NETWORKING I - The most important characteristics in an electronic cable are the following ones: impedance, attenuation, shielding and capacitance. Could you describe them? Impedance: The resistance to the movement of electrons in an AC circuit. Its represented by the letter Z.Like resistance, its unit of measurement is the ohm, represented by Omega. Attenuation: refers to the resistance to the flow of electrons, and why a signal becomes degraded(to fade away) as it travels along the conduit. Its unit of measurement is the db/m. Shielding: is normally specified as a cable construction detail.For example Shielded twisted pair STP which contains four pair copper wires and each pair is wrapped in metallic foil. The functions are to act as a barrier between the internal or external signals (to reduce the EMI) and to be part of the electrical circuit. Capacitance: How much energy it is hold in the cable. Its represented by the letter C and its unit of measurement is the picofarads pF - Give a physical description of the following cables: twisted pair, coaxial and optical fiber. Explain the role of each material. Twisted pair: It consists of four pairs of thin, copper wires covered in color-code plastic insulation that are twisted together. The wire pairs are twisted for two reasons. First, to provide protection against crosstalk, which is the noise generated by adjacent pairs of wires. Two magnetic fields occur in opposite directions and cancel each other out. Second, network data is sent using two wires in a twisted pair. One copy of the data is sent on each wire, and two copies are mirror images of each other(differentials signals). If the two wires are twisted together,
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NETWORKING I
- The most important characteristics in an electronic cable are the
following ones: impedance, attenuation, shielding and capacitance.
Could you describe them?
Impedance: The resistance to the movement of electrons in an AC
circuit. Its represented by the letter Z.Like resistance, its unit of
measurement is the ohm, represented by Omega.
Attenuation: refers to the resistance to the flow of electrons, and why a
signal becomes degraded(to fade away) as it travels along the conduit.
Its unit of measurement is the db/m.
Shielding: is normally specified as a cable construction detail.For
example Shielded twisted pair STP which contains four pair copper wires
and each pair is wrapped in metallic foil. The functions are to act as a
barrier between the internal or external signals (to reduce the EMI) and
to be part of the electrical circuit.
Capacitance: How much energy it is hold in the cable. Its represented by
the letter C and its unit of measurement is the picofarads pF
- Give a physical description of the following cables: twisted pair, coaxial
and optical fiber. Explain the role of each material.
Twisted pair: It consists of four pairs of thin, copper wires covered in
color-code plastic insulation that are twisted together.
The wire pairs are twisted for two reasons.
First, to provide protection against crosstalk, which is the noise
generated by adjacent pairs of wires. Two magnetic fields occur in
opposite directions and cancel each other out.
Second, network data is sent using two wires in a twisted pair. One
copy of the data is sent on each wire, and two copies are mirror images
of each other(differentials signals). If the two wires are twisted together,
noise seen on one wire is also seen on the other wire. When the data is
received, one copy is inverted, and the two signals are then compared.
In this manner the recover can filter out noise because the noise signals
cancel each other.
Coaxial cable, as show in Figure, consists of four parts:
-Copper conductor.
-Plastic Insulation.
-Braided copper shielding
-Outer jacket
At the center of the cable is a solid copper conductor. Surrounding that
conductor is a layer of flexible plastic insulation. A woven copper braid or
metallic foil is wrapped around the insulation. This layer acts as the
second wire in the cable. It also act as a shield for the inner conductor
and helps reduce the amount of outside interference. Covering this
shield is the outer cable jacket. The connector used on coaxial cable is
the BNC, short for British Naval Connector or Bayonet Neill Concelman,
connector.
Fiber-Optic cablle:Five parts typically make up each fiber-optic cable:
-The core
-The cladding
-A buffer
-A strengthening material
-An outer jacket
The core is the light transmission element at the center of the optical
fiber, and all the light signals travel through the core. This core is
typically glass made from a combination of silica and other elements.
Surrounding the core is the cladding, also made of silica but with a lower
index of refraction than the core. Light rays traveling through the fiber
core reflect off this core-to-cladding interface where the core and
cladding meet, which keeps light in the core as it travels down the fiber.
Surrounding the cladding is a buffer material, usually plastic, that helps
shield the core and cladding from damage.
The strengthening material surrounds the buffer, preventing the fiber
cable from being stretched when installers pull it. The material used is
often Kevlar, the same material used to produce bulletproof vests. The
final element, the outer jacket, surounds the cable to protect the fiber
against abrasions, solvents, and other contaminants. this outer jacket
composition can vary depending on the cable usage.
- What is the difference between UTP and STP? Any consequence?
The difference between UTP and STP is the shielding. The shielding
reduces unwanted electrical noise. This noise reduction provides a major
advantage of STP over unshielded cable. However, shielded cable is
more difficult to install than unshielded cable because the metallic
shielding needs to be grounded. If improperly installed, STP become
very susceptible to noise problems because an ungrounded shield acts
like an antenna, picking up unwanted signals. The insulation and
shielding considerably increase the size, weight, and cost of the cable.
Despite these disadvantages, shielded copper cable is still used as
networking media today, especially in Europe.
- When crimping RJ-45 UTP cables, how can you create straight-
through, crossover and loopback cables? Describe some situations in
which they are used.
Crossover cables provide a network connection between two similar
devices, such as computer to computer or switch to router. With
crossover cables you can connect 2 computers directly. It directly
connects two network devices of the same type to each other over
Ethernet. Ethernet crossover cables are commonly used when
temporarily networking two devices in situations where a network router,
switch or hub is not present. Crossover cables have the 1st and 3rd
wires crossed, and the 2nd and 6th wires crossed. Two devices in the same category use a crossover cable. As you can see as follows:
SWITCH PC (COMPUTER)
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
R+R-T+ T- T+ T-R+ R-
HUB ROUTERStraight cable Cat.1 &
Cat.2
SWITCH SWITCHCrossover cable (Cat.
1)
HUB HUBCrossover cable (Cat.
1)
SWITCH HUBCrossover cable (Cat.
1)
PC ROUTERCrossover cable (Cat.
2)
Loopback cable: A loopback cable redirects the output back into itself.
This effectively gives the NIC the impression that it is communicating on
a network, since its able to transmit and receive communications. You
must Redirect Pin 1 to Pin 3 and Pin 2 to Pin 6 to create Loopback
cable.
- What is the difference between single-mode and multimode fiber-optic
cable?
The part of an optical fiber through which light rays travel is called the
core of the fiber. Light rays cannot enter the core of an optical fiber at all
angles. The rays can enter the core only if their angle is inside the fiber's
numerical aperture: likewise, one the rays have entered the fiber's core,
a limited number of optical paths exist that a light ray can follow through
the fiber. These optical path are called modes. If the diameter of a fiver's
core is large enough so that many paths exist that light can take as it
passes through the fiber, the fiber is called multimode fiber. Single-mode
fiber has a much smaller core that allows light rays to travel along only
one path(one mode) inside the fiber.
- Describe the basic types of networks as far as their size is concerned:
PAN, LAN, CAN, MAN and WAN. Give some examples.
LAN or local area network is a group of interconnected devices that is
under the same administrative control. It is a network which covers a
small physical area. All local networks within a LAN are under one
administrative control group that governs the security and access control
policies. LANs allow users to have common access to data and
equipment such as printers.
WAN or Wide-area networks are networks that connect LANs in
geographically separated locations. It is a network which covers a wider
area, in which machines are usually connected via telephone lines or
radio. A WAN can be as small as two LANs which are connected, or as
big as the Internet. The Internet is a large WAN that is composed of
millions of interconnected LANs. Telecommunications service providers
(TSP) are used to interconnect these LANs at different locations.
MAN or metropolitan area network is a computer network that usually
spans a city or a large campus. A MAN usually interconnects a number
of local area networks (LANs) using a high-capacity backbone
technology, such as fiber-optical links. A MAN typically covers an area of
between 5 and 50 km diameter.
PAN: Personal Area Network (Ex: Bluetooth)
CAN: Campus Area Network (Ex: Hospitals, Universities, etc)
- Compare simplex, half-duplex and full-duplex transmissions.
Simplex: The capability of transmission is only one direction between a
sending station and a receiving station. Broadcast television is an
example of a simplex technology.
half-duplex: A capability for data transmission in only one direction at a
time between a sending station and receiving station.
full-duplex: The capability for simultaneous data transmission between a
sending station and receiving station.
- Data are delivered by means of packets in a network. Provide a
technical description of "packet".
A logical grouping of information that includes a header containing
control information and (usually) user data. Packets most often refer to
network layer units of data. The terms datagram, frame, message, and
segment also describe logical information groupings at various layers of
the OSI reference model and in various technology circles.
- Describe the OSI model.
A network architectural model developed by the ISO. This model
consists of seven layers, each of which specifies particular network
functions, such as addressing, flow control, error control, encapsulation,
and reliable message transfer.
The following sections briefly describe each layer in the OSI
reference model:
Layer 7. The Application Layer. Is the layer that is closest to the user. It
provides network services to the user's applications.
Layer 6. The Presentation Layer. Ensures that the information that the
application layer of one system sends out can be read by the application
layer of another system. If necessary, the presentation layer translates
among multiple data formats by using a common format. One of the
more important tasks of this layer is encryption and decryption.
Layer 5. The Session Layer. As its name implies, the session layer
establishes, manages, and terminates sessions between two
communicating hosts. The session layer provides its services to the
presentation layer. It also synchronizes dialogue between the two host's
presentations layers and manages their data exchange.
Layer 4: The Transport Layer: This layer is responsible for reliable
network communication between en nodes. The transport layer provides
mechanisms to establish, maintain, and terminate virtual circuits,
transport fault detection and recovery, and information flow control.
Layer 3: The Network Layer: This layer provides connectivity and path
selection between two ends systems. The network layer is the layer at
which routing occurs.
Layer 2:The Data Link Layer: This layer provides reliable transit of data
across a physical link. In so doing, the data link layer is concerned with
physical(as opposed to logical) addressing, network topology, network
access, error notification, ordered delivery of frames, and flow control.
Layer 1: Physical Layer: The physical layer defines the electrical,
mechanical, procedural, and functional specifications for activating,
maintaining, and deactivating the physical link between the systems.
- Describe the following application protocols in TCP/IP: SMTP, POP3,
IMAP4, FTP, HTTP, HTTPS and DNS.
SMTP: Simple mail transfer Protocol. The SMTP protocol transport e-
mail messages in ASCII format using TCP. When a mail server receives
a message destined for a local client, it stores that message and waits
for the client to collect the mail. Mail clients can collect their mail in
several ways:
They can use programs that access the mail servers files directly
or can use one of many network protocols. The most popular mail client
protocols are Post Office Protocol Version 3(POP3) and Internet
Messaging Access Protocol version 4 (IMAP4)
POP3: Post Office Protocol Version 3, which uses TCP port 110, is a
mail protocol that is responsible for holding e-mail until delivery. When a
SMTP servers sends an e-mail message to a POP3 server, POP3 holds
on to the message until a user makes a request to have the data
delivered. Thus, POP3 transfers mail files from a mail server to a mail
client.
IMAP4: Internet Message Access Protocol Version 4 allows a client to
access and manipulate electronic mail messages on a server. IMAP4
permits manipulation remote message folders, called mailboxes, in a
way that functionally equivalent to local mailboxes
FTP: is a fast connection-oriented, error free protocol that uses TCP
ports 20 and 21. FTP allows data to be transferred between servers and
clients. For FTP to connect to a remote server, IP address or host name
must be provided. FTP must be capable of resolving IP addresses to
host names to establish a connection.
HTTP: which uses TCP port 80, allows clients to transfer documents that
are written in Hypertext Markup Language(HTML) over the World Wide
Web for display by a browser. It's the universal display language of the
Internet.
HTTPS: Another protocol for transmitting data securely over the World
Wide Web, which is designed to transmit individual messages securely.
Technically, it is not a protocol in and of itself; rather, it is the result of
simply layering the Hypertext Transfer Protocol (HTTP) on top of the
SSL/TLS protocol, thus adding the security capabilities of SSL/TLS to
standard HTTP communications.
DNS: is a name-resolution service that resolves (associates) host names
to IP addresses. DNS keeps a record of IP addresses and host names in
a process called a domain. DNS provides services along hierarchical
chain, with a database design that is similar to a file tree structure. DNS
also services requests for host names that cannot be resolved locally.
Large inter-networks have several levels os DNS servers to provide
efficient name resolutions.
- Compare the two main transport protocols (i.e. TCP and UDP).
An advantage that UDP has over TCP is that, because it does not
concentrate on establishing a connection, it can transmit more
information in a smaller amount of time than TCP. TCP is useful for
transmitting large amounts of data reliably, but with the penalty of large
ACK overhead consuming bandwidth. UDP is useful for transmitting
small amounts of data when reliability is less crucial, UDP lacks the
overhead caused by ACKs.
- Describe the following Internet protocols: ARP, RARP, ICMP and IGMP.