Understanding network topologies
Objective - Understanding network topologies.
A network topology can be referred to as a plan or a layout on how each computer is connected together or the method used on how each computer will communicate together. These can also be referred to as Physical and Logical topologies.
The bus topology exists when all the systems in a network are connected to the same cable and at the end of the cable a terminator is connected. When a packet is sent each device will ignore the packet until it reaches the correct recipient.
In order to prevent collisions between computers on the network the bus topology uses CSMA/CD (Carrier Sense Multiple Access/Collision detection) which will monitor the network for any traffic before a packet is sent over the network.
- Easy to install
- Costs are low
- Easy to add systems to Network
- Good for small Network (Ideally up to 10 computers)
- Out of date technology
- If cable breaks, whole network is down
- Difficult to troubleshoot
- Difficult to manage large network
The star topology consists of all the network nodes connected to one central device being a hub or a switch. Through this central device all the nodes within the network will be able to communicate together, this means that if one node is having problems or is unavailable the network will still be able to continue functioning.
Advantages of Star Topology
- Easy to install
- Easy to add devices to network
- Network will remain stable if one device is not available
- Easier to troubleshoot since all devices are connected to a centralized system
Disadvantages of Star Topology
- Cost are high compared to ring network
- Network dependent on Centralized System
The hybrid/tree topology is made of a combination of other topologies for example 3 networks using a star topology connected to each other using the Bus topology.
When using a mesh topology, each system is connected to each other. When a device sends data over the network to another device it takes the shortest route possible. Since each device is connected to each other there are several redundant links available in case one of them fails.
Advantages of Mesh topology
- High Availability
Disadvantages of Mesh topology
- Cost of cabling and maintenance
- Difficult to administrator
- Difficult to troubleshoot
- Difficult to implement
In order to implement any of the topologies above, different types of media must be used.
Twisted Pair Cable.
UTP and STP cable can be used for telephone system to high speed network such as the star and hybrid topologies and can reach to a distance of a 100m. The cable has four pairs of wires protected with a plastic cover. Each pair is twisted together to help eliminate interference from adjacent pairs and other electronic devices.
The star topology requires a central unit for all the UTP cables to be connected into. The central unit is called a network switch.
A network switch is responsible for routing and processing data within the network. A network can support speeds from 10mbps up to 1gbps depending on the network card installed on the computer and the network cable used.
The bus topology will require thin coaxial cable as the backbone of the network with the computers connecting directly into the backbone with BNC Connections.
Coaxial cable has a single copper conductor at its center and a plastic layer that provides insulation between the center conduction and a braided metal sheet. This makes the cable highly resistant to signal interference and can support greater cable lengths then twisted pair cable.
Since the mesh topology is used in large networks, fiber optic is used to connect all the devices together.
The fiber optic cable consists of a center glass core surrounded by several layers of protection. As fiber optic transfers light and not electronic signals, electrical interference is now eliminated. This makes fiber cable ideal when installing in environments which contain a large amount of electrical interference.
Fiber optic cable can transmit signal over long distances then both coaxial and twisted pair at greater speeds. However when comparing costs, fiber optic is the greatest from all three and since it's made from glass and plastic fibers they are prone to damage quite easily.
Recommended topology to use for police department
Since the bus topology is now a legacy system and is slow to use and mesh will have more redundancy, however it is expensive to implement and maintain. The most suitable option for the police department will be the Star topology.
The network will be wired with Cat5 UTP for the internal LAN in each department, with a CAT6 STP Cable used as backbones between the switches in each department and one main switch in the HQ Server room.
(Note: CAT6 cable is used to have 1000mbps backbone between switches)
The police headquarters will require an internet connection that will be able to cater for the services used in the headquarters such as Email, administrative tasks, VOIP PABX and Research. For these purposes, the headquarters will require a high speed internet connection. There are various ways on how to connect to the Internet; however the most common are ADSL, Cable Internet and ISDN connection.
ISDN (Integrated Services Digital Network)
ISDN is a standard which allows the user to use their existing copper telephone to transfer data, voice and video at the same time. This was a huge improvement from the previous method to connect to the internet, which was the dial up method. This method used to transfer both voice and data, at very slow speeds, but not at the same time.
ISDN can reach to speeds between 64kbps to 128 kbps however has now been overtaken by new technologies such as ADSL and Cable internet.
ADSL (Asymmetric Digital Subscriber Line)
ADSL is a high-speed internet access service that uses existing copper telephone lines to send and receive data at speeds from 128kbs to 20mbs. An ADSL internet connection will require an Internet Service Provider (ISP), ADSL Modem and a telephone filter which will be able to filter out interference between the DSL Service and the normal telephone service.
Advantages of ADSL
- The telephone service remains available when the Internet Connection is in use
- Can reach high download speeds
- Will not require installing new telephone cable
Disadvantages of ADSL
- ADSL download speed is related to the distance from the user and the nearest provider.
- Faulty cables internal or external will result into many disruptions or no service.
- When using ADSL, download speeds are much fast then uploads speeds this is not suitable for business or home users who require uploading a large amount of data daily.
In Malta ADSL is provided by GO which also provides other services such as telephony and digital television.
Cable internet is an internet access that runs of the same network as cable television. It can reach up to similar speeds as ADSL.
Cable internet is transmitted by a local cable service provider which uses a designated television cable to pass data through. The installation will require a coaxial cable from the closed distribution box into the home or business which will then be connected to a Cable Modem which via a USB or RJ45 cable connects to the computer.
Advantages of Cable Internet
- Cable internet does not require a telephone line unlike ADSL
- Once connected remains always online
- Can be faster than ADSL
- Compared to telephone copper cable, coaxial cable has less chance of breakage which will allow the internet service stable
Disadvantages of Cable Internet
- Cable Internet will not always operate at the highest speed possible; this can be affected by the number of people connected at the same time
- Initial installation costs can be expensive
- Not all areas can access cable internet
- In Malta cable internet is provided by Melita who also provide Telephone and television services
When choosing an ISP for the Police headquarters I would suggest GO's ADSL internet services. For the following reasons:
- Can be installed on current telephone system
- Installation is cheaper
- Dedicated download speed unlike cable internet which slows down depending on number of users online
- Easy to install new ADSL connection
- 24x7 support
However it is highly suggested that due to the importance that the police station remains online all the time a redundant connection is made available by an alternative source such as cable internet.
A software firewall will be install to filter out any harmful packets addressed to the network and also be configured to limit any abuse that can be made by the users such as browsing, chat and downloading of files such as MP3 and video clips.
The police headquarters require a two way communication system in order for the officers on patrol can keep communication between themselves and their superior officers at the head quarters.
The two way communication system must be secure and reliable.
VOIP - Voice over Internet Protocol
VOIP also referred to as Internet telephony, uses the internet to send voice data instead of the Public Switched telephone network. When a user makes a call, the analog voice waves are transferred to a digital format which is then split up into packets and sent over an IP network. A reverse sequence is then used with the help of codec's at the other end of the call for the recipient to hear the message.
This method will require specialized equipment which will have inbuilt mechanisms to convert and reconvert the packets being sent over the IP network.
To use VOIP, the officers patrolling the City must use a mobile device. Latest mobiles that are being released come with a SIP Application installed such as XLITE. However there are a range of WIFI IP Phones that are on the market.
Since the Police officers on patrol will require network access to connect to the PBX System, a Wi-Fi Connection will be required to cover the area of Townsville.
Using the 802.11G standard, a number of Wi-Fi base stations such as the Vivato 802.11G outdoor Wi-Fi base station around the Townsville are installed around the town so a Wi-Fi connection will always be available for all the Wi-Fi devices.
The Vivato 802.11G outdoor Wi-Fi base station supports all encrypted methods for the connection to remain secure and it is important that the SSID's are disabled so no one from the public will be able to view the network.
VOIP over Wi-Fi Limitiatons
- Emergency calls - Since VOIP works with packets being transferred over an IP Network the location of the person making an emergency call will not be tracable.
- Dead Spots - Even though that the Wi-Fi Base stations will cover most of the areas in townsville there will always be area's where there will be no Wi-Fi coverage.
- Cost - Most IP Phones especially the Mobile IP phones are expensive.
- The whole telephony system will be relying on the VOIP PABX server within the headquaters. If the server is unavailable for some reason or another the whole two communication system will not be available.
- Security - Even though that most encryptions are supported by the Wi-Fi Outdoor Base Station there is still a risk that a hacker will be able to tap into the system.
- Availabilty - Wi-Fi does encounter problems when there are electronic devices or metal stuctures interfering.
Digital Two Way Radio
A two way radio system is a mobile device that transmits and recieves voice over the radio spectrum using the Ultra High Frequency (UHF) and Very High Frequency (VHF) Bands
Analog radio has been used succesfully in the past, however can be interupted by several enviromental factors which effects how the message is received. In order to hear a clear transmission, a quality booster is used which drains the radio's battery.
Digital radio includes a built-in error correction technique which rebuilds the voice transmission to near to perfect transmission without the use of quality boosters. This gives a longer life to the radio's battery. This is important for the Police who are patrolling the area as they will require a clear signal all the time and long battery life. Full duplex is also supported by Digital Radio Sets which allow the user to send and receive the same time.
The following hardware is required to implement a digital two way radio system:
The XPR 4550 mobile two-way radio features integrated GPS, 160 channels and an intuitive, menu-driven interface. It enables easy migration with operational capability in both analog and digital, and is available in UHF and VHF.
The XPR 6550 digital portable two-way radio with display has an integrated GPS modem for advanced location tracking. Theradio delivers reliable communication by combining two-way radio functionality with digital technology. It permits easy migration with operation in both analog and digital, and is available in UHF and VHF versions, both with 160 channels.
The XPRTM 8300 repeater is a continuous-duty unit that supports two simultaneous voice or data paths in digital TDMA mode. It mounts easily in wall or rack systems.
GTR 8000 Base Station/Repeater
GTR 8000 Base Radios support ASTRO® 25 (IV&D) and High Performance Data systems. The GTR 8000 Base Radio is available for IV&D systems in 700/800 MHz, UHF (435-524 MHz), and VHF (136-174 MHz), and for HPD systems in 700/800 MHz.
In Malta, a two-way radio license issued by the MCA is required for the operation of two-way radios, except for Private Mobile Radios (PMRs) which power up to 500 milliwatts (mW).
Disadvantages of Digital Two Way Radio
- Expensive to setup
- Must pay expensive license fees per device
- Repeaters to extend range which will also require license fee
- Equipment can be bulky
- Mobile equipment will require servicing
For the Police headquarters, the digital two way radio is recommended for the following reasons:
- Devices offer Global Positioning Service which will show the position of the user of a radio set.
- More battery life
- Can easily be implemented in Squad Cars
- Backward compatible with analog systems
- Distress Signal
- Technology has been proven and tested over a number of years
- Covers more ground
The police headquarters will require an internal telephone system in order to communicate between them and with the outside world.
Overview of a PABX System
In a large business or organization, a PABX will be required. With a PABX, telephone calls within the same office can be made for free as there will be no need to access the Public Switched Telephone Network (PSTN).
A PABX system will also have trunk lines which connect to the Public Switched Telephone Network (PSTN). When calling outside of the office the user will have to dial in a special number (for example "0") in order to access the PSTN and make a call.
Other features of a PBX system include:
Do Not Disturb
VOIP - IP PBX
The IP PBX is a telephone system which sends voice, data and messaging using the Internet Protocol and will also be able to connect to the Public Switch Telephone Network (PSTN).
A dedicated server which will act as a software based PBX there are several software packages one can use in order to run a software based PBX such as the 3cx software system and Asterix. Such applications will also offer services such as fax, voice mail boxes and chat. You will also be able to see the users that are online, marked as busy or currently on a call.
For use in the Headquarters the following must be installed.
When using normal analog telephone sets an ATA (Analog Telephone Adaptor) will be required. Basically the ATA will be the converter that connects a normal telephone set to the Network.
From the image below one can see a RJ11 plug can be connected into the adaptor and there is a RJ45 port which is used to connect a Network cable to the network.
Another alternative to the ATA adaptor will be to implement IP telephone sets. These telephone sets have an inbuilt ATA and can be connected directly into the network using a normal patch lead.
A soft phone is an application that can act as an IP Phone without installing any hardware. A headset can then be plugged in the user's computer in order to participate in a telephone discussion.
Features of using a VOIP PBX system running with 3CX
3CX Phone System for Windows is a software-based IP PBX that replaces a proprietary hardware PBX / PABX. 3CX's IP PBX has been developed specifically for Microsoft Windows and is based on the SIP standard - making it easier to manage and allowing you to use any SIP phone (software or hardware). A software-based IP PBX / PABX offer numerous benefits:
- Easier to install & manage via web-based configuration interface
- Far less expensive to purchase and expand than a hardware-based PBX / PABX
- Improve productivity with presence, desktop based call control and extension management
- No need for separate phone wiring - phones use computer network
- Delivery mobility by allowing employees to work from home using a remote extension
- Choose between popular IP hardware phones or soft phones - no vendor lock in
- Receive & Make calls via the standard PSTN using VOIP Gateways or cards
- Save on monthly call costs using SIP trunks, VoIP providers or the Skype gateway!
Hosted PBX system
Telephony providers can provide a remote PBX system hosted at a remote location and is made available either via the PSTN or the internet. This will mean that the police headquarters will not require buying or installing any PBX equipment and all maintenance is done by the provider, however will still be able to use all the features within a PBX.The Police headquarters will require access to a console that will allow an administrator to change settings, such as call forwarding and configuring voice mail, using a hosted PBX system will allow users to access the company's telephone system from there home's or abroad.
Other features included in a Hosted PBX system include:
- After Hours Auto-Attendant
- User Voice Mail
- Voice Mail to Email Alert
- Voice Mail Forwarding
- Send to Voice Mail
- Web Voice Mail
- Ring and Hunt Groups
- Music or Messages on Hold
- External Transfer
- Video Conference
When setting up the system the provider can also lease out the required number of pre configured telephone sets or use existing sets and connect them to an ATA which will act as an interface between the telephone set and the rest of the network.
Key Telephone System (KTS)
A key telephone system will allow a user to access the company's telephone lines without the use of a centralized unit with the aid of a number of buttons that indicate a number of lines which have will include a LCD which will flash when a telephone line is busy or a call is in waiting to be answered.
Other buttons will have different functions very similar to a PBX system such as:
- Calling another extension internally
- Access to the Public Switching Telephone System for external calls
- Keeping a call on hold while making another call externally or internally
- Transfer an external call to another extension
- Conference calling
- On Hold Music
- Voice mail functions
A hosted PBX system will sound as the natural choice when implementing a telephone system within the Police Headquarters. However due to the sensitivity of certain calls made been high chief constables and the deputies it is a must that the system is contained in house. This will also allow an administrator to directly create new rules and add new extensions to the system. The Key Telephone System is cost effective and the system will still remain in-house however a Key Telephone System caters for small offices which have up to 20 users.
The Townsville Police Headquarters has requested the following requirements for a new network system: a internet connection, two way communication method to keep in contact with the police officers patrolling the town and a telephone system to implement at the headquarters.
The initial phase of a new network will be to create a layout and have an understanding on how the data will flow through the network. Four topologies where chosen to investigate which one is appropriate for the police headquarters network. The topologies investigated were: bus, star, hybrid and mesh. Assuming that the police headquarters will have more the one story, a backbone (CAT6 STP cable) must be installed to join each switch together, hence Star Topology was recommended.
A high speed connection was required to cater for communication and research needs. Since there will be a large number of users browsing the internet, it is very important that the ISP chosen offers a high download speed rate.
It is also a must that the Internet service provider provides 24x7 specialist support for commercial entities.
As an ISP, GO was chosen as it caters both requirements. It is also suggested that a dual wan broadband router will be installed which will allow the police headquarters to install another internet connection for redundancy and load balancing.
I also made an assumption that a System administrator will monitor and restrict abuse on the internet connection.
As for the two way communication system, two system's have been evaluated which were the VOIP over Wi-Fi and the Digital Two Way radio system. VOIP is a technology which is improving and advancing security issues. However there are a number of concerns that can be a hazard for the Constable patrolling the town.
The main concerns where due to the fact that Wi-Fi will not be available in all areas, a Wi-Fi Phone is not traceable and there will remain a possibility that the Wi-Fi network is tapped into. Therefore for Digital Radio was chosen for a more stable and proven system for the Two Way radio communication method.
Regarding the telephone system, the PBX and key telephone system were the two technologies discussed about. The current trend is a VOIP PBX as it is easy to maintain and administer, SIP phones can be installed on computers, laptops and mobile devices avoiding installation of new telephone sets in the building.
Hosting PBX service sounds interesting and cheap to run as there is no need for any equipment besides for the telephone sets to be installed at the office. However, since the PBX system will not be hosted at the office one can never be sure who will be able to access the PBX system and tap into sensitive calls.
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