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CH/S6CA/Apr. 2009 SOHO Networking A network devices. Networking is the process of connecting computers that permits the transfer of data and programs between them. Today networking is the fastest growing area in information technology. is a communication system that connects two or more computers and their peripheral Need for networking There are several reasons why a company or an organisation would choose to implement a network. The four most important ones are: i. Fast response: a network system allows users at various locations to get immediate responses to inquiries. ii. Up-to-the-minute database: a major benefit of a network system is that a user can maintain an up-to-the-minute database. iii. Hardware resource sharing: Every computer on the network is allowed to use certain expensive devices. iv. Information resource sharing: It allows using a computer on the network to access data and software stored on any other computer or the host computer in the network. Distributed processing: Rather than using a large mainframe, several smaller computers can be used around the organisation and in each branch office, by connecting together in networks. A distributed system is a system in which a job is performed by a number of computers that are linked through a communication network.. Types of Networks - a geographical distribution (p. 68 – p. 69) Local Area Network (LAN). i. Local area networks are networks in which all the computers and terminals are located in the same offices or buildings. ii. They enable all the computers on the site to communicate with each other, and share peripherals such as hard disk, printers and external communications links. iii. LANs represent a low-cost solution for low-volume data transfer over relatively short distances. iv. Network models of a LAN a. b. Client / Server model It uses a central computer, known as the server , to maintain all the files used by the users. Each individual computer on the network can have access to the files on the server. Advantage: centralized management, higher security, handling large networks more efficiently. Peer-to-peer model It has no server. It allows any computer to share the hardware, software, or information located on any other computer in the network. It is commonly found in small organisations. Advantage: no single point of failure affecting the whole network, faster in distribution, low cost and easy to set up. SOHO NETWORKING page 1 CH/S6CA/Apr. 2009 Bit Torrent (BT) is a Peer-to-peer file sharing protocol used to distribute large amounts of data. The initial distributor of the complete file or collection acts as the first seed. Each peer who downloads the data also uploads them to other peers. Wide Area Network (WAN). i. Wide area networks (WANs) use telephone lines, microwave relays and satellites to reach data over long distances in the same country or outside country on different continents between various centres. ii. Under a WAN, each location needs to access resources locally but also needs to share data with the other locations. ** This is another type of network namely the Metropolitan Area Network (MAN) with its scale larger than LAN and smaller than WAN. Packet Most computer networks do not transfer data as an arbitrary string of continuous bits. Instead the network system divides data into small blocks called packet , which it sends individually. Computers are often called packet networks or packet switching network because they use packet technology. Reasons of using packets instead of sending data together. i. To ensure that all computers receive fair, prompt access to a shared communication facility, a network system cannot allow one computer to deny access to others. Using small packets helps ensure fairness. ii. Dividing the data into small blocks helps a sender and receiver determine which blocks arrive intact and which do not. There is no universal agreement on the exact format of a packet. The specific definition of packets used with a specific type of network is called a frame The sending and receiving computers must agree on how they will specify the beginning and end of each frame as follows. (soh : start of header, eot : end of transmission) In a data packet sent via the Internet, the data are preceded by header information such as the sender's and the recipient's IP addresses, the protocol governing the format of the data and several other formats. The header's format is specified in the Internet Protocol. In information technology, trailer refers to supplemental data placed at the end of a block of data being stored or transmitted, which may contain information for the handling of the data block, or just mark its end. header data trailer The IPv4 packet header consists of (reference only): i. 4 bits that contain the version, that specifies if it's an IPv4 or IPv6 packet, SOHO NETWORKING page 2 CH/S6CA/Apr. 2009 ii. 4 bits that contain the Internet Header Length which is the length of the header in multiples of 4 bytes. Ex. 5 is equal to 20 bytes. iii. 8 bits that contain the Type of Service, also referred to as Quality of Service (QoS), which describes what priority the packet should have, iv. 16 bits that contain the length of the packet in bytes, v. 16 bits that contain an identification tag to help reconstruct the packet from several fragments, vi. 3 bits that contain a zero, a flag that says whether the packet is allowed to be fragmented or not (DF: Don't fragment), and a flag to state whether more fragments of a packet follow (MF: More Fragments) vii. 13 bits that contain the fragment offset, a field to identify which fragment this packet is attached to, viii. 8 bits that contain the Time to live (TTL) which is the number of hops (router, computer or device along a network) the packet is allowed to pass before it dies (for example, a packet with a TTL of 16 will be allowed to go across 16 routers to get to its destination before it is discarded), ix. 8 bits that contain the protocol (TCP, UDP, ICMP, etc...) x. 16 bits that contain the Header Checksum, a number used in error detection, xi. 32 bits that contain the source IP address, xii. 32 bits that contain the destination address. TCP/IP A TCP/IP includes a common set of communication languages or standards allows different types of computers and communication devices to talk to each other. TCP is responsible for the provision of reliable end-to-end data transmission. IP specifies how packets are routed from computer to computer. IP address. (Act. 4: Pls. refer to http://computer.howstuffworks.com/internet-infrastructure5.htm ) protocol is a set of rules for the exchange of data between communication devices. i. 32 bits (divided into four sets of 8 bits (ranging from 0 to ii. e.g. 202.128.233.88 (For a Class C network: network ID – 255 ) 202.128.233 , host ID: 88 ) Classwork: Try type the following command in DOS prompt and drop down the result 1. ping www.cuhk.edu.hk SOHO NETWORKING page 3 CH/S6CA/Apr. 2009 2. ipconfig More on IP address The IP Address Hierarchy. i. An IP address is a unique 32-bit binary number assigned to a host and used for all communication with the host. ii. Each 32-bit IP address is divided into two parts: a level hierarchy is designed to make routing iii. The address prefix identifies the physical network to which the computer is attached, while the suffix identifies an individual computer on that network. iv. The IP address hierarchy guarantees two important properties: prefix efficiently. and suffix ; the two- a. Each computer is assigned a unique address. b. Although network number assignments must be coordinated globally, suffixes can be assigned locally without global coordination. Original Classes of IP Addresses i. The suffix need sufficient bits to permit each computer attached to a network to be assigned a unique suffix. Thus if the size of the network is larger, there should be more bits in the suffix. (8 bits for 256 computers, 9 bits for 512 computers, so on) ii. A single internet can contain a mixture of large and small networks. Thus the IP address designers chose a compromise addressing scheme that can accommodate a combination of large and small networks. iii. The original IP addressing scheme divides the IP address into three primary classes, where each class has a different size prefix and suffix. SOHO NETWORKING page 4 CH/S6CA/Apr. 2009 iv. Class D is used for multicasting, which allows delivery to a set of computers. To use IP multicasting, a set of hosts must agree to share a multicast address. Once a multicast group has been established, a copy of any packet sent to the multicast address will be delivered to each host in the set. v. Dotted decimal notation. Although IP addresses are 32-bit numbers, the software often uses a notation that is more convenient for humans to understand, i.e. the form expresses each 8-bit section of a 32-bit number as a decimal value and uses periods to separate the sections. It is called dotted decimal notation. E.g. 10.127.48.152. vi. More examples: vii. The class of the address can be found from the 1st 4 bits of the address. viii. Class work find the Class, prefix and suffix from the following IP addresses :IP address SOHO NETWORKING Class Prefix Suffix page 5 CH/S6CA/Apr. 2009 48.45.49.43 00110000 A 48 45.49.43 134.1.168.222 B 134.1 168.222 200.200.200.200 C 200.200.200 200 193.12.166.253 C 193.12.166 253 220.166.222.222 C 220.166.222 222 223.255.255.255 C 223.255.255 255 SOHO NETWORKING page 6 CH/S6CA/Apr. 2009 Division of the address space i. The IP class scheme does not divide the 32-bit address space into equal size classes. ii. The classes do not contain the same number of networks. iii. The number of networks and hosts per network in each of the three primary IP address classes are as follows: Authority for addresses i. For networks connected to the global Internet, an organization obtains network numbers from its Internet Service Provider (ISP). ii. ISPs coordinate with a central organization, the Internet Assigned Number Authority, to ensure that each network prefix is unique throughout the entire Internet. A classful addressing example i. Consider an organization that chooses to form a private TCP/IP internet which consists of four physical networks. ii. The network administrator estimates the ultimate size of each physical network, and uses that size to choose a prefix. iii. If the organization expects one small network, two medium networks, and one extremely large network, the administrator might choose to assign a class C prefix(e.g. 192.5.48), two class B prefixes (e.g., 128.10 and 128.211), and a class A prefix (e.g., 10) as shown in following figure. Note that the size of the cloud used to denote a physical corresponds to the number of hosts expected on the network. SOHO NETWORKING page 7 CH/S6CA/Apr. 2009 Subnet i. The original classful addressing scheme has a limitation as all networks had to choose one fo three possible sizes. ii. Subnet addressing. Also known as classless addressing. Instead of having three distinct address classes, the division between prefix and suffix to occur on an arbitrary bit boundary. iii. Under subnet addressing, a subnet mask is used. 1 bits mark the network prefix and zero bits mark the host portion. iv. When a router is given a destination address, D, and a pair (A, M) that represent a 32-bit IP address and a 32-bit address mask. To make a comparison, the router tests the condition: A == (D &M) v. The size of a subnet mask is represented in the IP address after ‘/’, e.g. 128.211.0.0/16. Special IP addresses SOHO NETWORKING page 8 CH/S6CA/Apr. 2009 Network Address – suffix all-0s (e.g. 128.211.0.0/16). i. It denotes a network. ii. It does not refer to any host computer attached to that network. Directed Broadcast Address – suffix all-1s (e.g. 128.211.255.255/16). i. When a packet is sent to a network’s directed broadcast address, a single copy of the packet travels across the internet until it reaches the specified network. ii. The packet is then delivered to all hosts on the network. Limited Broadcast Address – prefix all-1s, suffix all-1s (i.e. 255.255.255.255). i. IP will broadcast any packet sent to all hosts on the local network. ii. It is used during system startup by a computer that does not yet know the network number. This Computer Address – prefix all-0s, suffix all-0s (i.e. 0.0.0.0). i. The TCP/IP protocol suit contains protocols a computer can use to obtain its IP address automatically when the computer boots. ii. When using such startup protocols, a computer cannot supply a correct IP source address, This Computer Address is used. Loopback Address – prefix 127 (e.g. 127.0.0.1/8). i. It is used to test network applications. ii. To perform a loop back test, a programmer must have two application programs that are intended to communicate across a network. iii. Instead of executing program on a separate computer, the programmer runs both programs on a single computer and instructs them to use a loopback IP address when communicating. Routers and IP addresses i. A router has connections to multiple physical networks. ii. Each router is assigned two or more IP addresses each containing a prefix that specifies a physical network connected to it. SOHO NETWORKING page 9 CH/S6CA/Apr. 2009 Common protocols In order to ensure that different manufacturers’ products can be connected properly in one network, some international organisations set up a strict set of rules called for communication. It describes the following things: i. the language to be used. ii. the speed at which the data is sent. iii. odd or even parity check being used. iv. the types of codes that identify stations and data storage devices on the network. v. the type of line control in use i.e. how an individual station gains control of the communication line so that it can talk without interrupting others or being interrupted. There are many common protocols used in Networking, especially in the Internet. E.g. TCP, IP, HTTP, HTTPS, UDP, DNS, POP3, SMTP, IMAP, FTP, SNMP, DHCP, etc. Hypertext Transfer Protocol (HTTP) i. It is a communication protocol for the transfer of information on the Internet. Its use for retrieving interlinked text documents (hyperlink) led to the establishment of the World Wide Web. ii. HTTP is a request/response standard between a client and a server. A client is the end-user, the server is the web site.. SOHO NETWORKING page 10 CH/S6CA/Apr. 2009 iii. Hypertext Transfer Protocol over Secure Socket Layer (HTTPS). The HTTPS protocol provides for secure HTTP transfer. It uses encryption to guarantee confidentiality of the data being transferred in an HTTP session. The protocols for e-mail (SMTP, POP3, IMAP) i. Usually e-mail service from ISP requires you to maintain your e-mails using an e-mail client, e.g. Outlook, a dedicated e-mail software. In this case, the e-mail system consists of two different servers: (Act. 6 : Pls. refer to http://communication.howstuffworks.com/email.htm ) ii. SMTP server iii. a. SMTP stands for Simple Mail Transfer Protocol . b. It handles outgoing / incoming mail. c. Whenever you send a piece of e-mail, your e-mail client interacts with the SMTP server to handle the sending. The SMTP server on your host may have conversations with other SMTP servers to deliver the e-mail. POP3 server or IMAP server: a. POP stands for Post Office Protocol . IMAP stands for Internet Mail Access Protocol . b. They handle outgoing / incoming mail. c. Using a POP3 server, your e-mail client connects to the POP3 server and brings copies of your email messages to your local machine. Generally, it will then delete the messages from the server (unless you've told the e-mail client not to). d. Many users want their e-mail to remain on the server so as to allow users to connect from a variety of machines. e. With IMAP, your mail stays on the e-mail server regardless of which machine you use, you have access to all of your mail in all of your folders. Most email clients allow users to keep copies of email on their machine. SOHO NETWORKING page 11 CH/S6CA/Apr. 2009 iv. Your e-mail client allows you to add attachments of various file-types to e-mail messages you send, and also lets you save attachments from messages that you receive. Dynamic Host Configuration Protocol (DHCP) i. Protocol Configuration. Protocol software must be informed about the local computer and the local network environment. E.g. the network hardware type (e.g. Ethernet), the protocols to be used (e.g. TCP/IP), and the location of services (e.g. DNS). ii. The exact details of configuration information depend on the protocol stack. E.g. TCP/IP configuration details include IP address, default IP router address (default gateway), address mask, DNS server address and printer server address. iii. It is not possible for a network administrator enter the configuration information (e.g. host address) for each computer in a network if the set of computers remains rapidly. For example consider a WiFI access point in a restaurant that provides access to arbitrary customers. iv. DHCP provides a mechanism that allows a computer join a new network and obtain configuration information (including an IP address) automatically. v. It uses a client-server approach. vi. An administrator can configure a DHCP server to have two types of addresses: permanent addresses that are assigned to server computers and a pool of addresses to be allocated on demand. vii. The DHCP format: viii. When a computer boots, a. the computer broadcasts a DHCP request to a DHCP server. b. the DHCP server consults its database to find configuration information. If the database contains a specific entry for the computer, the DHCP server returns the information from the entry. SOHO NETWORKING page 12 CH/S6CA/Apr. 2009 If no entry exists for the computer in the database, the DHCP server chooses the next IP address from the pool. . ix. Addresses assigned on demand are not permanent. DHCP issues a lease on the address for a finite period of time. When the lease expires, the server returns the address to the pool of available addresses, which allows the address to be assigned to another computer. Different Types of Internet Connection Dialup Connection. One form for internet connection is a dialup connection . It is a temporary connection established using a modem to dialup the number for a linked to a remote computer. i. ii. To use this kind of connection, you must have a. a computer with a modem, b. a telephone line, c. software(s) for web browsing and E-mail management, and d. an account with an ISP. A modem is a device used to change the data from the digital form stored in the computer to the (analog form) wave form to be transmitted through telephone line. Permanent Leased Line Connection. i. Many organizations lease permanent, private and directly from ISPs. always-on connections ii. The connection speed mainly depends on the type of network connection. (T1 – 1.5Mbps, T3 – 44.7Mbps) Broadband Connection. Two types: digital subscriber line (DSL) and Cable TV connection. i. ii. DSL a. Asymmetric digital subscriber line used. (ADSL) together with and ADSL modem is b. It is asymmetric because the downstream rate (the receiving speed) to users is higher than the upstream rate (the upstream speed). Cable TV connection: Through the cable TV network using a cable modem. Hardware for Networking Telephone line. The telephone system, originally set up for voice communication, is used to handle data communications. Modem. i. Computers deal with data in digital form but most telephone lines are designed to carry analog signals. ii. An interface is needed to change the data from digital to analogue form and vice versa, i.e. a modem (for both modulator and demodulator). Coaxial cable. Coaxial cable contains electrical wire and is constructed to permit high-speed data transmission over this wire with a minimum of signal distortion. SOHO NETWORKING page 13 CH/S6CA/Apr. 2009 Unshielded Twisted Pair (UTP) Cable. i. Twisted pair cables were first used in telephone systems. ii. It is also the most common cable used in computer networking. Fibre optics cable. i. Eventually replacing the copper wire traditionally used in the telephone system. ii. A fibre optics cable consists of a number of thin tubes of glass which carry data as pulses of light. The pulses are very rapid, large in data capacity, more reliable and more secure at transmitting data. Microwave radio. Microwave transmission consists of high-frequency waves that travel in straight lines through the air rather than through wires. Communication satellite. In satellite communications, data are first transmitted to an Earth station, where giant antennas route signals to another Earth station via a communications satellite. Network Interface Card (NIC). i. An interface between a computer and the network. ii. It is an expansion card installed on the motherboard. iii. A network cable is plugged into the NIC from the wall socket. Figure: A sample network diagram with a router, a hub and a broadband connection. Connecting Devices. i. ii. iii. Hub . It connects all network cables together and broadcasts every packet of data to every port. Switch . It also connects all network cables together but it forwards each packet of data to the destined port only. Bridge SOHO NETWORKING . It connects different networks together. page 14 CH/S6CA/Apr. 2009 iv. Router . It also connects different networks. Network data is routed to different networks based on the program in it. Broadband Router v. . It connects small network at home to a broadband service. Network Topology Network topology is used to describe the physical organisation of the network. The basic patterns are usually classified as: i. Star topology. a. The central control unit handles the communication tasks of receiving and routing messages to the various workstations. b. Advantage: Easy to install No disruption to the network when connecting or removing devices Easy to detect faults by the central control unit. main resource Fig. (a) Star-network topology gives the Highest speed and security. ii. Ring topology. a. It involves computers that are approximately the same size, has all devices connected by a single communication cable that forms a closed circle. Ring topology is used for more local communication. b. There are usually two rings running in parallel. c. Advantage: East to connect computer or peripheral to a linear bus Requires less cable SOHO NETWORKING page 15 CH/S6CA/Apr. 2009 main resource Fig. (b) Ring-network based on a ‘continuous’ loop iii. Bus topology. a. All devices in the network are connected to and share a single cable. An advantage of the bus topology is that if one computer on the network fails, this does not affect the rest of the network. b. Advantage: Does not rely on a single hosting computer and can function even though one of the nodes is broken. main resource Fig. 9(c) Bus-network is versatile and cheap – but less secure. SOHO NETWORKING page 16 CH/S6CA/Apr. 2009 Ethernet Ethernet is a well-known and widely used network technology that employs bus. The Ethernet hardware operates at a rate ranging from 10 Mbps to 1 Gbps. Ethernet requires multiple computers to share access to a single medium. A sender transmits a signal, which propagates from the sender toward both ends of the cable as shown below: Thus while one computer transmits a frame to another all other computers must wait. More on Switches The difference between a hub and a switch arises from the devices operate: i. a hub simulates a single shared medium, while ii. a switch simulates a bridged LAN with one computer per segment as follows: Thus in a hub system, at most two computers can communicate through a hub at a given time. But in a switched LAN, each computer has a simulated LAN segment to itself – the segment is busy only when a frame is being transferred to or from the computer. More on Bridges A bridge is an electronic device that connects two LAN segments. A bridge is a digital device that uses the same network interface as a conventional computer and handles frames. SOHO NETWORKING page 17 CH/S6CA/Apr. 2009 When it receives a frame from one segment, the bridge verifies that the frame arrived intact and then forwards a copy of the frame to the other segment if necessary. Thus two LAN segments connect by a bridge behave like a single LAN. Frame filtering. A bridge does not forward a frame unless necessary. In particular, if a computer attached to one segment sends a frame to a computer on the same segment, the bridge does not need to forward a copy of the frame to the other segment. Most bridges are called adaptive or learning bridges because they learn the locations of computers automatically. When a frame arrives, the bridge extracts the physical source address information in the frames that arrive. Routers and Internet A router is a special purpose system dedicated to the task of interconnecting networks. A router can interconnect networks that use different technologies, including different media physical addressing schemes, or frame formats. An internet consists of a set of networks interconnected by routers. The internet scheme allows each organization to choose the number and type of networks, the number of routers to use to interconnect them, and the exact interconnection topology. SOHO NETWORKING page 18 CH/S6CA/Apr. 2009 The most widely used protocol suite in the Internet is TCP/IP. TCP/IP defines term host computer to refer to any computer system that connects to an internet and runs applications. A host can be as small as a personal computer or as large as a mainframe. Wireless LAN Using a wireless network, data are transmitted by radio wave. Wireless network connections perform the same functions as wired connection. Each computer that is to be part of the wireless network must have a wireless adapter / wireless NIC (or a wireless PC card for a notebook computer). A wireless access point Like, other LAN technologies, the wireless LANs use sharing, i.e. all the computers participating in a given wireless LAN are configurated to the same radio frequency. SOHO NETWORKING allows computer to access a wired network. page 19 CH/S6CA/Apr. 2009 A number of product of wireless LAN technologies, which are manufactured by several companies , are available under a variety of trade name: i. WiFi: an IEEE standard defines wireless LANs that operate at 11 Mbps using frequency in the 2.4 GHz range. ii. Bluetooth: a wireless technology designed for short distance. Software for Networking The network operating system (NOS) manages the resources on the network. It monitors the data exchange, ensure data security by incorporating users’ account and password system. Common samples of network OS: Windows NT, Windows 2000, Linux. A network operating system must have the ability to communicate with each network using common protocols. Common Network Service Simultaneous Data Access i. When separate copies of certain data are stored in different computer, it is difficult to determine when copy of the data is the most up-to-date. (data consistency) ii. Using a client/server network, a single copy of data stored in a server, which can be accessed through different workstations at the same time. Device Sharing i. When several people can share a device, e.g. printer, in a network, the use of the device becomes less expensive and easier to manage. ii. Spooling. a. Users send documents to a networked printer. b. Each document is temporarily stored in the printer server. c. The printer server stores the print job in a d. When the printer finishes the current job, its printer server sends the next print job of the queue to the printer. queue . Internet Access i. is Refer to Notes on Internet. Web Server i. A computer is responsible for accepting HTTP requests from web clients, which are known as web browsers, and serving them HTTP responses along with optional data contents, which usually are web pages such as HTML documents and linked objects (images, etc.). SOHO NETWORKING page 20 CH/S6CA/Apr. 2009 Web Hosting Web hosting refers to the provision of storage space and file maintenance service for web sites. Web pages of a web site are stored and maintained in a web server. Virtual Hosting i. Hosting a number of web sites in one web server. ii. Each web site has its own domain name. iii. The server has only one IP address. Non-virtual Hosting i. Hosting a number of web sites in one web server. ii. Each web site has a file directory name under the service provider’s host domain name, e.g. “www.hkedcity.net/ihouse/ktg-040234”. iii. The server has only one IP address. Dedicated Hosting i. Hosting one web site in one web server. ii. The site owner has full of the dedicated server including choice of hardware and OS. Collocated Hosting i. Locating the site owner’s dedicated web server with other ones’ servers on the premises of a web hosting service provider. ii. Those data centers are equipped with high Internet connection speed, physical security, airconditioning and power supply. In-house Hosting i. The firms or organizations host their own web servers. ii. Factors like information traffic volume can be controlled better. iii. The firms or organizations have to deal security problems, like hackers and virus attacks, on their own. Thus they have to employ more technical workers. SOHO NETWORKING page 21 CH/S6CA/Apr. 2009 Wireless Fidelity (Wi-Fi) Wi-Fi, also known as IEEE 802.11, is the most popular wireless LAN standard used in HK. It communicates using microwaves which have frequency ranges from app. 1 GHz to 300 GHz. Hardware Requirements i. ii. iii. a. It allows a computer to join a wireless network and communicate with other devices. b. Inside the adaptor, a microwave transceiver (i.e. transmitter and receiver) is used for transmission. Access point. a. It is a device that allows wireless communication devices (e.g. notebook computers, cell phone, etc) to connect to a wireless network using Wi-Fi. b. The access point itself will connect to another access point or more frequently, a wired network. c. In each access point, there is a service set identifier (SSID) which is used to identify one wireless LAN from another. Each wireless client must specify the SSID of the access point to which the client wants to connect. Wireless router. a. It combines the functions of a router and an access point. b. It can serve as a broadband router, a DHCP server and a firewall, provides a network address translation (NAT) service. 802.11 Architecture i. ii. Wi-Fi wireless network adaptor. Basic service set (BSS) a. A group of hosts communication using Wi-Fi. b. It may use an access point (infrastructure network) or does not use an access point (ad hoc network, i.e. a peer-to-peer network). Extended service set (ESS) a. Joining infrastructure mode BSSs together forms an ESS. b. In an ESS, the APs in each BSS are connected through a backbone which is usually a wired LAN using the Ethernet. Carrier Sense Multiple Access / Collisions Avoidance (CSMA/CA) i. When two or more wireless clients in a Wi-Fi send signals at the same, a data collision occurs and the signals sent by the two clients may be destroyed. ii. CSMA/CA avoid data collisions using the following method, SOHO NETWORKING page 22 CH/S6CA/Apr. 2009 a. Before sending out data, the sender will first sense the carrier (the wireless media) to see if anyone is sending data. b. If the carrier is being used, the sending process will be deferred, which is called the back-off period. c. If the carrier is free, the sender will continue to wait for a specified time, and then start transmission if the carrier is still free. d. On the receiver side, if the receiver receives a frame and the frame is error-free, the receiver will send an acknowledgement packet back to the sender. e. If the sender receivers the acknowledgement, it means that the transmission has been successful. Otherwise, it will retransmit the frame until an acknowledgment is received or after a certain number of retransmission. Four common 802.11 standards Standards Release date Op. Frequency Throughput (Typ) Net bit rate (Max) 802.11a October 1999 5 GHz 27 Mbit/s 54 Mbit/s 802.11b October 1999 2.4 GHz ~5 Mbit/s 11 Mbit/s 802.11g June 2003 2.4 GHz ~22 Mbit/s 54 Mbit/s 802.11n Jan 2010 (speculated) 5 GHz and/or 2.4 108Mbits GHz 600 Mbit/s Gross bit rate (Max) 72 Mbit/s Range (Indoor) ~35 m ~30 m 128 Mbit/s ~up to 100 m ~up to 300 m Note that all 802.11b, 802.11g and 802.11n use 2.4 GHz frequency band. Thus they are compatible but easier to be interfered by other devices. Hybrid Network. It connects an access point to a wired LAN and forms form a network with both wired and wireless equipment. Wi-Fi Hotspot Network. A venue that offers Internet access over a wireless LAN is called a hotspot. E.g. shopping centre, café, telephone booth, etc. SOHO NETWORKING page 23