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Unit 3 - Networks (A network is a number of computer systems connected…

- - - - Disadvantages
      - A network manager may need to be employed – expensive
      - Security problems – files sent between computers could spread a virus
      - Hackers can gain access to data more easily
      - If the server is down, all workstations on the network are affected
      - Initial cost of servers, communication devices, etc. can be expensive
- - - - The bus carries data packets along the cable. As the packets arrive at each computer system, it checks the destination address contained in the packet to see if it matches its own. If the address does not match, the computer system ignores the packet. If the address of the computer system matches that contained in the packet, it processes the data.
        
        Advantages
        
        Easy to implement and add more computer systems to the network
        
        Quick to set up – well suited for temporary networks
        
        Cost-effective – less cabling
        
        Disadvantages
        
        It is difficult to troubleshoot the bus
        
        Limited cable length and number of stations – performance degrades as additional computers are added
        
        If there is a problem with the main cable or connection, the entire network goes down
        
        Low security – all computers on the bus can see all data transmissions
        
        Proper termination is required
        
        Data collisions are more likely, which causes the network to slow down. A collision is when two computers try to send a packet at the same time
        
        In a ring network, computer systems are connected in a ring or a loop. Around the ring, packets are sent, being passed from one computer system to the next until they arrive at their destination.
        
        Advantages
        
        Data is quickly transferred without a bottleneck – consistent data transfer speeds
        
        The transmission of data is simple as packets travel in one direction only
        
        Adding additional nodes has very little impact on bandwidth
        
        It prevents network collisions.
        
        Disadvantages
        
        If any of the computer systems fail, the ring is broken and data cannot be transmitted efficiently
        
        a problem with the main cable or connection and the entire network goes down
        
        It is difficult to troubleshoot the ring
        
        Because nodes are wired together, to add another you must shut down the network
        
        In a star network, each computer system is connected to a central node, also known as a hub.
        
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- - - - Data are split into packets before transmission
      - Packets are sent in-order but might arrive out of order and are re-assembled at destination
      - Each node can route a packet along different routes according to its routing table
      - Data is reassembled at the destination
      - Benefits of transmitting packets using routers:
        
        Each packet can take a different route through network which therefore makes it secure as it is difficult to intercept all the packets
        
        Packets are less likely to be affected by network failure because they can simply take an alternative route
        
        Each packet can take a different route through network which means more efficient use of data lines as packet can use least busy route
        
        Each packet can take a different route through network which means a node failure does not stop the packet reaching its destination
- - - - These rules can include the following:
      - handshaking, where two devices establish their readiness to communicate
      - how the sending device will indicate that it has finished sending a message
      - how the receiving device will indicate that it has received a message
      - the type of error checking to be used
      - agreement on the data compression method to be used
      - There are many standard protocols used with computer systems
      - TCP/IP (Transmission Control Protocol/Internet Protocol)
        
        Two protocols that combine to allow communication between computer systems on a network. IP is a protocol that sets out the format of packets and an addressing system. TCP is a protocol that allows packets to be sent and received between computer systems
        
        HTTP (Hypertext Transfer Protocol)
        
        HTTP is a protocol than can be used to transfer multimedia web pages over the Internet.
        
        HTTPS (HyperText Transfer Protocol Secure)
        
        The “S” in HTTPS stands for “Secure”. It’s the secure version of the standard “hypertext transfer protocol” your web browser uses when communicating with websites.
        
        FTP (File Transfer Protocol)
        
        FTP is a protocol that can be used when copying a file from one location to another via a network or the Internet. It is typically used for the transfer of large files, as it allows broken communications to resume transferring a file rather than having to restart.
        
        SMTP (Simple Mail Transfer Protocol)
        
        Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission.
- - - - Email protocols
        
        To use email you must have an email client on your computer that has access to a mail server. Your Internet Service Provider (ISP) often supplies this server. The mail client and the mail server exchange information with each other using email protocols to transmit information.
        
        IMAP protocol - Internet Messaging Access Protocol (IMAP) is an email protocol that stores email messages on a mail server. It allows the email user to read and handle email messages as though they were stored locally on their own computer. The user can manage their email with facilities such as the ability to create folders to organise their messages, store draft messages in the server and delete unwanted messages.
        
        POP3 0 Post Office Protocol 3 (POP3) is the third version of a protocol for receiving email. POP3 receives email for a client and stores it in a single file on the mail server. When the email client logs onto the mail server the email is transfer to the users computer. There are no copies of the email stored permanently on the server after they have been downloaded.
        
        SMTP - The Simple Mail Transfer Protocol (SMTP) is used to deliver email from the sender to an email server or when email is delivered from one email server to another. SMTP can only be used to send emails but not to receive them.
- - - - The role of a Domain Name System (DNS) server is to translate meaningful names (e.g. wjec.co.uk) into IP addresses.
        
        The user types the URL
        
        Domain Name System (DNS) server has a list of domains and corresponding IP addresses.
        
        Checks ‘your’ domain server and if it does not hold the IP address it will pass the request along to a higher level DNS server, such as your Internet Server Provider’s (ISP) DNS server.
        
        Then it can query other domain name servers (hierarchical) for addresses.
        
        Summary - How a DNS works