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Introduction
• LAN, WAN, MAN Characteristics
• LAN Topologies
- Ring
- Bus
- Star
• Wan Architectures
- Point-to-Point
- Circuit Switching
- Packet Switching
- Cell Switching
• Simple and Fully meshed WAN
• WAN selection criteria
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LAN Characteristics
A local-area network (LAN) is a high-speed, fault tolerant
data network that covers a relatively small geographic area. It
typically connects workstations, personal computers, printers,
and other devices.
LANs offer computer users many advantages, including
shared access to devices and applications, file exchange
between connected users, and communication between users
via electronic mail and other applications.
LAN protocols function at the lowest two layers of the OSI
reference model: the physical layer and the data link layer.
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LAN topologies define the manner in which network devices are
organized. There are three commonly used LAN topologies: bus,
ring, and star.
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LAN protocols typically use one of two
methods to access the physical network
medium: Carrier sense multiple access
collision detect (CSMA/CD) and Token
Passing. These methods or algorithms
determine the order in which the connected
devices can use the LAN.
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A wide-area network (WAN) is a data
communications network covering a
relatively broad geographic area and often
using transmission facilities provided by
the common carriers (telephone
companies). Very often a WAN consists
of several bridged (connected) LANS. In
a WAN, you may see hybrid networks;
that means that there is a combination of
LANs of different topologies, connected
by using a WAN technology. A WAN can
span the globe.
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WAN technologies function at the lower
three layers of the OSI reference model: the
physical layer, the data link layer, and the
network layer.
There are four general classifications of
WAN technologies:
1. WAN point-to-point links
2. Circuit switched WANs
3. Packet switched WANs
4. Cell switched WANs
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1. A point-to-point link provides a single,
pre-established WAN communications path
from the customer premises, through a
carrier network (the telephone company), to
a remote network. Point-to-point links are
also known as leased lines. The established
path is permanent and is fixed for each
remote network reached through the carrier
facilities. Point-to-point links are reserved
by the carrier company for the private use
of the customer.
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2. Circuit switching is a WAN
switching method in which a dedicated
physical circuit through a carrier
network is established, maintained,
and terminated for each
communication session. Circuit
switching, used extensively in
telephone company networks, operates
much like a normal telephone call.
Integrated Services Digital Network
(ISDN) is an example of a circuitswitched WAN technology.
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3. Packet switching is a WAN switching
method in which network devices share a
single point-to-point link to transport
packets from a source to a destination
across a carrier network. For this purpose,
data to be transmitted needs to he broken
into smaller data packets. Statistical
multiplexing is used to allow devices to
share these circuits.
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4. Cell switching is sometimes considered as a
special packet switching method. Like with
packet switching, cell switching requires that
the data to be transmitted is broken into data
packets, but these packets are of a fixed, small
size, the cells. Because cells are fixed- length,
they can be processed and switched in
hardware at high speeds. Cell relay is the
basis for many high-speed network protocols
including ATM. ATM (Asynchronous Transfer
Mode) is a cell switching protocol.
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A metropolitan-area network (MAN) is a communications
network that serves an urban area An example of a MAN
would be the connection of several LANs in several
buildings of an. enterprise. There are no MAN-specific
protocols or definitions.
Nevertheless FDDI (Fiber Distributed Data Interface) could
be considered as a typical MAN protocol. FDDI specifies a
high-speed token-passing ring LAN, using fiber optic
media. FDDI was created to fill the need for a highbandwidth, secure, local-to-medium area network. See
module 5 for further specifications. On a campus site,
connecting buildings may not be possible using LAN
technologies due to the distance involved. Very often a
FDDI network is installed to provide a high-speed
backbone.
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The terms LAN, WAN and MAN are used to
distinguish the size of a network in terms of the
distance it spans. As there are different requirements
needed by the different media and devices depending
on the distances to be served, there are some protocols
that have been especially designed for short distance
networks, others for medium or long distance networks.
Consequently, they are usually considered as typical
LAN protocols or as typical for a WAN. In this sense
the terms LAN, WAN and MAN are used to categorize
the networks and the used protocols.
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The word LAN is used when we are talking about a
limited geographic area, as well as when we talk about
the typical protocols like Ethernet and Token Ring.
Generally, a MAN spans a larger geographic area than
a LAN, but a smaller geographic area than a WAN.
FDDI would be a typical MAN protocol.
A data communications network that serves users
across a broad geographic area and often uses
transmission devices provided by common carriers is
called a WAN. Frame Relay and X.25 are examples of
WANS.
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We will now look at the three common LAN topologies.
When using the terms Ring, Bus and Star, we are referring
to logical topologies (i.e. logically how the topology
functions). The physical topology may be different.
For an Ethernet environment, where a single coaxial cable
is connecting all systems on the network, the physical and
logical topology is identical. However, there are other
examples. The typical Token Ring cabling looks like a star
configuration, since one cable is connecting each system to
an access point of the ring. The same is true for another
type of Ethernet bus topology cabling, Ethertwist, which
uses the same configuration, a single cable from the system
to an access point of the bus.
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In a ring, the network cable passes from one system to
another, until they are interconnected to form a ring.
Between each neighboring system, there is a direct
point-to-point link. Sharing of the ring between the
systems is ensured by appropriate medium access
control algorithms. A typical network using ring
topology is Token Ring. In the Token Ring example, the
physical connection is made using an MSAU (MultiStation Access Unit) and appears as a physical STAR
connection.
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In the case of FDDI which uses a Logical Ring
(as for Token Ring) the physical representation
may be as:
• Physical Star (when using a FDDI
concentrator)
or
• Physical Ring (in the case of a dual attached
FDDI Ring).
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Mostly, a single network cable is passed along all
locations that require systems to be connected to the
network. Each system has a physical connection to the
cable so that the systems can access the network in
parallel. The available transmission bandwidth is
shared between the systems by the use of appropriate
medium access control algorithms.
A typical network using bus topology is Ethernet.
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In a star, a networking device in the center of the
network is connected to all systems via a direct
point-to-point link. Sharing of the star between
the systems is controlled by the networking
device in the center. A typical network using star
topology is 100VG AnyLAN.
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Token Ring networks are used mostly for technical and
office environments and for IBM Mainframe systems. In
a ring network, data flows from system to system,
always in one direction, preceded by a token, giving the
network its name. The characteristics of a Token Ring
network are illustrated in the slide.
Token Ring is deterministic, each station on the ring is
guaranteed an opportunity to transmit data at regular
intervals. It is a non-contention access control method,
and looks similar to a polling method as stations can
only transmit data when given authority to do so. As
only the station holding the token can transmit data,
token ring networks never experience collisions.
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The token (a special sequence of three bytes) constantly
circulates around the network from one station to the next. Only
the station holding the token is allowed to transmit data onto the
ring. The token circulates around the ring, in an idle state, until a
station wants to transmit data. The station waits to receive the
token, removes it from the network, and then transmits its data.
The recipient copies the data from the network and allows the
frame to carry on to the sender. If a node MAC address matches
the source address of the packet, it removes the packet from the
network and regenerates a new token.
The Token Ring Network uses a totally different access method
& protocol than the rest  special type of bridge is needed to
connect it to the rest of the network.
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Note: Hubs are
also called multiport repeaters.
Ethernet, Token
Ring, etc.,
describe how the
data is framed
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Ethernet is the most widespread bus network. It can be
implemented as a coaxial cable (ThinLAN) or using telephonestyle twisted pair cabling (Ethertwist). The characteristics of an
Ethernet network are illustrated in the slide.
Ethernet allows multiple stations to access the transmission
medium without prior coordination, using a carrier sense access
method (CSMA/CD) to govern access to the network.
Ethernet is a half-duplex, non-deterministic technology.
The non-deterministic operation of Ethernet networks can be fast
and efficient, but as the amount of traffic increases, collisions
increase, performance drops, and throughput decreases.
When a node is accessing the network, all other nodes must be in
receive mode. When two or more signals exist on the LAN (nodes
transmitting at the same time), a collision results; after a collision,
each node involved in the collision waits a random amount of time,
and then retransmits its information onto the LAN.
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100VG AnyLAN is a networking technology similar to
standard Ethernet running at much higher speed. It is
implemented using the same type of twisted pair cabling
as in the Ethernet network example. In a 100VG
AnyLAN network, access to the network is controlled by
intelligent network devices that allot transmission time to
the connected devices instead of having the systems
compete. AnyLAN brings increased overall bandwidth
(many users need to exchange a lot of information),
brings individual increased bandwidth (an application
exchanging very large amounts of information, e.g.
database, imaging, desktop publishing applications or
network printing) and allows time-sensitive applications
(real-time video requires continuous transfer of packets
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with minimal delay).
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A Wide Area Network, WAN, is a communications
network that can support data communications over a
large area such as across a country, or even across the
globe. A WAN is usually made up of a combination of
several LANs and/or other types of data communication
environments and, when properly implemented, should
appear to work in the same way as a LAN.
A LAN may be expanded to a WAN using components
such as Routers, Switches and Bridges.
Exception: In the case of Frame Relay, Frame Relay
Switches are used instead of Routers.
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The various data communication components are linked
together using communication links called WAN links
such as:
• Packet-switching networks
• Fiber-optic cable
• Microwave transmitters
• Satellite links
• Cable television coaxial systems
Wide area telephone networks are prohibitively
expensive for most private companies to utilize and
maintain and are often leased from service providers.
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A point-to-point link provides a single, pre-established WAN
communications path from the customer premises, through a
carrier network (the telephone company), to a remote network.
Point-to-point links are also known as leased lines. The
established path is permanent and is fixed for each remote
network reached through the carrier facilities. Point-to-point links
are dedicated transmission links reserved by the carrier company
for the private use of the customer.
Leased lines can make use of two different types of transmission
facilities:
- Analog transmission
- Digital transmission.
The picture shows point-to-point links using these types of
transmission. further definitions of Analog transmission and
Digital transmission are listed below.
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Analog transmission
One method by which LANs may be linked together is by using
modems across the telephone system (PSTN), using voice-grade
telephone lines. Voice-grade communications transmission
methods are slow, with modems reducing the transmission speed
even further.
Digital transmission
If the volume of WAN transmissions within an Organization is
high, analog transmission will become inefficient and expensive.
For digital transmissions, the device that connects the customer
equipment to the network is no longer a modem, but a device
called a CSU/DSU (Channel Service Unit/Data Service Unit).
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Digital lines are available in several forms including:
• T1 (USA and Japan)
• El (Europe)
• T3
• Fractional T1/El
T1 is a method of point-to-point transmission that uses two-wire
pairs (one pair to send and one to receive) to transmit a fullduplex signal at a rate of 1.544 Mbps. T1 is a very costly WAN
link, utilizing a very large bandwidth; if this bandwidth is not
required, it is possible to subscribe to one or more T1 channels in
64Kbps increments known as Fractional T-1 (FT-1).
El is a Wide area digital transmission scheme used in Europe that
carries data at the rate of 2.048Mbps. E1 lines can be leased for
private use from commercial carriers.
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T3 and Fractional T-3 provide voice and data-grade
services from 6Mbps to 45Mbps and are designed to
transmit large amounts of data, at high speed, between
two fixed points. A T3 line may be used to replace
several T1 lines.
Fractional T1/E1: Wide Area Service Providers offer
incremented digital services for leased lines. Speeds of
56Kbps (USA and Japan) or 64Kbps (Europe) are
supported by Digital Services level 0 (DS0). You may
sometimes hear services referred to as fractional T1/E1.
Fractional T1 service is available in increments of 64,
128, etc.
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Circuit switching is a WAN switching method in which
a dedicated physical circuit through a carrier network is
established, maintained, and terminated for each
communication session.
Circuit switching as opposed to point-to-point provides
multiple access to a network. In this particular case, we
do not need to deal with a pre-established dedicated link
between two physical locations, but we talk about a
circuit established on demand from one physical
location to any other location on the network whenever
a communication takes place.
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The public switched telephone network (PSTN) is a
circuit-switched network. With this type of network, a
communication line is re-opened each time that it is
used and, therefore, the exact route of the transmitted
data cannot be guaranteed. This type of
communication is also referred to as Dial-Up as you
will dial into the network to establish a
communication between two locations.
The quality of the data transmission is governed by
the quality of each of the links involved invariably
introducing inconsistencies in transmission quality
from one session to the next.
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Circuit switching facilities as previously stated for
point-to-point can also rely on analog transmission or
digital transmission.
An example of Circuit switching relying on digital
transmission facilities is ISDN (Integrated Services
Digital Network) which is covered in more detail on the
next page.
Another example of WAN technology using Circuit
switching is Frame Relay.
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ISDN was designed by CCITT as a set of digital
transmission protocols to allow users to send multiple
channels (voice, video, and data) across a single link. ISDN
was originally developed as the digital replacement for the
Public Switched Telephone Network (PSTN), to link
Businesses and Homes and produces a dial-up service only.
There are several reasons why ISDN is an attractive
communication medium: ISDN is a circuit switched
communication medium which means that you only pay for
the service when you use it. ISDN offers moderate to high
speed communication line rates which makes it attractive
for LAN-to-LAN applications. ISDN is generally available
throughout the world.
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Data Transmission
ISDN transfers data using time division multiplexed channels of which there are
two types:
1. B Channels
2. D channels
It is possible for the two B channels to be combined to provide a combined 128
Kbps data stream. If both end stations also support compression, a much higher
data throughput may be achieved.
ISDN divides its available bandwidth into the two types of data channels, as
detailed in the following table:
Channel
Type
Transmission
Rate
Data Types Supported
B
64 Kbps
Voice
Data
Images
D
16 Kbps
Signaling
Link Management Data
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ISDN Channel Types
There are two types of ISDN interfaces available:
1. Primary Rate Interface
2. Basic Rate Interface
Primary Rate Interface: Primary Rate ISDN divides the total
TI/El bandwidth to provide either: 23 B channels and 1 D
channel (America and Japan), or 30 B channels and 1 D channel
(Europe).
Basic Rate Interface: Basic Rate ISDN divides its available
bandwidth to provide 2 B channels and 1 D channel.
Note: Cable Modem and DSL are better now than ISDN
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Other WAN technologies have been developed over the
last few years in order to face the ever increasing
demand for more speed and bandwidth which is
imposed by applications like multimedia, backup and
software distribution over the network.
Synchronous Optical Network (SONET) is another
circuit switching example. It employs fiber-optic
technology and is capable of transmitting voice, data and
video at speeds of greater than one gigabit per second.
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SONET is a standard for optical transmission that has
been formulated by the Exchange Carriers Standards
Association (ECSA) for the American National
Standards Institute (ANSI), and has been incorporated
into the Synchronous Digital Hierarchy
recommendations of the CCITT (or International
Telecommunications Union-ITU), which sets the
standards for international telecommunications.
SONET defines optical carrier (OC) levels and
electrical equivalent synchronous transport signals
(STSs) for the fiber-optic based transmission hierarchy.
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SONET uses a basic transmission rate of STS- 1, which is
equivalent to 51.84 Mbps. However higher level signals are
achievable and are of integer multiples of the base rate. For
example, STS-3 is three times the rate of STS-1 (3 X 51.84 =
155.52 Mbps). An STS-12 would be at a rate of 12 X 51.84 =
622.08 Mbps.
SONET is flexible enough to be used as the underlying transport
layer for BISDN ATM cells. BISDN (Broadband Integrated
Services Digital Network) is a single ISDN network that can
support voice, data and video services. ATM is a CCITT standard
that supports cell-based voice, data, video and multimedia
communication in a public network under BISDN. The ATM
forum is aligning with SONET as the transport layer for cell-based
traffic.
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Synchronous Digital Hierarchy (SDH) is the European version of SONET. It is a
European standard that defines a set of rate and format standards which are
transmitted using optical signals over fiber. SDH is similar to SONET with a basic
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rate of 155.52Mbps (up to 688 Mbps).
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Packet-switching involves transmitting packets of data
from many different users over many different possible
paths. This method of networking is fast, efficient and
reliable, and may be used to transmit data globally.
The data to be transmitted is divided into small packets of
data, each of which is uniquely identified with information
including the destination address and details of its position
in the original data message.
The individual, labeled data packages are then relayed
through stations in a computer network, via the best route
currently available between the source and destination.
These networks are sometimes called any-to-any
connections.
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The data packets may be routed through different
paths, arriving at the destination at different times, and
possibly out of order. The additional information
linked to the data packets ensures that the original
message is reassembled correctly at the destination.
Packet-switching networks will require the computers
and software that control the data transmission to have
the intelligence to manage the routing, assembling and
disassembling of the data packets.
As the data packets are kept small, if a transmission
error should occur, only the one packet will need to be
re-sent, saving on transmission time and call-costs.
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Virtual circuits are used by many packet-switching networks.
These circuits are composed of a series of logical, as opposed to
physical, connections between the originating and destination
computer. These connections may last either as long as the
conversation (temporary connection) or as long as both computers
are up and running (permanent connection). The circuit is
bandwidth allocated on demand and is established and maintained
once the two computers exchange the relevant information to
define the communication parameters. These parameters are
required to ensure reliability and include:
• Maximum message size
• Data transmission path
• Acknowledgements
• Flow control
• Error control
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Switched virtual circuits (SVCs)
These circuits are also known as point-to-many-point
connections. Network resources are dedicated to the route
defined between the end computers and the connection is
maintained until either one of the computers is switched
off (Needs destination address, establishes connection,
sends data, then disconnects [releases line]).
3 types of SVCs exist:
• Incoming SVC [analogy: phone that can be used only to
answer calls]
• Outgoing SVC [analogy: phone that can be used only to
dial out]
• 2-way SVC
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Permanent virtual circuits (PVCs)
PVCs are in effect similar to leased lines that are
permanent or virtual, except that the customer is only
charged for the time that the line is in use (permanent
connection  no need to send an address).
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X.25 is a set of protocols employed by a packet-switching
network to transport data over which other protocols may be
routed or bridged. The packet-switching network provides the
best transmission route available at any time using switches,
circuits and routes.
The original X.25 networks transmitted data over unreliable
telephone lines, resulting in many errors, therefore, extensive
error-checking facilities were incorporated into the X.25
protocol, which appears to slow the transmission of data further.
X.25 protocol now defines the interface between synchronous
packet-mode host and the public data network (PDN) over a
dedicated (leased) line. This interface is referred to as a Data
Terminated Equipment/Data Communications Equipment
interface (DTE/DCE) interface.
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Examples of DTEs (each having the PDN as the DCE
part of the interface) include the following:
• A host computer with an X.25 interface
• A packet assembler/disassembler (PAD)
PADs receive asynchronous characters from a low-speed
terminal and assembles them into packets to be
transmitted over the network.
PADs also disassemble packets received from the
network and delivers them as characters to the terminals.
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Asynchronous transfer mode is a method of packet
switching where fixed-size packets of data are
transmitted over broadband and baseband LANs or
WANS. ATM uses fixed size cells of 53 bytes.
ATM was defined by the CCITT (Consultative
Committee for International Telephony & Telegraphy)
as part of the broadband integrated services digital
network (BISDN). It is capable of transmitting data at
very high speeds, typically between 155 Mbps and 622
Mbps. ATM will support the transmission of the
following data types:
• Data
• Voice
• Video
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Packet switching involves breaking the original data into
smaller, variable-length data packets; ATM also divides
the original data, with the size of the data packets being
fixed at 53 bytes. These 53 byte cells consist of 48 bytes
of application data and 5bytes of ATM header data.
Network equipment can switch, route and transmit these
uniform cells much more quickly than random-sized
data packets. The uniform cells use the buffers more
efficiently and, therefore, improve the efficiency in the
processing of the incoming data.
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Most ATM boards currently transmit data at about 155
Mbps, but has the theoretical capability of transmitting
at 1.2 gigabits per second.
AT'M may be used in both LANs and WANs at
approximately the same speed.
This is layer 2 switching (frames). ATM switches
constantly send out 53 byte cells. No sequence
numbers are assigned. Its job is to deliver the data (and
deliver them fast). It is up to the above layers to notify
the sender if something wrong (error) happens.
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In a simple mesh network (every switch in a forward
sense is connected to at least one other switch), the
nodes are organized in a mesh topology with some
nodes having either a physical or virtual circuit
connecting them to each node in the network, while
other nodes may be connected, (either physically or
virtually) to only one or two nodes.
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In a full mesh network (every switch is connected to
every other switch in the network), the nodes are
organized in a mesh topology with each node having
either a physical or virtual circuit connecting them to
each other node in the network as illustrated in the slide.
• Full mesh networks provide a great deal of
redundancy, however they can be expensive to
establish and maintain. Some WAN backbone networks
implement this type of topology.
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WAN Selection Criteria
* Point-to-point versus multi-point communications
* Speed
* Reliability
* Cost
Note: ISDN 64K to 128K is for BRI (Basic Rate
Interface - Defines an ISD network service consisting
two B channels and one D channel. The PRI ISDN
gives us basically T1 capability (Primary Rate
Interface - defines an ISDN network service
consisting of 23 or 30 channels and one D channel).
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