1.6
Identify the purposes, features and functions of the following network
components:
> Hubs
A hub or
concentrator is a device used to connect all of the computers on a star or ring
network. A hub, is nothing more than a box with a series of cable connectors in
it. Hubs are available in a variety of sizes, from four- and five-port devices
designed for home and small business networks to large rack-mounted units with
up to 24 ports or more. Installing a single hub is simply a matter of
connecting it to a power source and plugging in cables connected to the network
interface adapters in your computers. However, it's important for a network
technician to understand what goes on inside a hub.
Like network interface adapters, hubs are
associated with specific data-link layer protocols. Ethernet hubs are the most
common, because Ethernet is the most popular data-link layer protocol, but
Token Ring MAUs are hubs too, and other protocols, such as the Fiber
Distributed Data Interface (FDDI) also use hubs.
Stackable hubs
|
Small
Hub |
Ethernet
Hubs: An Ethernet hub
is also called a multiport repeater. A repeater is a device that amplifies a signal
as it passes through it, to counteract the effects of attenuation. If, for
example, you have a thin Ethernet network with a cable segment longer than the
prescribed maximum of 185 meters, you can install a repeater at some point in
the segment to strengthen the signals and increase the maximum segment length.
This type of repeater only has two BNC connectors, and is rarely seen these
days. The hubs used on UTP Ethernet networks are repeaters as well, but they
can have many RJ45 ports instead of just two BNC connectors.
When data enters
the hub through any of its ports, the hub amplifies the signal and transmits it
out through all of the other ports. This enables a star network to have a
shared medium, even though each computer has its own separate cable. The hub
relays every packet transmitted by any computer on the network to all of the
other computers, and also amplifies the signals. The maximum segment length for
a UTP cable on an Ethernet network is 100 meters. A segment is defined as the
distance between two communicating computers. However, because the hub also
functions as a repeater, each of the cables connecting a computer to a hub port
can be up to 100 meters long, allowing a segment length of up to 200 meters
when one hub is inserted in the network.
> Switches
Switches are a
special type of hub that offers an additional layer of intelligence to basic,
physical-layer repeater hubs. A switch must be able to read the MAC address of
each frame it receives. This information allows switches to repeat incoming
data frames only to the computer or computers to which a frame is addressed.
This speeds up the network and reduces congestion.
Switches operate
at both the physical layer and the data link layer of the OSI Model.
> Bridges
A bridge is used
to join two network segments together, it allows computers on either segment to
access resources on the other. They can also be used to divide large networks
into smaller segments. Bridges have all the features of repeaters, but can have
more nodes, and since the network is divided, there is fewer computers
competing for resources on each segment thus improving network performance.
Bridges can also
connect networks that run at different speeds, different topologies, or
different protocols. But they cannot, join an Ethernet segment with a Token
Ring segment, because these use different networking standards.
Bridges operate
at both the Physical Layer and the MAC sublayer of the Data Link layer. Bridges
read the MAC header of each frame to determine on which side of the bridge the
destination device is located, the bridge then repeats the transmission to the
segment where the device is located.
> Routers
Are networking devices used to extend or segment networks by forwarding packets from one logical network to another. Routers are most often used in large internetworks that use the TCP/IP protocol suite and for connecting TCP/IP hosts and local area networks (LANs) to the Internet using dedicated leased lines.
Are networking devices used to extend or segment networks by forwarding packets from one logical network to another. Routers are most often used in large internetworks that use the TCP/IP protocol suite and for connecting TCP/IP hosts and local area networks (LANs) to the Internet using dedicated leased lines.
Routers work at
the network layer (layer 3) of the Open Systems Interconnection (OSI) reference
model for networking to move packets between networks using their logical
addresses (which, in the case of TCP/IP, are the IP addresses of destination
hosts on the network). Because routers operate at a higher OSI level than
bridges do, they have better packet-routing and filtering capabilities and
greater processing power, which results in routers costing more than bridges.
Routers contain
internal tables of information called routing tables that keep track of all
known network addresses and possible paths throughout the internetwork, along
with the cost of reaching each network. Routers route packets based on the
available paths and their costs, thus taking advantage of redundant paths that
can exist in a mesh topology network. Because routers use destination network
addresses of packets, they work only if the configured network protocol is a
routable protocol such as TCP/IP or IPX/SPX. This is different from bridges,
which are protocol independent.
Static
routers: These must have
their routing tables configured manually with all network addresses and paths
in the internetwork.
Dynamic
routers: These
automatically create their routing tables by listening to network traffic.
You can use
routers, to segment a large network, and to connect local area segments to a
single network backbone that uses a different physical layer and data link
layer standard. They can also be used to connect LAN's to a WAN's.
> Gateways
A gateway is a
device used to connect networks using different protocols. Gateways operate at
the network layer of the OSI model.
In order to
communicate with a host on another network, an IP host must be configured with
a route to the destination network. If a configuration route is not found, the
host uses the gateway (default IP router) to transmit the traffic to the
destination host. The default t gateway is where the IP sends packets that are
destined for remote networks. If no default gateway is specified, communication
is limited to the local network.
Gateways receive
data from a network using one type of protocol stack, removes that protocol
stack and repackages it with the protocol stack that the other network can use.
Examples
· E-mail gateways—for example, a gateway that
receives Simple Mail Transfer Protocol (SMTP) e-mail, translates it into a
standard X.400 format, and forwards it to its destination
· Gateway Service for NetWare (GSNW), which
enables a machine running Microsoft Windows NT Server or Windows 2000 Server to
be a gateway for Windows clients so that they can access file and print
resources on a NetWare server
· Gateways between a Systems Network Architecture
(SNA) host and computers on a TCP/IP network, such as the one provided by
Microsoft SNA Server
· A packet assembler/disassembler (PAD) that
provides connectivity between a local area network (LAN) and an X.25
packet-switching network
> CSU / DSU (Channel Service Unit / Data Service
Unit)
A CSU/DSU
is a device that combines the functionality of a channel service unit (CSU) and
a data service unit (DSU). These devices are used to connect a LAN to a WAN,
and they take care of all the translation required to convert a data stream
between these two methods of communication.
A DSU
provides all the handshaking and error correction required to maintain a
connection across a wide area link, similar to a modem. The DSU will accept a
serial data stream from a device on the LAN and translate this into a useable
data stream for the digital WAN network. It will also take care of converting
any inbound data streams from the WAN back to a serial communication.
A CSU is similar
to a DSU except it does not have the ability to provide handshaking or error
correction. It is strictly an interface between the LAN and the WAN and relies
on some other device to provide handshaking and error correction.
> NICs (Network Interface Card)
Network
Interface Card, or NIC is a hardware card installed in a computer so it can
communicate on a network. The network adapter provides one or more ports for
the network cable to connect to, and it transmits and receives data onto the
network cable.
Wireless Network
Interface Card
Interface Card
Network Interface
Card
Card
Every networked
computer must also have a network adapter driver, which controls the network
adapter. Each network adapter driver is configured to run with a certain type
of network adapter.
A networked
computer must also have one or more protocol drivers (sometimes called a
transport protocol or just a protocol). The protocol driver works between the
upper-level network software and the network adapter to package data to be sent
on the network.
In most cases,
for two computers to communicate on a network, they must use identical
protocols. Sometimes, a computer is configured to use multiple protocols. In
this case, two computers need only one protocol in common to communicate. For
example, a computer running File and Printer Sharing for Microsoft Networks
that uses both NetBEUI and TCP/IP can communicate with computers using only
NetBEUI or TCP/IP.
> ISDN (Integrated Services Digital Network)
adapters
Integrated
Services Digital Network adapters can be used to send voice, data, audio, or
video over standard telephone cabling. ISDN adapters must be connected directly
to a digital telephone network. ISDN adapters are not actually modems, since
they neither modulate nor demodulate the digital ISDN signal.
Like standard
modems, ISDN adapters are available both as internal devices that connect
directly to a computer's expansion bus and as external devices that connect to
one of a computer's serial or parallel ports. ISDN can provide data throughput
rates from 56 Kbps to 1.544 Mbps (using a T1 carrier service).
ISDN hardware
requires a NT (network termination) device, which converts network data signals
into the signaling protocols used by ISDN. Some times, the NT interface is
included, or integrated, with ISDN adapters and ISDN-compatible routers. In
other cases, an NT device separate from the adapter or router must be
implemented.
ISDN works at
the physical, data link, network, and transport layers of the OSI Model.
> WAPs (Wireless Access Point)
A wireless
network adapter card with a transceiver sometimes called an access point,
broadcasts and receives signals to and from the surrounding computers and
passes back and forth between the wireless computers and the cabled network.
Access points
act as wireless hubs to link multiple wireless NICs into a single subnet.
Access points also have at least one fixed Ethernet port to allow the wireless
network to be bridged to a traditional wired Ethernet network..
> Modems
A modem is a
device that makes it possible for computers to communicate over telephone
lines. The word modem comes from Modulate and Demodulate. Because standard
telephone lines use analog signals, and computers digital signals, a sending
modem must modulate its digital signals into analog signals. The computers
modem on the receiving end must then demodulate the analog signals into digital
signals.
Modems can be
external, connected to the computers serial port by an RS-232 cable or internal
in one of the computers expansion slots. Modems connect to the phone line using
standard telephone RJ-11 connectors.
> Transceivers (media converters)
Transceiver
short for transmitter-receiver, a device that both transmits and receives
analog or digital signals. The term is used most frequently to describe the
component in local-area networks (LANs) that actually applies signals onto the
network wire and detects signals passing through the wire. For many LANs, the
transceiver is built into the network interface card (NIC). Some types of
networks, however, require an external transceiver.
In Ethernet
networks, a transceiver is also called a Medium Access Unit (MAU).
Media converters
interconnect different cable types twisted pair, fiber, and Thin or thick coax,
within an existing network. They are often used to connect newer 100-Mbps,
Gigabit Ethernet, or ATM equipment to existing networks, which are generally
10BASE-T, 100BASE-T, or a mixture of both. They can also be used in pairs to
insert a fiber segment into copper networks to increase cabling distances and
enhance immunity to electromagnetic interference (EMI).
> Firewalls
In computing, a
firewall is a piece of hardware and/or software which functions in a networked
environment to prevent some communications forbidden by the security policy,
analogous to the function of firewalls in building construction.
A firewall has
the basic task of controlling traffic between different zones of trust. Typical
zones of trust include the Internet (a zone with no trust) and an internal
network (a zone with high trust). The ultimate goal is to provide controlled
connectivity between zones of differing trust levels through the enforcement of
a security policy and connectivity model based on the least privilege
principle.
There are
three basic types of firewalls depending on:
· whether the communication is being done between
a single node and the network, or between two or more networks
· whether the communication is intercepted at the
network layer, or at the application layer
· whether the communication state is being tracked
at the firewall or not
With
regard to the scope of filtered communication there exist:
· personal firewalls, a software application which
normally filters traffic entering or leaving a single computer through the
Internet.
· network firewalls, normally running on a
dedicated network device or computer positioned on the boundary of two or more
networks or DMZs (demilitarized zones). Such a firewall filters all traffic
entering or leaving the connected networks.
The latter
definition corresponds to the conventional, traditional meaning of
"firewall" in networking.
In
reference to the layers where the traffic can be intercepted, three main
categories of firewalls exist:
· network layer firewalls An example would be
iptables.
· application layer firewalls An example would be
TCP Wrapper.
· application firewalls An example would be
restricting ftp services through /etc/ftp access file
These
network-layer and application-layer types of firewall may overlap, even though
the personal firewall does not serve a network; indeed, single systems have
implemented both together.
There's also the
notion of application firewalls which are sometimes used during wide area
network (WAN) networking on the world-wide web and govern the system software.
An extended description would place them lower than application layer
firewalls, indeed at the Operating System layer, and could alternately be called
operating system firewalls.
Lastly,
depending on whether the firewalls track packet states, two additional
categories of firewalls exist:
· stateful firewalls
· stateless firewalls
Network
layer firewalls
Network layer
firewalls operate at a (relatively low) level of the TCP/IP protocol stack as
IP-packet filters, not allowing packets to pass through the firewall unless
they match the rules. The firewall administrator may define the rules; or
default built-in rules may apply (as in some inflexible firewall systems).
A more
permissive setup could allow any packet to pass the filter as long as it does
not match one or more "negative-rules", or "deny rules".
Today network firewalls are built into most computer operating system and
network appliances.
Modern firewalls
can filter traffic based on many packet attributes like source IP address,
source port, destination IP address or port, destination service like WWW or
FTP. They can filter based on protocols, TTL values, netblock of originator,
domain name of the source, and many other attributes.
Application-layer
firewalls
Application-layer
firewalls work on the application level of the TCP/IP stack (i.e., all browser
traffic, or all telnet or ftp traffic), and may intercept all packets traveling
to or from an application. They block other packets (usually dropping them
without acknowledgement to the sender). In principle, application firewalls can
prevent all unwanted outside traffic from reaching protected machines.
By inspecting
all packets for improper content, firewalls can even prevent the spread of the
likes of viruses. In practice, however, this becomes so complex and so
difficult to attempt (given the variety of applications and the diversity of
content each may allow in its packet traffic) that comprehensive firewall
design does not generally attempt this approach.
> Proxies
A proxy device
(running either on dedicated hardware or as software on a general-purpose
machine) may act as a firewall by responding to input packets (connection
requests, for example) in the manner of an application, whilst blocking other
packets.
Proxies make
tampering with an internal system from the external network more difficult, and
misuse of one internal system would not necessarily cause a security breach
exploitable from outside the firewall (as long as the application proxy remains
intact and properly configured). Conversely, intruders may hijack a
publicly-reachable system and use it as a proxy for their own purposes; the
proxy then masquerades as that system to other internal machines. While use of
internal address spaces enhances security, crackers may still employ methods
such as IP spoofing to attempt to pass packets to a target network.
1.7
Specify the general characteristics (For example: carrier speed, frequency,
transmission type and topology) of the following wireless technologies:
> Infrared
Infrared (IR)
radiation is electromagnetic radiation of a wavelength longer than that of
visible light, but shorter than that of microwave radiation. The name means
"below red" (from the Latin infra, "below"), red being the
color of visible light of longest wavelength.
> Bluetooth
Is an industrial specification for wireless
personal area networks (PANs). Bluetooth provides a way to connect and exchange
information between devices like personal digital assistants (PDAs), mobile
phones, laptops, PCs, printers and digital cameras via a secure, low-cost,
globally available short range radio frequency.
|
|
802.11
|
802.11x
|
Infrared
|
Bluetooth
|
|
Speed
|
500
Kbps
|
802.11a
> 54 Mbps
802.11b > 11 Mbps 802.11g > 54 Mbps |
115.2
Kbps
|
1.2
> 720 Kbps
2.0
> 2.1 Mbps
|
|
Frequency
|
Radio
Wave
|
Radio
Wave
802.11a
> 5 GHz
802.11b > 2.4 GHz 802.11g > 2.4 GHz |
Light
Wave
|
Radio
Wave
2.45
GHz. In order to avoid interfering with other protocols which use the 2.45
GHz band, the Bluetooth protocol divides the band into 79 channels (each 1
MHz wide) and changes channels up to 1600 times per second.
|
|
Transmission
|
Light
(modulated, switched on and off, to encode the data.)
|
|||
|
Topology
|
Various
|
Various
|
Various
|
Various
|
FHSS
Frequency-hopping spread spectrum is a spread-spectrum method of transmitting
radio signals by rapidly switching a carrier among many frequency channels,
using a pseudorandom sequence known to both transmitter and receiver.
Spread-spectrum
transmission offers these advantages over a fixed-frequency transmission:
· Highly resistant to noise and interference.
· Signals are difficult to intercept. A
Frequency-Hop spread-spectrum signal sounds like a momentary noise burst or
simply an increase in the background noise for short Frequency-Hop codes on any
narrowband receiver except a Frequency-Hop spread-spectrum receiver using the
exact same channel sequence as was used by the transmitter.
· Transmissions can share a frequency band with many
types of conventional transmissions with minimal interference. As a result,
bandwidth can be utilized more efficiently.
DSSS direct-sequence spread spectrum is a modulation
technique where the transmitted signal takes up more bandwidth than the information
signal that is being modulated, which is the reason that it is called spread
spectrum.
Comparison of
DSSS and Frequency Hopped SS
DSSS
· Flexible support of variable data rates
· High capacity is possible with enhancements
(interference cancellation, adaptive antenna, etc.)
· Suffers from near-far effect
FHSS
· Suitable for ad hoc networks (no near-far
problem)
· Robust to interference
· Limited data rate
OFDM Orthogonal
frequency-division multiplexing, also called discrete multitone modulation
(DMT), is a transmission technique based upon the idea of frequency-division
multiplexing (FDM).
· Used in some wireless LAN applications,
including WiMAX and IEEE 802.11a/g
· Used in many communications systems such as:
ADSL, Wireless LAN, Digital audio broadcasting.
1.8
Identify factors which affect the range and speed of wireless service (For
example: interference, antenna type and environmental factors).
> 802.11g
Suffers from the
same interference as 802.11b in the already crowded 2.4 GHz range. Devices
operating in this range include microwave ovens, Bluetooth devices, and
cordless telephones.
Since the 2.4
GHz band is heavily used, using the 5 GHz band gives 802.11a the advantage of
less interference. However, this high carrier frequency also brings
disadvantages. It restricts the use of 802.11a to almost line of sight,
necessitating the use of more access points; it also means that 802.11a cannot
penetrate as far as 802.11b since it is absorbed more readily, other things
(such as power) being equal.
> 802.11a
Transmits radio
signals in the frequency range above 5 GHz. This range is
"regulated," meaning that 802.11a gear utilizes frequencies not used
by other commercial wireless products like cordless phones. In contrast,
802.11b utilizes frequencies in the unregulated 2.4 GHz range and encounters
much more radio interference from other devices.
> IEEE 802.11a / IEEE 802.11h
This is also a
physical layer enhancement. IEEE 802.11a provides significantly higher
performance than 802.11b, at 54 Mbps. Unlike 802.11b, the 802.11a standard
operates within the frequency range of 5.47 to 5.725 GHz and is not subject to
the same interference from other commercial electronic products. This higher
frequency band allows significantly higher speeds of communication over the 2.4
GHz range.
802.11g APs are
backward compatible with 802.11b APs. This backward compatibility with 802.11b
is handled through the MAC layer, not the physical layer. On the negative side,
because 802.11g operates at the same frequency as 802.11b, it is subject to the
same interferences from electronic devices such as cordless phones. Since the
standard’s approval in June 2003, 802.11g products are gaining momentum and
will most likely become as widespread as 802.11b products. Table II-1 displays
basic 802.11b/a/g characteristics.
The common range
of operation for 802.11b is 150 feet for a floor divided into individual
offices by concrete or sheet-rock, about 300 feet in semi-open indoor spaces
such as offices partitioned into individual workspaces, and about 1000 feet in
large open indoor areas. Disadvantages of 802.11b include interference from
electronic products such as cordless phones and microwave ovens.
Range
The layout
of your building can reduce the range.
· A lot of concrete walls can reduce your range.
· The size of the antenna and the placement
greatly affect the range of their signals
· The weather and amount of water vapor in the air
can affect your signals strength
Speed
· The layout of your building can reduce the speed
· The size of the antenna and its signal can affect
your speed
· The weather and amount of water vapor can weaken
the signal and affect your speed
