A) Introduction
(201 – 227)
a. Local area network (LAN) – a communication
network that interconnects a variety of data communications devices within a
small geographic area and transmits data at high data transfer rates
B) Primary
Function of Local Area Networks
a. File server – when a network performs
file serving and is connected to a workstation with a large storage disk drive
that acts as a central storage repository
b. Print server – provides workstations
with authorization to access a particular printer, accepts and queues print
jobs, prints cover sheets, and allows users access to the job queue for routine
administrative functions
C) The First
Local Area Network: The BUS/Tree
a. Bus/tree local area network (bus LAN) – first physical
design when LANs became commercially available. It is simply a linear coaxial
cable which multiple devices or workstations tap
i. Tap – it is a passive device, as it
does not alter the signal and does not require electricity to operate
ii. Network interface card (NIC) –
electronic device, sometimes in the form of a computer circuit board or part of
a larger circuit board, that performs the necessary signal conversions and
protocol operations that allow the workstation to send and receive data on the
network
iii. Baseband technology
is bidirectional, meaning that when
the signal is transmitted from a given workstation, the signal propagates away
from the source in both directions on the cable
iv. Trees – splitting and joining broadband
cables and signals to create configurations more complex than a single linear
bus. These more complex bus topologies consisting of multiple interconnected
cable segments are called trees
D) A More
Modern LAN
a. Star-wired bus LAN – most popular
configuration for a local area network today
i. Logical design – a network
determines how the data moves around the network from workstation to
workstation
ii. Physical design – pattern formed
by the locations of the elements of the network, as it would appear if drawn on
a sheet of paper
iii. Hub – nonintelligent device that
simple and immediately retransmits the data it receives from any workstation
out to all other workstations (or devices) connected to the hub
iv. Shared network – when two
or more hubs are interconnected and a workstation transmits data, all the
workstations connected to all the hubs receive the data. All devices on the
network are sharing the one bandwidth
v. Medium access control protocol – software that
allows a device to place data onto a hub-based local area network (as well as
other networks that require their workstations to compete for access to the
network)
1. Contention-based
protocols – carrier sense multiple access with collision detection
2. Round-robin
protocols – such as token passing
b. Contention-based protocols – first come,
first served protocol. The first station to recognize that no other stations is
transmitting data and places it data onto the medium is the first station to
transmit
i. Carrier sense multiple access with collection
detection (CSMA/CD) – only one workstation at a time can transmit and
because of this, it is a half-duplex protocol
ii. Collision – occurs when two or more
workstations listen to the medium at the same moment, hear nothing, and then
transmit their data at the same moment
iii. Collision window – a workstation
will not hear a collision until its data has, on average, traveled halfway down
the bus, collided with the other workstation’s signal, and then propagate back
down the bus to the first workstation, this interval is the collision window
iv. Nondeterministic
protocol – one in which you cannot calculate the time at which a workstation
will transmit
E) Switches – uses addresses and processing
power to direct a frame out a particular port, thus reducing the amount of
traffic on the network
a. Components
of a switch
i. Transparent – switches that learn by
themselves which NICs are on their network
ii. Backward learning – observing
the location from which a frame has come
iii. Backplane – the main hardware of the
switch which must be fast enough to support the aggregate or total bandwidth of
all the ports
iv. Hot swappable – possible to insert and
remove cards while the power to the unit is still on
v. Cut-through architecture – the data
frame begins to exit the switch almost as soon as it begins to enter the
switch. It does not store the data frame, it just forwards it
vi. Store-and-forward device – holds in
the entire frame for a small amount of time while various fields of the frame
are examined, a procedure that diminishes the overall network throughput
vii. Shared segment network – a switch
may be connected to a hub (or several hubs), which then connects multiple workstations,
because they are connected to a hub, they are sharing one channel, thus
limiting bandwidth
viii. Dedicated segment network – a switch
may be directly connected to one or more workstations. Each workstation then
has a private or dedicated connection
b. Isolating Traffic patterns and providing multiple access
c. Full-duplex switch – allow for
a CSMA/CD network to simultaneously transmit and receive data to and from a
workstation
d. Virtual LAN (VLAN) – logical
subgroup within a local area network created via switches and software rather
than by manually moving wiring from one network device to another
e. Link aggregation – allows you
to combine two or more data paths, or links, into one higher-speed link
f. Spanning tree algorithm – consists
of four steps
i. First,
designate a root switch
ii. Second,
visit each switch, at each switch, identify the port/connection that leads you
back to the root switch in the fewest number of hops
iii. Third,
visit each local area network, at each network, identify the port/connection
that leads you back to the root switch in the fewest number of hops
iv. Finally,
the ports that remain without either an RP designation or a DP designation can
be removed, which means updating it so that data is not passed through those
ports
v. Rapid spanning tree protocol (RSTP) – can recompute
the spanning tree algorithm in roughly a few seconds
g. Quality of
service
F) Wired Ethernet
a. Ethernet – first commercially available
local area network system and remain the most popular today
b. Power over Ethernet (PoE) – sending electrical
power over the Ethernet connection
G) Wired
Ethernet Frame Format
a. Medium access control (MAC) sublayer – works closely
with the physical layer and contains a header, computer (physical) addresses,
error-detection codes, and control information
b. Logical link control (LLC) sublayer –
responsible for logical addressing and providing error control and flow control
information
c. Runts (frame fragments) – frames that are
shorter than 64 byes and are automatically discarded
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