Signals

The Transmission of wireless signal waves takes place in the electromagnetic (EM) spectrum. The carrier frequency of the data is expressed in cycles per second called hertz (Hz). Low frequency signals can travel for long distances through many obstacles but cannot carry a high bandwidth of date while high frequency signals waves can travel for shorter distances through few obstacles and carry a narrow bandwidth. “Waves that belong to the wireless spectrum (waves used for broadcasting, cellular phones, and satellite transmission) are neither visible nor audible, except by the receiver”. Regan (2004)

There are four categories of Wireless Signals and are as follows;

1. Radio: 10 KHz to 1 GHz is broken into various bands including AM, FM, and VHF bands. The Federal communications Commission (FCC) regulates the assignment of these frequencies. Frequencies for unregulated use 902 to 928 MHz, these include cordless phones and remote controls. Mobile companies’ use 2.4 GHz 5.72 to 5.85 GHz radio signals to communicate, an example would be a mobile service that exchanges information using two-way radios.

2. Microwave: Microwaves are used to link networks over long distances but the two microwave towers must have a line of sight between them. The frequency is usually 4-6GHz or 21-23GHz. Speed is often 1-10Mbps. The signal is normally encrypted for privacy. Two nodes may exist. Microwave signals are used as a backbone or backhaul carriers in cellular technology, communication with satellites, or a Microwave radio relay link for television and telephone service providers.

3. Satellite: A satellite orbits at 22,300 miles above the earth which is an altitude that will cause it to stay in a fixed position relative to the rotation of the earth using stations on the ground to send and receive signals from the satellite. The signal can have broadcast delays between 0.5 to 5 seconds due to the distances that are involved. The transmission frequency is 11to14GHz with transmission speeds in the range of 1to10Mbps. With communication being such an integral and critical part of the military, Satellite Communication is responsible for the lines of communication which is used to transmit messages over vast distances.

4. Infrared: The Infrared signal uses a visible range of light, with the transmission frequency of 100GHz to1000THz with the distance of transmission in 10's of meters and uses a light emitting diode (LED) or laser to transmit the signal. Infrared signals use point to point transmission and are limited to line of sight transmission because signals cannot travel through objects. The broadcast speed is 100Kbps to 16Mbps and the signal is dispersed so several units may receive the signal at the same time. The unit used to disperse the signal may be reflective material or a transmitter that amplifies and retransmits the signal. Normally the speed is limited to 1Mbps. Installation is easy and the cost is comparatively inexpensive for wireless networking, and it is difficult to eavesdrop on infrared transmissions. Wireless infrared communication systems can be characterized by the application for which they are designed, for example; building to building connections for high-speed network access or metropolitan or campus area networks may use the infrared technology.

Figure 1: The properties of the four signal types

Media		Frequency Range		Cost	Ease of installation		Capacity Range		Attenuation	Immunity for interference and signal capture
Radio	Low power single frequency		Entire RF, a high GHz is most common		Moderate depending on the equipment	Simple		<1 to 10 Mbps		High	Extremely low
	High power single frequency		Entire RF, a high GHz is most common		Moderately expensive	Difficult		<1 to 10 Mbps		Low	Extremely low
	Spread spectrum radio		Entire RF is 902 to 928 in the U.S. and 2.4 is the most common		Moderate depending on the equipment	Simple to moderate		2 to 6 Mbps		High	Moderate
Microwave	Terrestrial Microwave		Uses a low GHz, 4 to 6 or 21 to 23 is the most common		Moderate to high depending on the equipment	Difficult		<1to 10 Mbps		Variable	Low
Satellite	Satellite Microwave		Uses a low GHz, 11 to 14 GHz is the most common		High	Extremely difficult		<1to 10 Mbps		Variable	Low
Infrared	Point to Point Infrared		Uses 100 GHz to 1000 THz		Low to Moderate	Moderate to difficult		<1 to 16 Mbps		Variable	Moderate
	Broadcast Infrared		Uses 100 GHz to 1000 THz		low	Simple		<1 Mbps		High	Low
										Regan (2004)

Conclusion

Today, the use of wireless technology is widespread throughout the United States and growing. About 71 % of America's 108 million households own at least one cell phone, according to Forrester Research Inc. More than 25 million households now own laptop computers, according to Forrester. And 5.3 million households have wireless Internet access and is up from zero a couple of years ago. That is rapid growth. Ask (2008)

The OSI model

The OSI model

The OSI model typifies the stream of data in a network, from the physical connections at the lowest layer up to the layer that has the user’s applications. Data passing through the network is passed from layer to layer, and each layer has the ability to communicate with the layer directly above it and the layer directly below it. Each layer written is a capable streamlined software component and when a layer receives a packet of information, it checks the target address and if its corresponding address is not there it passes the packet to the following layer. Merkow, M. & Breithaupt, J. (2006)

The seven layers of the OSI model

Physical layer

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers.

Data link layer

The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link.

Network layer

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors.

Transport layer

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

Session layer

The session layer allows session establishment between processes running on different stations. It provides session establishment, maintenance and termination which allow two application processes on different machines to establish, use and terminate a connection, called a session, and session support which performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

Presentation layer

The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, and then translate the common format to a format known to the application layer at the receiving station.

Application layer

The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions like resource sharing and device redirection, remote file access, remote printer access, inter-process communication, network management, directory services, electronic messaging (such as mail), and Network virtual terminals. Kretchmar (2003)

At which layers of the OSI model do packet-filtering routers and firewalls reside and how do packet-filtering routers and firewalls protect a network?

Packet filtering firewalls and routers examine both the source and destination address of the incoming data packet and operates at different layers which use different criteria’s to restrict traffic. The lowest layers at which a firewalls and routers can work is layer three and four, the network and transport layers. The network layer is concerned with routing packets to their destination. At this layer a firewall can determine whether a packet is from a trusted source, but cannot be concerned with what it contains or what other packets it is associated with. Firewalls that operate at the transport layer know a little more about a packet, and are able to grant or deny access depending on more sophisticated criteria. At the application level, firewalls know a great deal about what is going on and can be very selective in granting access. Kretchmar (2003)

Packet filtering is the simplest packet screening method. A packet filtering firewall does exactly what its name implies, it filters packets. The most common implementation is on a router. The packet filtering process is accomplished in the following manner. As each packet passes through the firewall, it is examined and information contained in the header is compared to a pre-configured set of rules or filters, allowing or denying decisions are made based on the results of the comparison. Each packet is examined individually without regard to other packets that are part of the same connection. A Packet filtering firewall uses firewall rules set to allow or deny packets. Packet filtering routers and firewalls are often called network layer firewalls because the filtering is primarily done at the network layer (layer three) or the transport layer (layer four) of the OSI reference model. Lipták (2002)

Conclusion

A network security domain is a contiguous region of a network that operates under a single, uniform security policy. Whenever domains intersect, there is a potential need for security to control traffic allowed into the network. Firewall technology in the OSI model can be used to filter this traffic. The most common boundary where firewalls are applied is between an organization’s internal network and the internet.

References

Instrument engineers' handbook, Béla G. Lipták, ISA--The Instrumentation, Systems, and Automation Society 2002 Retrieved July 3, 2009

Chapter 12, Telecommunications, Network, and Internet Security Information Security: Principles and Practices, by Mark S. Merkow and Jim Breithaupt. Copyright © 2006 by Pearson Education, Inc. Retrieved July 3, 2009

Open Source Network Administration, Prentice Hall Series in Computer Networking and Distributed Systems, James M. Kretchmar 2003, Retrieved July 4, 2009