Chapter 7
Local Area Networks:
The Basics
Primary Function of a LAN
 File serving – large storage disk drive acts as a
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central storage repository
Print serving – Providing authorization to access a
particular printer, accept and queue print jobs, and
user access to print queue to perform administrative
duties
Video transfers – High speed LANs are capable of
supporting video image and live video transfers
Manufacturing support – LANs can support
manufacturing and industrial environments
Academic support – In classrooms, labs, and
wireless
E-mail support
Interconnection between multiple systems
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Advantages of LAN
 Ability to share hardware and software resources
 Individual workstation might survive network
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failure
Component and system evolution are possible
Support for heterogeneous forms of hardware and
software
Access to other LANs and WANs
Private ownership
Secure transfers at high speeds with low error rates
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Disadvantages of LAN
 Equipment and support can be costly
 Level of maintenance continues to grow
 Private ownership?
 Some types of hardware may not interoperate
 Just because a LAN can support two different kinds
of packages does not mean their data can
interchange easily
 A LAN is only as strong as it weakest link, and there
are many links
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Basic LAN Topologies
 Bus/tree
 Star-wired bus
 Star-wired ring
 Wireless
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Bus/Tree Topology
 The original topology.
 Workstation has a network interface card (NIC) that
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attaches to the bus (a coaxial cable) via a tap.
Data can be transferred using either baseband
digital signals or broadband analog signals.
Baseband signals are bidirectional (broadcast) and
move outward in both directions from the
workstation transmitting.
Broadband signals are usually uni-directional and
transmit in only one direction. Because of this,
special wiring considerations are necessary.
Buses can be split and joined, creating trees.
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Baseband
Broadband
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Star-wired Bus Topology
 Logically operates as a bus, but physically looks like a star
 Star design is based on hub. All workstations attach to hub
 Unshielded twisted pair usually used to connect workstation to
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hub
Hub takes incoming signal and immediately broadcasts it out
all connected links
Hubs can be interconnected to extend network size
Modular connectors and twisted pair make installation and
maintenance of star-wired bus better than standard bus
Hubs can be interconnected with twisted pair, coaxial cable, or
fiber optic cable
Biggest disadvantage: when one station talks, everyone hears
it. This is called a shared network. All devices are sharing the
network medium
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Star-wired Ring Topology
 Logically operates as a ring but physically appears
as a star
 Based on MAU (multi-station access unit) which
functions similarly to a hub
 Where a hub immediately broadcasts all incoming
signals onto all connected links, the MAU passes the
signal around in a ring fashion
 Like hubs, MAUs can be interconnected to increase
network size
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Wireless LANs (I)
 Not really a specific topology
 Workstation in wireless LAN can be anywhere as long as within
transmitting distance to access point
 Several versions of IEEE 802.11 standard defines various forms
of wireless LAN connections
 Two basic components necessary:
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Client Radio - usually PC card with integrated antenna installed in
a laptop or workstation
Access Point (AP) - Ethernet port plus transceiver
 AP acts as bridge between wired and wireless networks
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Can perform basic routing functions
 Single-cell - Workstations reside within a basic service set
 Multiple-cell - Multiple basic service sets create an extended service
set
 Ad-hoc - Wireless LANs configured without access point
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Wireless LANs (II)
IEEE 802.11 – The original wireless standard, transmitting data at 2
Mbps
 IEEE 802.11b – The second wireless standard, transmitting data at 11
Mbps
 IEEE 802.11a – One of the more recent standards, transmitting data
at 54 Mbps using 5 GHz frequency range
 IEEE 802.11g – The other recent standard, also transmitting data at
54 Mbps but using the same frequencies as 802.11b (2.4 GHz)
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IEEE 802.11n (100 Mbps) is last standard that has been widely
implemented.
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Available at both 2.4 & 5 GHz
Latest wireless Ethernet is using MIMO technology (multiple input
multiple output)
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Backwards compatible with 802.11b
Sender and receiver have multiple antennas for optimum reception
IEEE 802.11ac is the latest standard that is gaining momentum
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Operates only on 5 GHz band with data rate up to 6.9 Gbps
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Medium Access Control Protocols
 How does a workstation get its data onto the LAN
medium?
 Medium access control protocol - software that
allows workstations to “take turns” at transmitting
data
 Two basic categories:
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Contention-based protocols
Round robin protocols
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Contention-Based Protocols (I)
 Essentially first come first served
 Most common example:
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Carrier sense multiple access with collision detection (CSMA/CD)
 If no one is transmitting, a workstation can transmit
 If someone else is transmitting, workstation “backs off” and
waits
 If two workstations transmit at same time, collision occurs
 When two workstations hear collision, they stop
transmitting immediately
 Each workstation backs off a random amount of time and
tries again
 Hopefully, both workstations do not try again at exact
same time
 CSMA/CD is an example of a nondeterministic protocol
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Contention-Based Protocols (II)
 Wireless CSMA/CA (Collision avoidance)
 Protocol does not listen and detect collisions
 Instead, tries to avoid collisions before they happen
 How does CSMA/CA do this?
 All devices, before they transmit, must wait an amount of
time called an interframe space (IFS)
 Some applications have a short IFS, while others have a
long IFS
 If two applications want to transmit at same time, the
application with shorter IFS will go first. If medium is idle
after IFS, a random backoff counter is selected and
transmission starts after the countdown.
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Round Robin Protocols
 Each workstation takes turn transmitting: turn is
passed around the network from workstation to
workstation
 Most common example is token ring LAN in which a
software token is passed from workstation to
workstation
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Token ring is an example of a deterministic protocol
Token ring more complex than CSMA/CD
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What happens if token is lost? Duplicated? Hogged?
Token ring LANs are losing the battle with CSMA/CD LANs
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IEEE 802
 To better support local area networks, data link
layer of the OSI model was broken into two
sublayers:
1.
2.
Logical link control sublayer
Medium access control sublayer
 Medium access control sublayer defines the frame
layout
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More closely tied to specific medium at physical layer
 Thus, when people refer to LANs they often refer to
its MAC sublayer name, such as 10BaseT
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IEEE 802 Frame Formats
 IEEE 802 suite of protocols defines frame formats
for CSMA/CD (IEEE 802.3), CSMA/CA (IEEE 802.11),
and token ring (IEEE 802.5)
 Each frame format describes how data package is
formed
 If a CSMA/CD network connects to a token ring
network, frames have to be converted from one to
another
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Frame Formats
IEEE 802.3 CSMA/CD
IEEE 802.11 CSMA/CA
IEEE 802.5 Token Ring
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LAN Systems
 Ethernet or CSMA/CD
 IBM Token Ring
 FDDI (Fiber Distributed Data Interface)
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Ethernet
 Originally, CSMA/CD was 10 Mbps.
 Then 100 Mbps was introduced. Most NICs sold
today are 10/100 Mbps.
 Then 1000 Mbps (1 Gbps) was introduced.
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Transmission is full duplex (separate transmit and receive),
thus no collisions.
Prioritization is possible using 802.1p protocol.
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Topology can be star or mesh (for trunks).
Cabling can be either UTP or optical.
Where 10 Mbps Ethernet has less than 30% utilization due
to collisions, 1000 Mbps is limited only by traffic queuing.
Distance with 10 Mbps is limited by CSMA/CD propagation
time, whereas 1000 Mbps is limited only by media.
 10 Gbps is now beginning to appear.
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Power & Ethernet
 What if you have a remote device that has an
Ethernet connection?
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It will require a power connection
 What if you don’t have an electrical outlet nearby?
 Use PoE
 Power to drive Ethernet NIC is sent over wiring along with
usual Ethernet signals
 Ethernet over power line
 Uses existing power lines in the building
 No new wiring needed
 Slower
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IBM Token Ring
 Deterministic LAN offered at speeds of 4, 16 and 100
Mbps.
 Very good throughput under heavy loads.
 More expensive components than CSMA/CD.
 Losing ground quickly to CSMA/CD. May be extinct
soon.
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FDDI
 Based on the token ring design using 100 Mbps fiber
connections.
 Allows for two concentric rings - inner ring can
support data travel in opposite direction or work as
backup.
 Token is attached to the outgoing packet, rather
than waiting for the outgoing packet to circle the
entire ring.
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Interconnection
 Necessary to connect a local area network to
another local area network or to a wide area
network.
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LAN-to-LAN connections are often performed with a
bridge-like device.
LAN-to-WAN connections are usually performed with a
router.
A switch can be used to interconnect segments of a local
area network.
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Why Segment or Interconnect?
 To separate / connect one corporate division with
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another
To connect two LANs with different protocols
To connect a LAN to the Internet
To break a LAN into segments to relieve traffic
congestion
To provide a security wall between two different
types of users
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Hubs
 Interconnects two or more workstations into a local
area network.
 When a workstation transmits to a hub, the hub
immediately resends the data frame out all
connecting links.
 A hub can be managed or unmanaged.
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A managed hub possesses enough processing power that it
can be managed from a remote location.
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Hub issues
 Maximum distance between devices (100m in
10Base-T)
 Must avoid loops between connected hubs
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message would circulate endlessly
 Number of devices on network increases collision
risks
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collisions during peak traffic periods can crash the network
(200 devices)
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Bridges
 Connect two similar LANs, such as two CSMA/CD
LANs.
 Connect two closely similar LANs, such as a
CSMA/CD LAN and a token ring LAN.
 Examines the destination address in a frame and
either forwards this frame onto the next LAN or
does not.
 Examines the source address in a frame and places
this address in a routing table, to be used for future
routing decisions.
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Transparent Bridge
 Does not need programming but observes all traffic and builds
routing tables from observation.
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The observation is called backward learning.
 Each bridge has two connections (ports) and there is a routing
table associated with each port.
 Observes each frame that arrives at a port, extracts the source
address from the frame, and places that address in the port’s
routing table.
 Found with CSMA/CD LANs.
 Can also convert one frame format to another.
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Sometimes refereed to as a gateway or sometimes a router.
 Removes the headers and trailers from one frame format and
inserts (encapsulates) the headers and trailers for the second
frame format.
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Remote Bridge
 Passing a data frame from one LAN to another when the two
LANs are separated by a long distance and there is a wide area
network connecting the two LANs.
 Takes the frame before it leaves the first LAN and encapsulates
the WAN headers and trailers.
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When the packet arrives at the destination remote bridge, that
bridge removes the WAN headers and trailers leaving the original
frame.
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Switches (I)
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Combination of a hub and a
bridge.
Can interconnect two or more
workstations, but like a
bridge, it observes traffic flow
and learns.
When a frame arrives at a
switch, the switch examines
the destination address and
forwards the frame out the
one necessary connection.
Workstations that connect to a
hub are on a shared segment.
Workstations that connect to a
switch are on a switched
segment.
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Switches (II)
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The backplane of a switch is fast
enough to support multiple data
transfers at one time.
A switch that employs cut-through
architecture is passing on the
frame before the entire frame has
arrived at the switch.
Multiple workstations connected to
a switch use dedicated segments.
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This is a very efficient way to isolate
heavy users from the network.
A switch can allow simultaneous
access to multiple servers, or
multiple simultaneous connections
to a single server.
Using a pair of routers, it is
possible to interconnect to
switched segments, essentially
creating one large local area
network
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Virtual LANs
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Logical subgroup within a LAN that is created via switches and software rather
than by manually moving wiring from one network device to another
Even though employees and their actual computer workstations may be
scattered throughout the building, LAN switches and VLAN software can be
used to create a “network within a network”
A relatively new standard, IEEE 802.1Q, was designed to allow multiple
devices to intercommunicate and work together to create a virtual LAN
Instead of sending technician to a wiring closet to move a workstation cable
from one switch to another, an 802.1Q-compliant switch can be remotely
configured by a network administrator
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Full Duplex Switches
 Allows for simultaneous
transmission and reception
of data to and from a
workstation
 This full duplex connection
helps eliminate collisions
 To support a full duplex
connection to a switch, at
least two pairs of wires are
necessary
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One for the receive
operation
One for the transmit
operation
Most people install four
pairs today, so wiring is
not problem
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Link Aggregation
 Combining multiple physical connection into
one logical connection
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Increase connection speed
Fault tolerance
IEEE 802.3ad-2000
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Spanning Tree Algorithm
 In large network, a loop can be created where a
frame can circle through the network and back to
the originating device
 The spanning tree algorithm (used in Spanning Tree
Protocol and now Rapid Spanning Tree Protocol)
runs in switches and can identify loops and remove
them
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Identify a switch as the root switch
Visit each switch and identify the one port (RP) that has
the shortest path back to the root switch.
Visit each LAN and identify the port (DP) that provides the
shortest path back to the root switch.
Mark the remaining unidentified ports as Removed in the
forwarding tables.
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Quality of Service (QoS)
 Set priority for each frame
 The 802.1p adds a 3-bit field (PCP) to each Ethernet
frame
PCP Value
0
1
2
3
4
5
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Traffic Type
Best effort
Background (lowest priority)
Excellent effort
Critical applications
Video
Voice
Internetwork control
Network control (highest priority)
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Routers
 Router - device that connects a LAN to a WAN or a
WAN to a WAN
 Router:
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Accepts outgoing packet
Removes any LAN headers and trailers
Encapsulates necessary WAN headers and trailers
 Because router has to make wide area network
routing decisions  router has to dig down into the
network layer of the packet to retrieve network
destination address
 Routers are often called “layer 3 devices”
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Operate at the third layer, or OSI network layer, of the
packet
 Often incorporate firewall functions
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