CIT 513: Introduction to Computer Communications

1. CIT 513: Introduction to Computer Communications Mohammed A. Saleh http://ifm.ac.tz/staff/msaleh/CIT513.html

2. WAN Technologies • Many technologies are used to create today's wide area networks (WANs) • Each of these technologies has pros and cons, making some of them well suited for certain environments and completely impractical in others • Each of the technologies varies in terms of media, speed, availability, and cost • WANs span great distances compared to LANs , which are restricted in a single area • One of the most important element of WAN technologies are the switching methods.

3. Switching Methods • In order for systems to communicate on a network, there has to be a communication path or multiple paths between which the data can travel • To communicate with another entity, these paths move the information from one location to another and back • This is the function of switching • Switching provides communication pathways between two endpoints and manages how data is to flow between these endpoints • Two of the more common switching methods used today include: • Packet switching • Circuit switching

4. Packet Switching • Messages are broken down into smaller pieces called packets • Each packet is assigned source, destination, and intermediate node addresses • Packets are required to have this information because they do not always use the same path or route to get to their intended destination • The biggest advantage of packet switching is independent routing • Independent routing allows for a better use of available bandwidth by letting packets travel different routes to avoid high-traffic areas

5. Cont … • It also allows packets to take an alternate route if a particular route is unavailable for some reason. • In a packet-switching system, when packets are sent onto the network, the sending device is responsible for choosing the best path for the packet • This path might change in transit, and it is possible for the receiving device to receive the packets in a random or non sequential order • When this happens, the receiving device waits until all the data packets are received, and then it reconstructs them according to their built-in sequence numbers.

6. Cont … • The receiving computer reassembles the packets into the original message Packet-switching

7. Cont … • Two types of packet-switching methods are used on networks: • Virtual-Circuit Packet Switching • A logical connection is established between the source and the destination device • This logical connection is established when the sending device initiates a conversation with the receiving device • The logical communication path between the two devices can remain active for as long as the two devices are available or can be used to send packets once • After the sending process has completed, the line can be closed.

8. Cont … • Datagram Packet Switching • Datagram packet switching does not establish a logical connection between the sending and transmitting devices • The packets in datagram packet switching are independently sent • they can take different paths through the network to reach their intended destination • To do this, each packet must be individually addressed to determine where its source and destination are. • This method ensures that packets take the easiest possible routes to their destination and avoid high-traffic areas.

9. Circuit Switching • It requires a dedicated physical connection between the sending and receiving devices. • Most commonly used in a telephone conversation in which the parties involved have a dedicated link between them for the duration of the conversation. • When either party disconnects, the circuit is broken and the data path is lost • The sending system establishes a physical connection • The data is transmitted between the two, and when the transmission is complete, the channel is closed. • The primary advantage is: • after a connection is established, there is a consistent and reliable connection between the sending and receiving device • This allows for transmissions at a guaranteed rate of transfer.

10. Cont … • Circuit switching has downsides: • a dedicated communication line can be very inefficient • After the physical connection is established, it is unavailable to any other sessions until the transmission is complete

11. Cont … Circuit-switched network

12. Fiber Distributed Data Interface • This is a topology standard that uses fiber-optic cable and token-passing media access. • Transmissions speeds of up to 100Mbps at distances of more than 2 kilometers • FDDI combines the strengths of Token Ring, the speed of Fast Ethernet, and the security of fiber-optic cable • Such advantages make FDDI a strong candidate for creating network backbones and connecting private LANs to create MANs and WANs. • Unlike the regular 802.5 network standard, FDDI uses a dual-ring configuration

13. Cont … • The first, or primary, ring is used to transfer the data around the network, and the secondary ring is used for redundancy and fault tolerance • the secondary ring waits to take over if the primary ring fails • If the primary ring fails, the secondary ring kicks in automatically, with no disruption to network users. • The other advantage is it using fiber-optic cable as its transmission media • resistance to EMI, the security offered by fiber, and the longer distances available with fiber cable

14. Cont … • Strong points of FDDI • Fault-tolerant design • By using a dual-ring configuration, FDDI provides some fault tolerance • If one cable fails, the other can be used to transmit the data throughout the network • Speed • Use of multiple tokens Unlike the IEEE 802.5 standard, FDDI uses multiple tokens, which increase the overall network speed. • Beaconing • FDDI uses beaconing as a built-in error-detection method, making finding faults, such as cable breaks, a lot easier.

15. Cont … • Downsides of FDDI • High costs • The costs associated with FDDI and the devices and cable needed to implement an FDDI solution are very costly; too costly for many small organizations. • Implementation • FDDI setup and management can be very complex • Requires trained professionals with significant experience to manage and maintain the cable and infrastructure.

16. Internet Access Technologies • It has become an integral part of modern business. • There are several ways to obtain Internet access • The type chosen will often depend on the cost as well as what technologies are available in the area you are located • We will also look at the terms related to the Internet • We will explore the some of the more common methods of obtaining Internet access.

17. xDSL Internet Access • DSL is an Internet access method that uses a standard phone line to provide high-speed Internet access • Most commonly associated with high-speed Internet access • because it is less expensive • often used in homes and small businesses. • With DSL, a different frequency can be used for digital and analog signals • which means that you can talk on the phone while you're uploading data.

18. xDSL Internet Access • DSL is an Internet access method that uses a standard phone line to provide high-speed Internet access • Most commonly associated with high-speed Internet access • because it is less expensive • often used in homes and small businesses. • With DSL, a different frequency can be used for digital and analog signals • which means that you can talk on the phone while you're uploading data. • Use skype while uploading an email document

19. Variations of xDSL • Asymmetric DSL (ADSL) • The most common of the DSL varieties is ADSL • ADSL uses different channels on the line: • One channel is responsible for analog traffic • The second channel is used to provide upload access • The third channel is used for downloads • With ADSL, downloads are faster than uploads. • Symmetric DSL (SDSL) • Offers the same speeds for uploads and for downloads • Most suitable for business applications such as Web hosting, intranets, and e-commerce • Not widely implemented in the home/small business environment and cannot share a phone line.

20. Variations of xDSL • Integrated Services Digital Network DSL (IDSL) • This is a symmetric type of DSL that is commonly used in environments where SDSL and ADSL are unavailable • IDSL does not support analog phones • Rate Adaptive DSL (RADSL) • This is a variation on ADSL that can modify its transmission speeds based on the signal quality • RADSL supports line sharing • Very High Bit Rate DSL (VHDSL) • This is an asymmetric version of DSL and, as such, can share a telephone line. • High Bit Rate DSL (HDSL) • This is a symmetric technology that offers identical transmission rates in both directions. • HDSL does not allow line sharing with analog phones

21. Cont … • Why are there are so many DSL variations? • Each flavor of DSL is aimed at a different user, business, or application. • Businesses with high bandwidth needs are more likely to choose a symmetric form of DSL • Budget-conscious environments such as home offices are likely to opt for an option that allows phone line sharing at the expense of bandwidth

22. Maximum Speeds of DSL

23. Cable Internet • This is an always on Internet access method that is available in areas that have digital cable television • Attractive to many small businesses and home office users because it is both inexpensive and reliable • Most cable providers do not restrict how much use is made of the access • Connectivity is achieved by using a device called a cable modem • it has a coaxial connection for connecting to the provider's outlet and an Unshielded Twisted Pair (UTP) connection for connecting directly to a system or to a hub, switch or WAP. • Providers often supply a cable modem free of charge • You are paying for the rental of the modem in a monthly service fee

24. Cont … • Most cable modems supply a 10Mbps Ethernet connection for the home LAN. • You wouldn't expect the actual Internet connection to reach these speeds • The actual speed of the connection can vary somewhat depending on the utilization of the shared cable line in your area • In day-to-day application, data rates range from 1.5Mbps to 3Mbps. • One of the biggest disadvantages of cable access • You share the available bandwidth with everyone else in your cable area

25. Cont … • Most cable modems supply a 10Mbps Ethernet connection for the home LAN. • You wouldn't expect the actual Internet connection to reach these speeds • The actual speed of the connection can vary somewhat depending on the utilization of the shared cable line in your area • In day-to-day application, data rates range from 1.5Mbps to 3Mbps. • One of the biggest disadvantages of cable access • You share the available bandwidth with everyone else in your cable area • As a result, during peak times, performance of a cable link might be poorer than in low-use periods • In residential areas, busy times are evenings and weekends

26. Broadband Security Considerations • Whether using DSL or cable Internet access, there are a few things to keep in mind • Each of these technologies offers always on service • This means that even when you are away from your computer, it is still on the Internet • This creates a security risk • The longer you are online, the more chance someone has of remotely accessing your system. • The operating systems we use today all have some security holes through which some people are waiting to exploit • Combining OS security holes with an always on Internet technology is certainly a dangerous mix

27. Cont … • DSL and cable Internet connections have to be protected by mechanisms such as firewalls to protect the system • The firewall system will offer features such as packet filtering • The firewall can be a third-party software application installed on the system, or it can be a hardware device • It is equally important to ensure that the operating system you are using is completely up-to-date in terms of service packs and security updates • Today's client systems typically offer automatic update features that will alert you when a new security update is available. • Following a few safety rules, both DSL and cable Internet can provide safe Internet access

28. Satellite Internet Access • DSL and cable Internet access technologies are not offered everywhere • For areas where cheaper broadband options are not available • There are a limited number Internet options • One of the primary ones is Internet via satellite. • Satellite access provides a viable Internet access solution for those who cannot get other methods of broadband • Satellite Internet offers an always on connection with theoretical speeds advertised anywhere from 512Kbps upload speeds to 2048Kbps download speeds

29. Cont … • One of the primary drawbacks to satellite Internet is the cost • Even with the high price tag, it is not as fast as DSL or cable modem • Satellite Internet is slower and more costly than DSL or cable, it offers some very attractive features • Portability - wherever you go, you can have Internet access, this is beneficial for remote users and clients • Satellite internet access is also considered as a wireless technology where atmospheric conditions can significantly affect the performance of satellite Internet access

30. Wireless Internet Access • It is provided by a Wireless Internet Service Provider (WISP) • The WISP provides public wireless Internet access known as hotspots • Hotspots provide Internet access for mobile network devices such as • laptops, handheld computers, and cell phones in airports, coffee shops, conference rooms, and so on • A hotspot is created using one or many wireless access points near the hotspot location • As of today, hotspots are not everywhere, but finding them is not difficult • airports, hotels, and coffee shops will advertise that they offer Internet access for customers or clients

31. Cont … • Establishing a connection to a wireless hotspot is a straightforward process. • If not equipped with built-in wireless capability, laptops will require an external wireless adapter card • With the physical requirements of the wireless card taken care of, connect as follows: • When you arrive at the hotspot site, power up your laptop. In some instances, you might need to reboot your system if it was on standby to clear out old configuration settings • The card might detect the network automatically. If this is the case, configuration settings, such as the SSID, will be automatically detected, and the wireless Internet will be available. If Internet access is free, there is little else to do; if it is a paid-for service, you will need to enter a method of payment. One thing to remember is to verify that you are using encryption for secure data transfer.

32. Cont … • When you arrive at the hotspot site, power up your laptop. In some instances, you might need to reboot your system if it was on standby to clear out old configuration settings • The card might detect the network automatically. If this is the case, configuration settings, such as the SSID, will be automatically detected, and the wireless Internet will be available. If Internet access is free, there is little else to do; if it is a paid-for service, you will need to enter a method of payment. One thing to remember is to verify that you are using encryption for secure data transfer. • If for some reason the wireless settings are not automatically detected, you will need to open up your wireless NICs configuration utility and manually set the configurations. These settings can include setting the mode to infrastructure, inputting the correct SSID, and setting the level of encryption used.

33. Cont … • Some companies such as hotels and cafes will provide wireless Internet access by connecting a wireless router to a DSL or cable Internet connection • The router becomes the wireless access point to which the users connect • It allows clients to connect to the Internet through the broadband connection • The technology is based on the 802.11 standards, typically 802.11b/g, and client systems require only an internal or external wireless adapter.

34. Questions