Overcoming Polarization Mode Dispersion using Special Single-Mode Fibers+

By Neil Lynch

          Polarization-mode dispersion is due to differential group delay as a result of chromatic (material and waveguide) refraction stemming from the asymmetry in the fiber core and cladding’s refractive indices; the cladding’s refractive index is lower than the core’s refractive index. This article looks at dispersion compensation; how polarization-mode dispersion is overcome using special single-mode fibers.

          It first discusses the core, cladding and their refractive indices, the nature of the interface between them (the core and cladding) which determines their modal properties; we’ll briefly look at the input signal into the fiber via the proper region of the acceptance angle. We’ll also look at dispersion as light propagates along the fiber and the combining effect of material and waveguide dispersions (chromatic dispersion) on its outcome, followed by orthogonal Polarization, asymmetrical cores, birefringence (discussed in the video)  and 4-wave mixing. We’ll then conclude with dispersion compensation: special single-mode fibers, chromatic dispersion compensation (dispersion compensation filters module, tunable dispersion compensators) and soliton transmission.

        “The two key elements of an optical fiber—from an optical standpoint—are its core and cladding. The core is the inner part of the fiber, which guides light. The cladding surrounds it completely, as shown in Figure 1. The refractive index of the core is higher than that of the cladding, so light in the core that strikes the boundary with the cladding at a glancing angle (as shown in figure 2) is confined in the core by total internal reflection” (Hecht, 26). The size of the core and cladding and the nature of the interface between them determine the fiber’s modal properties and how it transmits light at different wavelengths. The simple types of fiber have a step-index structure, where the refractive index changes sharply at the abrupt boundary between a high-index core and a low index-cladding; of the various transmission modes, single mode transmission is cleaner and simpler, and it’s also preferred for fiber-optic systems. The main limitation is that the core[1] of the fiber must be small enough to restrict transmission to a single mode, yet large enough to collect most of the input optical signal via the proper region of the acceptance angle, preferably the half acceptance region above or below the fiber-axis as shown in figure 3.

   “Dispersion is the spreading out of light pulses as they travel along a fiber. It occurs because the speed of light through a fiber depends on its wavelength and the propagation mode. The differences in speed are slight, but like attenuation, they accumulate with distance” (Hecht, 103).

        The pulse spreading arises because the velocity of light through a fiber depends on its wavelength. Material dispersion causes different wavelengths to travel at different speeds as a result of variations in the refractive index of the fiber core with wavelength. Some of the light also travels in the cladding of the fiber, which has a different refractive index and propagates light at a different speed than the core; an effect known as waveguide dispersion. Material and waveguide dispersion combined to produce an overall effect called "chromatic dispersion" (chromatic dispersion-bb.csis ch.5; web).

Polarization … can be important in single-mode fibers. The reason is that what we call single-mode fibers actually carry two modes with orthogonal polarization. Fibers with circularly symmetric cores can’t differentiate between the two linear polarizations…; the two modes are degenerate, meaning they’re functionally identical and can’t be told apart by the fiber, so light can shift easily between the two polarization modes.

If the circular symmetry of fibers were perfect, polarization would have little practical impact for communications. However, fiber symmetry is never absolutely perfect… As a result, the two polarization modes may experience slightly different conditions and travel along the fiber at slightly different speeds. This effect is called differential group delay  (double-click to see video above, escape to return (Yao, YouTube)), which averaged over time becomes polarization-mode dispersion, as shown in figure4. It can cause problems in high performance systems, such as those transmitting time-division multiplexed signals faster than about 2.5 Gbit/s.

      For a positive dispersion fiber, pulse broadening takes place due to the spectral components at the leading edge moving faster than the components at the trailing edge. In the case of a negative dispersion fiber, components at the leading and trailing edge act in the opposite sense, implying that the components move toward one another, which subsequently leads to pulse compression. Compression will continue to dominate until a point of maximum compression is reached, at which point a change in sign of the chirp[2]   will subsequently lead to pulse broadening. The significance of the initial pulse compression differentiates positive and negative dispersion fibers (positive/negative dispersion-bb.csis ch.5; web).

       Standard Single Mode Fiber (SSMF) – SSMF, also known as non-dispersion shifted fiber, is the most widely used fiber. SSMF is optimized for minimal dispersion in the 1310 NM band, while the high dispersion at 1550 NM prevents 4-wave-mixing[3].  With the assistance of dispersion-compensators[4] (discussed later), long range fiber-transmission distances can be 100 km at OC-192 bit rates and 525 km at OC-48 bit rates. 

        The amount of chromatic dispersion experienced in optical fiber is dependent on the wavelength at which light is being transmitted, and a graph showing this for regular single-mode fiber is shown below. It is worth noting that there is a "slope" to the dispersion; meaning that each wavelength experiences a different amount of dispersion.

Dispersion slope tells how dispersion changes with wavelength.
Normally this change is very small over the range of wavelengths generated by a single laser transmitter. However, it’s important in wavelength-division multiplex systems, which carry many optical channels spanning tens of nanometers in wavelength.

Specification sheets typically do not plot chromatic dispersion directly as a function of wavelength, but give the chromatic dispersion that may be found at a range of wavelengths (Hecht, 110).  

 

Special Single-Mode Fibers

Special single-mode fibers can control the polarization of light they transmit. There are two types: true single-polarization fiber and polarization-maintaining fiber (see video below). Both intentionally avoid circular symmetry, so they transmit vertically and horizontally polarized light differently…

                                                                                                                                                             Single-polarization fibers can remove the undesired polarization in order to achieve, a transmission … almost as well as standard single-mode fiber … where only the desired polarization remains in the end.

Single-polarization fiber has a different attenuation for light of different polarization. It transmits light of one polarization well but strongly attenuates light with the orthogonal polarization. “The figure below illustrates the right-angle (perpendicular or orthogonal) relationship between the electric and magnetic field in a light wave” (Orthogonal Polarization, bb.csis chapter 4, web)



 

Under the proper conditions, a single-polarization fiber attenuates the undesired polarization by a factor of 1000 to 10,000 within a few meters but transmit the desired polarization almost as well as standard single-mode fiber.

Polarization-Maintaining Fiber (Yao, YouTube)   double-click for video, escape to exit

 

… Polarization entanglement could be significantly decohered (degraded) during fiber transmission due to two polarization effects in optical fibers: polarization mode dispersion (PMD) and polarization-dependent loss (PDL); fiber symmetry is never absolutely perfect… as a result, these polarizations may experience slightly different conditions and travel along the fiber at slightly different speeds.

By introducing PMD in a controlled way and performing tomography for various levels of PMD in each fiber,… PMD induced degradations:

Loss of polarization entanglement in a fiber-optic system with polarization mode dispersion in one optical path, and either reduce or increase decoherence depending on the relative orientation of two PMD elements. Sometimes in the latter case the entanglement disappears completely, which is a manifestation of the sudden death arising naturally during photon propagation in fibers (Hecht, 86).

… Pulse dispersion is cumulative, building up along the length of a fiber system. In general, this means that adding more fiber only makes pulse dispersion worse. However, it is possible to reduce total chromatic dispersion by adding a length of fiber with chromatic dispersion of the opposite sign. For example, you could add a length of fiber with negative chromatic dispersion at 1550 nm to a system containing fiber with positive dispersion in that band. According to Downing, “…  if the existing fiber dispersion parameter is -8 ps, then the total dispersion can be brought to zero by adding a length with net dispersion of +8 ps” (281). This idea is similar to using waveguide dispersion to offset material dispersion, but in this case the compensation is done by splicing together two fibers with different chromatic dispersion[5].

        The dispersion-compensation fiber could be added in a length of cable, but it’s often installed in modular form in an equipment rack near a receiver or optical amplifier. In long-distance systems, length of the two types of fibers alternate, so chromatic dispersion does not build up to excessive levels before being reduced.

Chromatic Dispersion Compensation

        The demand for DCMs [(chromatic) Dispersion Compensation Modules] will quickly grow in the near future, driven by the development of ultra-long-haul and 40 Gbps systems. Both applications require greater numbers of DCMs per link and higher priced, higher performance devices having 100 percent slope compensation, and in some cases, dynamically tunable slope compensation (see tunable dispersion compensator below).

 

CS-TDCMX - Compact Tunable Dispersion Compensation Module
 

 

n Modul

This small form factor tunable dispersion compensator is a fully integrated module with a wide bandwidth and a large dispersion tuning range to support any 40 Gb/s modulation format. Based on TeraXion's well-established FBG technology, this ready-to-use module integrates a circulator and all the electronics, making it the smallest in the industry. With its embedded channel selector, the ClearSpectrum^TM-TDCMX allows the user to optimize dispersion in a link for any channel, regardless of the channel plan. Therefore, system flexibility is increased and DWDM systems can be operated at higher speeds with improved design margins.(TereXion, web).

 

             In optical fibers, waveforms lengthen over long distances, making these signals difficult to interpret when they reach the receiving end. The result is distorted data signals that represent transmission errors at the intended receiver.

            As network speeds and span lengths increase; (data rates and coverage areas of telecom systems) … new technical challenges are appearing, including the adverse effects of signal broadening caused by chromatic dispersion. To counteract theses phenomenon’s the importance of tunable dispersion compensation should not be underestimated, especially in solving chromatic dispersion in fast growing OC 192 networks and the developing area of polarization mode dispersion. Tunability is critical in allowing networks of the future to adapt to variable path factors, environmental changes, and configurations that are themselves in constant change.

 Multichannel capability - In order to realize the potential of OC 192 and OC 768 DWDM networks, DCM solutions featuring multichannel capability are vital. Because OC 192 and OC 768 can have up to several dozen channels, a decreasing amount of space on telecommunications racks requires multichannel capability in dispersion compensation solutions.

        In the search for dispersion compensation solutions, DCMs are a great potential in solving the challenges associated with multichannel, high-speed networks. Recent innovations with components such as fiber-Bragg-gratings[6], coupled with important developments in tunability and multichannel capabilities, are important factors for the industry's future.

Dispersion Compensating Filter Modules are passive optical fiber devices used in various locations within the optical fiber communications network link.

Telecommunication System operators are upgrading their links toward 10 Gbps transmission speeds and the use of dense wavelength division multiplexing (DWDM), to increase their network capacity. Often, chromatic dispersion develops into a critical variable in expanding the capacity, since existing (installed) standard optical fiber with varying quality, is optimized for single wavelength transmission at 1310 NM

Dispersion compensating filter modules will be used as an alternative to installing a specific length of dispersion compensating optical fiber (DCF) in the link ( DCFs are usually installed in small spools).

Dispersion compensating filter modules will also be used in conjunction with Optical Add/Drop Multiplexers (OADM). The OADM segments wavelengths at specified locations in the network link, and these wavelengths require dispersion compensation before reaching the receiver (Dispersion Compensation Modules - bb.csis chapter 5, web).

Solition Transmission

            According to (blackboard csis document), there is a relatively new fiber optic data transmission scheme that utilizes something called Soliton Pulses. These are very short bursts of light generated in an Erbium-doped Fiber LASER. Soliton light can be used to transmit data at rates in excess of 50 Gb/s, at distances over 19,000 km of Dispersion-Shifted Fiber, requiring no repeaters, and with no errors. This data rate is the equivalent of sending 6,200 bibles per second. At this rate, one bible could be sent to everyone on earth--6 billion people--in about 10 days.

·        The soliton is a wave that exists in nature which can propagate over long distances without any distortion of its waveform. Optical solitons in optical fiber are maintained by the balance of the nonlinear optical index and the group velocity dispersion of the fiber. Ideal solitons, which propagate without waveform distortion, can exist only in a transmission line with no energy loss and no fluctuation of the grou        Soliton pulses are very short pulses of

 

                                                                References

Hecht, Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Light Guiding, p.26)

 

Fiber Optic Cables. Illustration: Figure 1 and 2. Retrieved from http://www.legrand.us/cablofil/tech_resources/fiber-optic-cable.aspx

 

Hecht, Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Dispersion, p.103)

 

Chromatic Dispersions. Blackboard Course-Documents Chapter 5. Retrieved from            http://bb.csis.pace.edu/course/1/AIT371127a02/content/_162494_1/dir_chapter05.zip/ch5.htm

 

Yao, Colin. Polarization Mode Dispersion. YouTube.  Retrieved from.  http://www.youtube.com/watch?v=J4-     wCa_VNfA&feature=player_embedded#

 

Hecht, Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Polarization Mode Dispersion, p.113)

 

Positive and Negative Dispersions. Blackboard Course-Documents Chapter 5. Retrieved from            http://bb.csis.pace.edu/course/1/AIT371127a02/content/_162494_1/dir_chapter05.zip/ch5.htm

 

Hecht, Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (4-Wave Mixing, p.117)

 

Hecht, Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Dispersion Slope, p.110)

 

Orthogonal Polarization. Blackboard Course-Documents Chapter 4. Retrieved from            http://bb.csis.pace.edu/course/1/AIT371127a02/content/_162486_1/dir_chapter04.zip/ch4.htm

 

Yao, Colin. Polarization Maintaining Fiber. YouTube.  Retrieve from. http://www.youtube.com/watch?v=7rrb-_Iin-g

 

Hecht, Jeff. Understanding Fiber Optics 5/e. New Jersey: Pearson Education, Inc., 2006. Print. (Polarization Mode Dispersion, p.86)

 

Downing, James. Fiber Optic Communications: Delmar/Cengage, 2005. Print. (Dispersion Compensation, 281)

 

CS-TDCMX - Compact Tunable Dispersion Compensation Module. Retrieved from

http://www.teraxion.com/en/cs-tdcmx?gclid=CJCthLni1rMCFQGFnQodxCcAbg

 

 

Dispersion Compensation Modules. Blackboard Course-Documents Chapter 5. Retrieved from            http://bb.csis.pace.edu/course/1/AIT371127a02/content/_162494_1/dir_chapter05.zip/ch5.htm

 

Soliton Transmission. Blackboard Chapter 19 Document. Retrieved from http://bb.csis.pace.edu/courses/1/AIT371127a02/content/_162560_1/ch19overview_1.htm

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[1]The most common dimensions of optical fibers are 9/125, 50/125, 62.5/125 and 100/140 microns.

[2]  A chirp is a signal in which the frequency increases ('up-chirp') or decreases ('down-chirp') with time. In some sources, the term chirp is used interchangeably with sweep signal … In optics, ultra short laser pulses also exhibit chirp due to the dispersion of the materials they propagate through.

[3] 4-Wave mixing: multiple optical channels passing through the same fiber interact with each other only very weakly, making wavelength-division multiplexing possible. However, these weak interactions in glass can become significant over long fiber-transmission distances. The most important is four wave mixing (sometimes called four-photon mixing) in which three wavelengths interact to generate a fourth (Hecht, 117).

 

[4] Dispersion compensation: as data rates of telecom systems increase from 10 to 40 Gbit/s, new technical challenges appear, including the adverse effects of signal broadening caused by chromatic dispersion. This physical phenomenon originates from the wavelength dependence of the propagation velocity in the transport optical fiber. In such a material, the blue part of an optical pulse propagates faster than its red part, resulting in progressive pulse broadening.

Fortunately, it is easy to recompress the optical pulses by providing a device that does just the opposite: providing a longer propagation time for the blue part than for the red part of the optical pulses. This scheme is referred to as dispersion compensation and actually works better than trying to eliminate the chromatic dispersion in the transport fiber. For example, dispersion-shifted fiber (DSF) was developed to provide negligible chromatic dispersion, but brings new problems, including larger nonlinear effects (Avants, web).

[5]  Chromatic dispersion is measured in Ps/NM/km. This means for every km of fiber traveled through, a pulse with a 1 NM spread of wavelengths will disperse by 1 Ps (Ps = picosecond = 1 x 10^-12 second) for a dispersion of 1 Ps/NM/km. Therefore you can see that with a 1 Ps/NM/km chromatic dispersion, a 10-Gbit/s pulse with a 0.2nm spectral width will have spread by a whole bit period (100 Ps) after 500 km of fiber and will then be completely indistinguishable (chromatic dispersion, bb.csis chapter 5; web).

[6] Fiber-Bragg-grating compensators:

Proposed for dispersion compensation two decades ago, a fiber Bragg grating (FBG) consists of a longitudinal index modulation in the core of an optical fiber.1 The light is reflected by the FBG when its wavelength satisfies the interference condition dictated by the modulation period. For dispersion-compensation purposes, the modulation period varies along the fiber axis such that the blue part and red part of an optical pulse are reflected back at the far and front parts of the FBG, respectively.

 

Since the FBG-compensation idea was first conceived-and especially over the past five years-significant advances have been made. This technology is now mature enough for dispersion compensation and represents the first deployed compensation technology besides DCF. The major advantage of FBG technology is its ability to provide tunable dispersion compensation-a critical feature required for 40 Gbit/s communication systems that cannot be met by DCF. Avants:  Laserfocusworld. Retrieved from http://www.laserfocusworld.com/articles/print/volume-43/issue-1/features/dispersion-compensation-fbgs-enhance-dispersion-compensation.html

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Hardening the Hardware Firewall

by Neil Lynch

Hardening the Hardware Firewall
              Inevitably, some attacks will get through the network firewall and reach individual hosts which can be devastating to a company. In this project I’ll briefly discuss the hacker’s motives and partial techniques, explain the function of a firewall, its vulnerabilities, and proven (tested) recommendations to harden it from the probing attacker.
             The malicious hacker’s motive, as opposed to the hacker who breaks-in for the thrill, (to demonstrate a company’s vulnerabilities) entails:
Breaking into corporate networks
Hacking into accounts without authorization
Stealing confidential data
Doing damage to critical infrastructure
Disrupting business continuity (day-to-day revenue generating operations)
Denial-of-service attacks, etc.
            “Hacked computer systems remain one of the most dangerous and frightening fears of the modern era … often it is unclear that a computer system has been hacked until it is too late” (laws.com).
The hacker studies his victim:
             Once the hacker has completed his reconnaissance and decides on his victim, he then penetrates the vulnerable firewall in one of many ways: via holes in the Access Control List (ACL), Zero-day exploits, etc., to acquire the target IP address range where he’ll starts probing the network for vulnerable hosts.
He sends probe packets into a network designed to elicit replies from internal hosts and routers within the address range. When a host receives an ICMP Echo reply message from an IP address, it knows there is a live host at that IP address. Once the hacker knows the IP address of live hosts, he needs to know what program the identified hosts are running because most attacks rely on the vulnerabilities in specific programs… The attacker sends port scanning probes to each identified host in order to determine which application the host is running. Once the attacker knows what programs the identified host is running, he can exploit the vulnerabilities in these specific programs. If the exploit succeeds, the attacker "owns" at least an account and may "own" the computer itself … Attackers take steps to make their actions more difficult to detect or analyze; hackers spoof their IP address, so as not to be identified. (Boyle et al. 32-33) 
             The hardware firewall, programmed by an administrator, contain preventative measures … designed to block unauthorized entry into the company’s network; the firewall, physically located between the internet and the network, filters all traffic before it enters the network … (ingress filtering) and also filters all packets before it leaves the network … (egress filtering), using a firewall technology called Stateful Packet Inspection.
             SPI uses different specific examination methods depending on the state of the connection; it is important to check for different things during different states.
According to Rouse, Stateful inspection, also known as dynamic packet filtering, is a firewall technology that monitors the state of active connections and uses this information to determine which network packets to allow through the firewall.
Stateful inspection has largely replaced an older technology, static packet filtering. In static packet filtering, only the headers of packets are checked -- which means that an attacker can sometimes get information through the firewall simply by indicating, "reply" in the header. Stateful inspection, on the other hand, analyzes packets down to the application layer. By recording session information such as IP addresses and port numbers, a dynamic packet filter can implement a much tighter security posture than a static packet filter can.
Stateful inspection monitors communications packets over a period of time and examines both incoming and outgoing packets. Outgoing packets that request specific types of incoming packets are tracked and only those incoming packets constituting a proper response are allowed through the firewall.
In a firewall that uses stateful inspection, the network administrator can set the parameters to meet specific needs. In a typical network, ports are closed unless an incoming packet requests connection to a specific port and then only that port is opened. This practice prevents port scanning, a well-known hacking technique. (techtarget.com)
           “The hardware firewall allows a large organization to have a central solution for its firewall needs…. Configuration changes to a hardware firewall affect all the computers on that network; thus, those changes do not have to be carried out on each and every computer” (Ezeobika, Yahoo!).
             It’s the responsibility of the network administrator to set parameters and counter-measures, to harden the firewall from relentless hack-attacks. For example,
A firewall examines each packet passing through it. If the packet is a provable attack packet, the firewall drops the packet. If the packet is not a provable attack packet, the firewall passes the packet on to its destination. In firewalls, this is called a pass/deny decision. Note that a firewall passes all packets that are not provable attack packets. This means that it will pass any true attack packet that is not a provable attack packet. This will allow attack packets to get through to their target. (Boyle et al. 314)
             So if an attacker gains the ability to turn it off or even to manipulate a firewall to allow certain traffic, the result could be disastrous.
For example, assume that the subnet 200.1.1.1/24 is considered malicious and that the security administrator has dutifully configured the (ACL) to block all inbound and outbound traffic to that subnet. If a nefarious individual successfully obtains management access to the firewall, they could wreak havoc with authorized network traffic, and certainly foster all sorts of malicious traffic and system requests. Altering the ACL to permit traffic to the above-mentioned subnet is merely an academic exercise as all manner of havoc can be wreaked by said nefarious individuals. (Casey, techtarget.com)
              “Consequently, it is important to harden hosts to protect them against attack packets that the firewall does not drop” (Boyle et al. 314).
             Here are a few reasons (vulnerabilities) requiring the administrator’s perpetual vigilance and countermeasures:
Zero-day exploits
Holes in ACL rules (passing any true attack packets that is not a provable attack packet)
Improper configuration (rendering a firewall useless)
Outdated virus definitions and security updates, etc.
Outdated firmware devices
Unpatched Operating Systems
             To harden. these vulnerabilities the administrator should already be in possession of an executable/ contingency plan. In addition to, (using proven “successfully tested” recommendations):
Upgrading firmware devices
Downloading and installing patches for operating system vulnerabilities (Windows Server does this automatically).
Before upgrading firmware and installing patches … back-up, back-up, back-up, your system.  
             We should also be running the most recent version of our licensed product and have the most up-to-date security content and virus definitions. Reading operating-system log files regularly to look for and counter … suspicious activities is essential; “one of the first things many hackers do after taking over a device is to delete or at least disable event logging” (Boyle et al. 570). To prevent this intrusion firewall administrator must write more filtering rules and in actual cases of attacks, (like zero-day, SQL-injection, etc.) respond quickly to maintain business continuity.
             Moreover, by eliminating other unnecessary services, which can create vulnerabilities for the host (utilizing ram and processing power), we can increase the performance, hence the safety.
             Other recommendations for hardening the system:
Managing Users and Groups – Every user must have an account, individual accounts can be consolidated into groups.
Managing permissions – Specify and assign permissions.
Create strong passwords – Password policies must be long and complex; at least 8-characters, upper and lower case letters, numbers, and special characters.
Black holing  attacker’s IP addresses
Testing for vulnerabilities – Mistakes will be made in hardening; do vulnerability testing, follow security baseline … it ensures uniformity in hardening (Boyle et al. 378).
             It’s an enduring battle for the firewall administrator, counteracting the exploits of the hacker’s morphing motives and newly acquired ways (of counteracting the firewall administrator’s preventative measures). It’s a vicious cycle. 
            As stated above, the centrally located hardware firewall and attempts at hardening it from the malicious hacker, required administrative programming of the centrally located firewall: ingress and egress filtering of packets using SPI technology, which rejects (provable attacks packets), externally initiated connection-opening attempts via ACL signatures, etc.; (SPI replaced SPF). Egress filtering prevents replies to probe packets from leaving the network, deterring the hacker.
             Administrating the ACL and configuring the centrally located firewall affects all computers on the network (this is good, security-wise, programming-wise, and administratively). Based on connect-type (packet state) and ACL scrutiny, provable attack packets are dropped, not-provable (potential attack packets) are passed. 
             Countering vulnerabilities like zero-day exploits, holes in the ACL, improper configuration, etc., and hardening vulnerabilities using executable contingency plans, proven recommendations (upgrading firmware devices, backing-up then installing patches, eliminating unnecessary services, etc.), should’ve given us a heightened understanding of the hardware firewall, its logging abilities, and the administrator’s relentless efforts  to decipher these logs while shielding the network.














                                                                       Work Cited
Laws.com  Purpose of Hacking.
            < http://criminal.laws.com/computer-crime/hacking/hacking-purpose >
Boyle, Randall J., and Raymond. R. Panko. Corporate Computer Security. New Jersey: Prentiss
             Hall, 2013. Print
Ezeobika M.D.,  Chukwumah. The Advantages and Disadvantages of Hardware and Software  
             Firewalls. Hardware Vs. Software Firewalls: Which is Better? Yahoo, 25 Feb. 2011 .    
 < http://voices.yahoo.com/the-advantages-disadvantages-hardware-software-7927614.html >
Casey, Brad. Network Security. Enterprise firewall protection: Where it stands, where it's
             headed .Web. Published: 02 Dec 2013 
< http://searchsecurity.techtarget.com/tip/Identifying-and-preventing-router-switch-and-firewall-vulnerabilities >
Rouse, Margaret. Stateful Inspection. Web. Oct. 2009.     
              < http://searchnetworking.techtarget.com/definition/stateful-inspection >
               <http://whatis.techtarget.com/contributor/Margaret-Rouse >

MPEG-4

By Neil Lynch

              MPEG-4 is a convergence of MPEG-1, MPEG-2, etc. It incorporates a wide range of new technologies for video compression … such as custom semiconductors and increase processing-power advancements. MPEG-4 has its biggest advantage on video compression in general and IP video transport in particular; … both developments require significantly more calculations to be made in both the encoder and the decoder. Together, these innovations have allowed a 50-percent reduction in bandwidth for equivalent video quality as compared to MPEG-2 and have enabled new applications such as HD video delivery over the Internet and by-way-of Blu-ray and IPTV networks. MPEG-4 achieves many of its advances in compression efficiency through the introduction of new video objects[1][1] (Simpson); in MPEG-4 each element of a broadcast could be generated separately and then transmitted as separate data units inside the MPEG-4 stream, the decoder ... would process each of the different signals elements and then combine them to form a video signal that is sent to the viewer's display. In MPEG-2, all of the separate elements would be combined together at the broadcaster's facility and then compressed and transmitted to the viewer.  Synthetic-image technology in MPEG-4 reduces bandwidth and gives users control over items being displayed. MPEG-2 is an older compression-version and do not have these innovations; it is less bandwidth efficient when sending natural signals (brightness and color data) of thousand of pixels.

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[1][1]New video objects … created by the encoder from the natural sources, such as video cameras and audio microphones, that capture input from the natural world, or they can be created as completely new objects created from synthetic sources that are generated through computer graphics or other means; much less bandwidth is consumed when synthetic signal are sent as compared to natural signals … due primarily to the innate complexity of natural signals and the need to reproduce accurately the pixels that make up a natural image(Simpson).

Simpson, Wes. Video Over IP. Burlington: Elsevier, 2008. Print.

 

Cocoa Application Layer

By Neil Lynch

            OSX has a layered architecture with key technologies belonging to each layer; this article focuses on the Cocoa (Application) layer, its importance, special features, architecture in OS X, and frameworks.

[1]Cocoa is an application environment most specific to Mac OS X, as Cocoa-based applications run

only on Mac OS X. Cocoa is based on the Object-C object-oriented programming language. Often,

developers must use the Cocoa environment if they want to take advantage of 64-bit services. For this

reason, most of the built-in system software and new third-party software is developed for the Cocoa

environment (White 334); Cocoa’s high-level [2]APIs make it easy to add animation, networking, and

the native platform appearance and behavior to your application with only a few lines of code.

It’s also the only application environment for [3]iOS … Most of the Application you see in OS X and

iOS, including Mail and Safari, are Cocoa applications. An integrated development environment

called [4]Xcode supports application development for both platforms.  The combination of this

development environment and Cocoa makes it easy to create a well-factored, full-

featured application  (What is Cocoa, Web).  "As with all application environments, Cocoa presents

two faces; it has a runtime aspect and a development aspect…. the development aspect … is the more

interesting one to programmers.

Cocoa’s integrated suite of object-oriented software components -- classes -- enables you to

rapidly create robust, full-featured OS X and iOS applications. These classes are reusable and

adaptable software building blocks; you can use them as-is or extend them for your specific

requirements. Cocoa classes exist for just about every conceivable development necessity, from user-

interface objects to data formatting. Where a development need hasn't been anticipated, you can

easily create a subclass of an existing class that answers that need" (What is Cocoa, Web).

 The Cocoa (Application) layer includes the frameworks described in the following sections.

Fig. 1 Cocoa Application Layer

Fig.1 Framework illustration (Cocoa Application layer, Web)

According to (OS X Cocoa, web), Cocoa includes primary

frameworks such as [5]AppKit and Core- Foundation that

provide common building blocks for all Mac applications,

as well as specialized frameworks for everything from

networking and data to graphics and professional audio processing. These frameworks range from

high-level Objective-C APIs that can create amazing effects in a few lines of code, down to low-level

frameworks to manipulate every aspect of the core system. Here is a small sampling of available

frameworks provided by Cocoa:

An example, Audio and Video:

OS X delivers a rich audio and video experience through a comprehensive set of system-level

frameworks and technologies. Powerful APIs that streamline your development process make it easy

to incorporate professional-grade audio, music, media, and video functionality into your application.

“Architecturally, OS X is a series of software layers going from the foundation of Darwin to the

various application frameworks and the user experience they support. The intervening layers

represent the system software largely (but not entirely) contained in the two major umbrella

frameworks, Core Services and Application Services. A component at one layer generally has

dependencies on the layer beneath it. Figure 2 below, situates Cocoa in this architectural setting.



 Figure 2. Cocoa in the architecture of OS X

For example, the system component that is largely responsible for rendering the Aqua user interface,

Quartz (implemented in the Core Graphics framework), is part of the Application Services layer. And

at the base of the architectural stack is Darwin; everything in OS X, including Cocoa, ultimately

depends on Darwin to function” (What is Cocoa, web).          

Based on the aforementioned (Cocoa’s (Application) layer, its importance, special features, architecture in OS X, and frameworks), Cocoa’s attractiveness to third-party app developers is apparent: the advantage of 64-bit services; its high-level Application Program Interface (API); its integrated development aspect called Xcode and its development aspect is the reason why most software --- built-in and third-party --- are developed for the Cocoa environment.

                                                                                                                                           

                                                                              References

White, Kevin M. Mac OS X Support Essentials v10.6 California: Peachpit  Press, 2010. Print.  

Objective –C/Cocoa Xcode Tutorial http://www.youtube.com/watch?v=pReuTG_w_ME

What is Cocoa?       http://developer.apple.com/library/mac/documentation/cocoa/conceptual/cocoafundamentals/index.html

OS X Cocoa https://developer.apple.com/technologies/mac/cocoa.html

Cocoa Application Layer http://developer.apple.com/library/mac/#documentation/MacOSX/Conceptual/OSX_Technology_Overview/CocoaApplicationLayer/CocoaApplicationLayer.html

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[1] From its introduction as NeXTSTEP in 1989 to the present day, it has been continually  refined and tested… The Cocoa frameworks consist of libraries, APIs, and runtimes that form the development layer for all of Mac OS X. Your application will automatically inherit the great

behaviors and appearances of Mac OS X, with full access to the underlying power of the UNIX operating system. https://developer.apple.com/technologies/mac/cocoa.html

[2] API, an abbreviation of application program interface, is a set of routines, protocols, and tools for building software applications. A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together. http://www.webopedia.com/TERM/A/API.html

[3] iOS is the operating system that runs on iPhone, iPod touch, and iPad devices. The operating system manages the device hardware and provides the technologies required to implement native apps. http://developer.apple.com/library/ios/#documentation/miscellaneous/conceptual/iphoneostechoverview/Introduction/Introduction.html

[4] The Xcode developer tools package provides everything you need to create great applications for Mac, iPhone, and iPad. https://developer.apple.com/technologies/tools/

[5] “AppKit, one of the application frameworks, provides the objects an application displays in its user interface and defines the structure for application behavior, including event handling and drawing. This is the only framework of the three that is actually in the Cocoa layer” (What is Cocoa, Web).

“AppKit is the key framework for Cocoa apps. The classes in the AppKit framework implement the user interface (UI) of an app, including windows, dialogs, controls, menus, and event handling. They also handle much of the behavior required of a well-behaved app, including menu management, window management, document management, Open and Save dialogs, and pasteboard (Clipboard) behavior.

In addition to having classes for windows, menus, event handling, and a wide variety of views and controls, AppKit has window- and data-controller classes and classes for fonts, colors, images, and graphics operations. A large subset of classes comprises the Cocoa text system, described in “Text, Typography, and Fonts.” Other AppKit classes support document management, printing, and services such as spellchecking, help, speech, and pasteboard and drag-and-drop operations” (Cocoa Application layer, Web).