CIT 524: Introduction
to Computer Networks
Mohammed A. Saleh
http://ifm.ac.tz/staff/msaleh/CIT524.html
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WAN Technologies
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Many technologies are used to create today's wide area
networks (WANs)
Each of these technologies has pros and cons, making
some of them well suited for certain environments and
completely impractical in others
Each of the technologies varies in terms of media,
speed, availability, and cost
WANs span great distances compared to LANs , which
are restricted in a single area
One of the most important element of WAN technologies
are the switching methods.
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Switching Methods
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In order for systems to communicate on a network, there
has to be a communication path or multiple paths
between which the data can travel
To communicate with another entity, these paths move
the information from one location to another and back
This is the function of switching
Switching provides communication pathways between
two endpoints and manages how data is to flow between
these endpoints
Two of the more common switching methods used today
include:
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Packet switching
Circuit switching
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Packet Switching
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Messages are broken down into smaller pieces called
packets
Each packet is assigned source, destination, and
intermediate node addresses
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Packets are required to have this information because they do
not always use the same path or route to get to their intended
destination
The biggest advantage of packet switching is
independent routing
Independent routing allows for a better use of available
bandwidth by letting packets travel different routes to
avoid high-traffic areas
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Cont …
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It also allows packets to take an alternate route if a
particular route is unavailable for some reason.
In a packet-switching system, when packets are sent
onto the network, the sending device is responsible for
choosing the best path for the packet
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This path might change in transit, and it is possible for the
receiving device to receive the packets in a random or non
sequential order
When this happens, the receiving device waits until all the data
packets are received, and then it reconstructs them according to
their built-in sequence numbers.
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Cont …
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The receiving computer reassembles the packets into
the original message
Packet-switching
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Cont …
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1.
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Two types of packet-switching methods are used on
networks:
Virtual-Circuit Packet Switching
A logical connection is established between the source
and the destination device
This logical connection is established when the sending
device initiates a conversation with the receiving device
The logical communication path between the two
devices can remain active for as long as the two devices
are available or can be used to send packets once
After the sending process has completed, the line can be
closed.
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Cont …
2.
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Datagram Packet Switching
Datagram packet switching does not establish a logical
connection between the sending and transmitting
devices
The packets in datagram packet switching are
independently sent
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they can take different paths through the network to reach their
intended destination
To do this, each packet must be individually addressed
to determine where its source and destination are.
This method ensures that packets take the easiest
possible routes to their destination and avoid high-traffic
areas.
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Circuit Switching
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It requires a dedicated physical connection between the
sending and receiving devices.
Most commonly used in a telephone conversation in
which the parties involved have a dedicated link between
them for the duration of the conversation.
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When either party disconnects, the circuit is broken and the data
path is lost
The sending system establishes a physical connection
The data is transmitted between the two, and when the
transmission is complete, the channel is closed.
The primary advantage is:
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after a connection is established, there is a consistent and
reliable connection between the sending and receiving device
This allows for transmissions at a guaranteed rate of transfer. 9
Cont …
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Circuit switching has downsides:
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a dedicated communication line can be very inefficient
After the physical connection is established, it is unavailable to
any other sessions until the transmission is complete
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Cont …
Circuit-switched network
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Fiber Distributed Data Interface
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This is a topology standard that uses fiber-optic cable
and token-passing media access.
Transmissions speeds of up to 100Mbps at distances of
more than 2 kilometers
FDDI combines the strengths of Token Ring, the speed
of Fast Ethernet, and the security of fiber-optic cable
Such advantages make FDDI a strong candidate for
creating network backbones and connecting private
LANs to create MANs and WANs.
Unlike the regular 802.5 network standard, FDDI uses a
dual-ring configuration
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Cont …
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The first, or primary, ring is used to transfer the data
around the network, and the secondary ring is used for
redundancy and fault tolerance
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the secondary ring waits to take over if the primary ring fails
If the primary ring fails, the secondary ring kicks in automatically,
with no disruption to network users.
The other advantage is it using fiber-optic cable as its
transmission media
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resistance to EMI, the security offered by fiber, and the longer
distances available with fiber cable
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Cont …
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Strong points of FDDI
Fault-tolerant design
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Speed
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By using a dual-ring configuration, FDDI provides some fault
tolerance
If one cable fails, the other can be used to transmit the data
throughout the network
Use of multiple tokens Unlike the IEEE 802.5 standard, FDDI
uses multiple tokens, which increase the overall network speed.
Beaconing
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FDDI uses beaconing as a built-in error-detection method,
making finding faults, such as cable breaks, a lot easier.
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Cont …
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Downsides of FDDI
High costs
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The costs associated with FDDI and the devices and cable
needed to implement an FDDI solution are very costly; too
costly for many small organizations.
Implementation
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FDDI setup and management can be very complex
Requires trained professionals with significant experience to
manage and maintain the cable and infrastructure.
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Internet Access Technologies
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It has become an integral part of modern business.
There are several ways to obtain Internet access
The type chosen will often depend on the cost as well
as what technologies are available in the area you are
located
We will also look at the terms related to the Internet
We will explore the some of the more common methods
of obtaining Internet access.
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xDSL Internet Access
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DSL is an Internet access method that uses a standard
phone line to provide high-speed Internet access
Most commonly associated with high-speed Internet
access
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because it is less expensive
often used in homes and small businesses.
With DSL, a different frequency can be used for digital
and analog signals
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which means that you can talk on the phone while you're
uploading data.
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xDSL Internet Access
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DSL is an Internet access method that uses a standard
phone line to provide high-speed Internet access
Most commonly associated with high-speed Internet
access
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because it is less expensive
often used in homes and small businesses.
With DSL, a different frequency can be used for digital
and analog signals
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which means that you can talk on the phone while you're
uploading data.
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Use skype while uploading an email document
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Variations of xDSL
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Asymmetric DSL (ADSL)
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The most common of the DSL varieties is ADSL
ADSL uses different channels on the line:
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One channel is responsible for analog traffic
The second channel is used to provide upload access
The third channel is used for downloads
With ADSL, downloads are faster than uploads.
Symmetric DSL (SDSL)
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Offers the same speeds for uploads and for downloads
Most suitable for business applications such as Web hosting,
intranets, and e-commerce
Not widely implemented in the home/small business
environment and cannot share a phone line.
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Variations of xDSL
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Integrated Services Digital Network DSL (IDSL)
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Rate Adaptive DSL (RADSL)
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This is a variation on ADSL that can modify its transmission
speeds based on the signal quality
RADSL supports line sharing
Very High Bit Rate DSL (VHDSL)
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This is a symmetric type of DSL that is commonly used in
environments where SDSL and ADSL are unavailable
IDSL does not support analog phones
This is an asymmetric version of DSL and, as such, can share a
telephone line.
High Bit Rate DSL (HDSL)
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This is a symmetric technology that offers identical transmission
rates in both directions.
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HDSL does not allow line sharing with analog phones
Cont …
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Why are there are so many DSL variations?
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Each flavor of DSL is aimed at a different user, business, or
application.
Businesses with high bandwidth needs are more likely
to choose a symmetric form of DSL
Budget-conscious environments such as home offices
are likely to opt for an option that allows phone line
sharing at the expense of bandwidth
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Maximum Speeds of DSL
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Cable Internet
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This is an always on Internet access method that is
available in areas that have digital cable television
Attractive to many small businesses and home office
users because it is both inexpensive and reliable
Most cable providers do not restrict how much use is
made of the access
Connectivity is achieved by using a device called a
cable modem
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it has a coaxial connection for connecting to the provider's
outlet and an Unshielded Twisted Pair (UTP) connection for
connecting directly to a system or to a hub, switch or WAP.
Providers often supply a cable modem free of charge
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You are paying for the rental of the modem in a monthly service
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fee
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Most cable modems supply a 10Mbps Ethernet
connection for the home LAN.
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The actual speed of the connection can vary somewhat
depending on the utilization of the shared cable line in
your area
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You wouldn't expect the actual Internet connection to reach
these speeds
In day-to-day application, data rates range from 1.5Mbps to
3Mbps.
One of the biggest disadvantages of cable access
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You share the available bandwidth with everyone else in your
cable area
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Cont …
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Most cable modems supply a 10Mbps Ethernet
connection for the home LAN.
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The actual speed of the connection can vary somewhat
depending on the utilization of the shared cable line in
your area
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You wouldn't expect the actual Internet connection to reach
these speeds
In day-to-day application, data rates range from 1.5Mbps to
3Mbps.
One of the biggest disadvantages of cable access
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You share the available bandwidth with everyone else in your
cable area
As a result, during peak times, performance of a cable link
might be poorer than in low-use periods
In residential areas, busy times are evenings and weekends
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Broadband Security Considerations
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Whether using DSL or cable Internet access, there are
a few things to keep in mind
Each of these technologies offers always on service
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This means that even when you are away from your computer,
it is still on the Internet
This creates a security risk
The longer you are online, the more chance someone has of
remotely accessing your system.
The operating systems we use today all have some
security holes through which some people are waiting to
exploit
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Combining OS security holes with an always on Internet
technology is certainly a dangerous mix
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Cont …
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DSL and cable Internet connections have to be
protected by mechanisms such as firewalls to protect
the system
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The firewall system will offer features such as packet filtering
The firewall can be a third-party software application installed
on the system, or it can be a hardware device
It is equally important to ensure that the operating system you
are using is completely up-to-date in terms of service packs and
security updates
Today's client systems typically offer automatic update features
that will alert you when a new security update is available.
Following a few safety rules, both DSL and cable Internet can
provide safe Internet access
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Satellite Internet Access
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DSL and cable Internet access technologies are not
offered everywhere
For areas where cheaper broadband options are not
available
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There are a limited number Internet options
One of the primary ones is Internet via satellite.
Satellite access provides a viable Internet access
solution for those who cannot get other methods of
broadband
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Satellite Internet offers an always on connection with theoretical
speeds advertised anywhere from 512Kbps upload speeds to
2048Kbps download speeds
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Cont …
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One of the primary drawbacks to satellite Internet is the
cost
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Satellite Internet is slower and more costly than DSL or
cable, it offers some very attractive features
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Even with the high price tag, it is not as fast as DSL or cable
modem
Portability - wherever you go, you can have Internet access,
this is beneficial for remote users and clients
Satellite internet access is also considered as a
wireless technology where atmospheric conditions can
significantly affect the performance of satellite Internet
access
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Wireless Internet Access
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It is provided by a Wireless Internet Service Provider
(WISP)
The WISP provides public wireless Internet access
known as hotspots
Hotspots provide Internet access for mobile network
devices such as
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laptops, handheld computers, and cell phones in airports,
coffee shops, conference rooms, and so on
A hotspot is created using one or many wireless access
points near the hotspot location
As of today, hotspots are not everywhere, but finding
them is not difficult
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airports, hotels, and coffee shops will advertise that they offer
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Internet access for customers or clients
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Establishing a connection to a wireless hotspot is a
straightforward process.
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If not equipped with built-in wireless capability, laptops will
require an external wireless adapter card
With the physical requirements of the wireless card
taken care of, connect as follows:
1.
2.
When you arrive at the hotspot site, power up your laptop. In
some instances, you might need to reboot your system if it was
on standby to clear out old configuration settings
The card might detect the network automatically. If this is the
case, configuration settings, such as the SSID, will be
automatically detected, and the wireless Internet will be
available. If Internet access is free, there is little else to do; if it
is a paid-for service, you will need to enter a method of
payment. One thing to remember is to verify that you are using
encryption for secure data transfer.
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Cont …
1.
2.
3.
When you arrive at the hotspot site, power up your laptop. In
some instances, you might need to reboot your system if it was
on standby to clear out old configuration settings
The card might detect the network automatically. If this is the
case, configuration settings, such as the SSID, will be
automatically detected, and the wireless Internet will be
available. If Internet access is free, there is little else to do; if it
is a paid-for service, you will need to enter a method of
payment. One thing to remember is to verify that you are using
encryption for secure data transfer.
If for some reason the wireless settings are not automatically
detected, you will need to open up your wireless NICs
configuration utility and manually set the configurations. These
settings can include setting the mode to infrastructure, inputting
the correct SSID, and setting the level of encryption used.
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Cont …
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Some companies such as hotels and cafes will provide
wireless Internet access by connecting a wireless router
to a DSL or cable Internet connection
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The router becomes the wireless access point to which the
users connect
It allows clients to connect to the Internet through the
broadband connection
The technology is based on the 802.11 standards, typically
802.11b/g, and client systems require only an internal or
external wireless adapter.
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Questions