations
• 1. The bridge makes no modifications to the
content or format of the frames it receives.
• 2. The bridge should contain enough buffer
space to meet peak demands.
• 3. The bridge must contain addressing and
routing intelligence.
• 4. A bridge may connect more than two LANs.
• Note: Bridges can be more complex and have
special functionality
Bridge Protocol Architecture
• The IEEE 802 committee has produced
specifications for bridges.
• These devices are called MAC-level relays.
• Fig. 11.7 illustrates the architecture and
operation.
Routing with Bridges
• Figure 11.8 illustrates the concept of alternate
routes.
• Three Strategies
– Fixed Routing
– Spanning Tree (IEEE 802.1)
– Source Routing (IEEE 802.5)
Routing with Bridges (p.2)
• Fixed Routing
– A route is selected for each source-destination
pair of LANs in the internet.
– If alternative routes exist, then the route with the
fewest hops in chosen and placed in a routing
table.
– Widely used; simple and requires minimal
processing.
– Too limited for a dynamically changing internet.
Routing with Bridges (p.3)
• The Spanning Tree Approach
– Three mechanisms
• Frame Forwarding
• Address Learning
• Loop Resolution
Routing with Bridges (p.4)
• The Spanning Tree Approach (cont.)
– Frame Forwarding
• The bridge maintains a forwarding database for each
port attached to a LAN.
• The database indicates the station addresses for
which frames should be forwarded through that port.
Routing with Bridges (p.5)
• The Spanning Tree Approach (cont.)
– Address Learning
• When a frame arrives at a particular port, the source
address can be checked.
• If the source address is not in the database for that port it
can be added.
• Each time an element is added to the database, a timer
can be set.
• When the timer expires, then the element will be
removed from the database.
• If the element is already in the database, the timer is
reset.
Routing with Bridges (p.6)
• The Spanning Tree Approach (cont.)
– Spanning Tree Algorithm--Loop Problems
• The above procedures work fine when the topology
is a tree, but problems occur when alternate routes
exist.
• Consider Fig. 11.9.
– When A transmits to B, both bridges will update their
databases and relay the frame.
– However, they will receive each others relay and update
the databases again.
– B then cannot transmit to A.
15.3 Routing with Bridges (p.7)
• The Spanning Tree Approach (cont.)
– Spanning Tree Algorithm--Some Assumptions
• 1.Each bridge is assigned a unique identifier.
• 2.There is a special group MAC address that means
"all bridges on this LAN".
• 3. Each port of a bridge is uniquely identified within
the bridge.
• These assumptions allow the bridges to exchange
routing information in order to obtain a spanning
tree.
11.4 Hubs and Switches
• Hubs
– The active central element of a star layout.
– Each station is connected to the hub with two
lines, one for transmitting and one for receiving.
– The system is essential a logical bus, since a
transmission from any one station is transmitted
to all other stations.
– Multiple levels of hubs are possible (Fig. 11.10.)
– Hubs are usually placed in a wiring closet.
– Stations are about 100 meters away, using
twisted pair, or 500 meters with optical fiber.
11.4 Hubs and Switches (p.2)
• Layer 2 Switches (Fig. 11.11)
– A shared medium hub (like a shared medium bus)
has collisions when more than one station is
transmitting at the same time.
– A layer 2 switch takes an incoming frame and
transmits it only on the destination station’s line.
– Two types of switches:
• Store-and-Forward--packets are buffered.
• Cut-through--headers are read and switching occurs
immediately--but no error checking.
11.4 Hubs and Switches (p.3)
• Layer 2 switches may function as a
multiport bridge--the differences are:
– Bridge frames are handled in software, while
layer 2 switches have hardware that performs
address recognition and frame forwarding.
– A bridge handles one frame at a time, while a
switch can handle multiple frames at a time.
– A bridge uses store and forward operations,
while cut-through operations are possible with
layer 2 switches.
11.5 Virtual LANS
• Figure 11.12,illustrates a typical LAN
configuration.
• Consider a single MAC frame from X.
• Assume that X wants to transmit to Y—the
local switch transmits it to Y.
• Alternatively, assume that X wants to
transmit to W or Z—then the local switch
routes the frame accordingly—unicast
addressing.
VLANS (p.2)
• Broadcasting is also possible using a
broadcast address.
• One approach to efficient transmission—
partition the LAN into separate broadcast
domains.
• Figure 11.13 illustrates the use of a router
for partitioning a LAN—IP addresses are
used for routing—this may not be efficient
either.
The Use of VLANs
• VLAN logic is implemented in LAN
switches and functions at the MAC layer.
• A VLAN is a logical subgroup within a
LAN that is created by software rather than
by physical partitioning.
• Figure 11.14 illustrates a VLAN
Configuration.
VLANS (cont.)
• From a business view, the VLAN provides
the ability to be physically dispersed while
maintaining its group identity.
Defining VLANs
• A VLAN is a broadcast domain consisting
of a group of end stations that are not
constrained by their physical locations.
• Approaches
– Membership by Port Group
– Membership by MAC Address
– Membership based on Protocol Information
Membership by Port Group
• Each switch has two types of ports.
– Trunk ports will connect switches and end
ports will connect workstations to the switch.
– A VLAN can be defined by assigning each end
port to a particular VLAN
• Advantage—easy to configure.
• Disadvantage—Network manager must take
care of configurations manually.
Membership by MAC Address
• MAC Addresses on in the hardware network
interface cards (NICs).
• If a network manager physically moves a
machine, the device automatically retains its
VLAN membership.
• Disadvantage—VLAN membership is assigned
initially, which is difficult in large
organizations. There is also a problem when
docking stations are used—they contain the
NICs.
Membership Based on Protocol
Information
• IP addresses can be used to assign VLAN
membership.
• Or, transport protocol information could be
used (or even higher protocol information.)
• Advantage—flexible.
• Disadvantage—issues related to performane
and the processing of MAC addresses and
other addressing.