Network Media and Topologies

wire crimper

A wire crimper is a tool that you use to attach media connectors to the ends of cables. For instance, you use one type of wire crimper to attach RJ-45 connectors on Unshielded Twisted Pair (UTP) cable, and you use a different type of wire crimper to attach Bayonet Neill Concelman (BNCs) to coaxial cabling.

Wire Map Testers

A wire map tester is a device that is similar in principle to the tone generator and locator, except that it tests all the wire connections in a UTP cable at once. This device also consists of two parts, which you connect to the opposite ends of a cable. The unit at one end transmits signals over all the wires, which are detected by the unit at the other end. A wire map tester can detect transposed wires, opens, and shorts, just as a tone generator and locator can, but it does all the tests simultaneously and provides you with a simple readout telling you what's wrong

Multifunction Cable Testers

Multifunction cable testers are handheld devices, that perform a variety of tests on a cable connection and compare the results to standard values that have been programmed into the unit. The result is that these are devices that anyone can use. You simply connect the unit to the cable, press a button, and the device comes up with a list of pass or fail ratings for the individual tests. Multifunction cable testers can test any of the following:

  • Length The most common method for determining the length of a cable is called time domain reflectometry (TDR), in which the tester transmits a signal over the cable and measures how long it takes for the signal's reflection to return. Using the nominal velocity of propagation (NVP) for the cable, which is the speed at which signals travel through the cable (supplied by the manufacturer) you can compute the length of the cable. This function also enables you to determine the location of a break in a cable.
  • Attenuation By comparing the strength of a signal at the far end of a cable to its strength when transmitted, the tester determines the cable's attenuation (measured in decibels).
  • Near end crosstalk (NEXT) Testing for near end crosstalk is a matter of transmitting a signal over one of a cable's wires and then detecting the strength of the signal that bleeds over into the other wires near the end of the cable where the transmitter is located.
  • Power sum NEXT (PSNEXT) This is a measurement of the crosstalk generated when three of the four wire pairs are carrying signals at one time. This test is intended for networks using technologies like Gigabit Ethernet, which transmit signals over several wire pairs simultaneously.
  • Equal level far end crosstalk (ELFEXT) This is a measurement of the crosstalk at the opposite end of the cable from the transmitter, corrected to account for the amount of attenuation in the connection.
  • Power sum ELFEXT (PSELFEXT) This is a measurement of the crosstalk generated at the far end of the cable by three signal-carrying wire pairs, corrected for attenuation.
  • Propagation delay This indicates the amount of time required for a signal to travel from one end of a cable to the other.
  • Delay skew This is the difference between the lowest and the highest propagation delay measurements for the wires in a cable. Because the wire pairs inside a UTP cable are twisted at different rates, their relative lengths can differ, and the delay skew measurement quantifies that difference.
  • Return loss This is a measurement of the accumulated signal reflection caused by variations in the cable's impedance along its length. These impedance variations are typically caused by untwisting too much of the wire pairs when making connections.

Tone Generator

tone generator

One of the most basic ways to identify and test a cable connection is to use a tone generator and locator cable tester. The tone generator is a device that you connect to a cable at one end, and which transmits a signal over the cable. The tone locator is a separate device that has a probe capable of detecting the generator's signal, either by touching it to the conductor in the cable, or simply by touching it to the insulation on the outside of the cable. When the locator detects the generator's signal, it emits an audible tone. You can use this type of device to test an entire cable, or to test the individual wire connections inside a UTP cable.

Tone generators are most commonly used to identify the cable belonging to a particular connection.

If you're performing an internal cable installation, and you forget to label one of your cables, you can connect the tone generator at the wall plate end and touch the probe to each of the cables at the patch panel end until you find the one that produces a tone. You can also use a tone generator and locator to test the individual wire connections inside a UTP cable.

  • Connect the generator to a single wire or connector contact using alligator clips
  • Then touch the locator to each wire or contact at the other end of the cable.

Using this method, you can test for any major wiring faults that affect internal UTP cable installations.

  • If you fail to detect a signal on the contact to which you have the generator connected at the other end, you have an open circuit.
  • If you detect a signal on the wrong contact, you have punched down the wires to the wrong contacts.
  • If you detect a signal on two or more wires, you have a short.

Tone generator and locator Pros:

  • Simple to use
  • Most inexpensive type of cable tester
  • Useful for troubleshooting a single cable connection.

Tone generator and locator Cons:

  • Testing each of the wires in a UTP cable individually is time consuming
  • You also need two people to use the equipment, one at the generator end and one at the locator end (unless you don't mind running back and forth from one end of your cable connections to the other)

Purpose, benefits and characteristics of using a firewall.

A firewall is a system or group of systems that enforces an access control policy between two networks. How this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms to either block or permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy.

Firewall techniques:

  • Packet filter looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules.
  • Application gateway applies security mechanisms to specific applications, such as FTP and Telnet servers.
  • Circuit-level gateway applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.

Network layer firewalls
Network layer firewalls operate at a 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. 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 also 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 without acknowledgement to the sender. Application firewalls can prevent all unwanted outside traffic from reaching protected machines.Firewalls can’t protect against attacks that don’t pass through it.

Types of firewalls

The term firewall is rather broad, because the features and effectiveness of any particular firewall vary greatly. However, firewalls in general can be classified into three basic forms, as outlined here:

  • A dedicated hardware device
  • A router with traffic filtering/firewall capabilities built in
  • A software based system normally running on a server, PC, or MAC

Purpose, benefits and characteristics of using a proxy service.

A proxy server provides numerous advantages for connecting a local area network to the Internet. Acting as an Application-Layer Gateway, the proxy provides a strong defense from the outside world. Performing the duties of a firewall, however, is just one benefit of a proxy server. The proxy can also provide caching services to increase performance, logging services to track Internet use, tools to maximize the use of precious bandwidth, and content filtering to help keep unwanted data off the local network. The proxy can also utilize multiple connection types to easily provide redundancy and automatic failover in the event of a primary line failure.
The primary security features of Proxy Server are:

  • It blocks inbound connections.
  • LAN clients can initiate connections to Internet servers, but Internet clients cannot initiate connections to LAN servers.
  • It can restrict outbound connections.