2. Data Transmission We need to know about the following: Synchronous & Asynchronous Error Checking CSMA/CD Wireless Connections Network Interface Cards

3. Data Transmission There are 2 main types of data transmission: Synchronous Asynchronous Serial Involves clocks to make blocks transmit at regular time intervals Used in ISDN connections (64-128kbps) Used in T carrier connections (about 1.5Mbps) Serial No clock – beginning and end marked by start & stop bits Each character has its own start & stop bit. So high overhead or “bit redundancy”

4. Error Checking There are two methods of error checking that we need to know about: Parity Cyclic Redundancy Check (Page 85)

5. Error Checking Odd Parity Each character is made up of bits,7 because 1 bit of each byte is the parity bit. Suppose we want to send 72 dec In binary, that’s: 1001000 To get an odd number of ones, we would assign a parity bit of 1, so what we would actually send is: 11001000 parity bit 11001001 Suppose it was corrupted during transmission, and was received. The number of ones is no longer an odd number. The error is spotted, and the sending computer is asked to re-transmit. More About Parity

6. Error Checking Even Parity Even parity is just the same as odd parity except that the parity bit is used to make the number of ones even. Advantages of parity checks: Simple Disadvantages of parity checks: High bit redundancy – 3/11 of the bits are “wasted”. Can only find “odd numbers” of errors. Even numbers of errors cancel out. if used with asynchronous transmission, Cyclical Redundancy Check Back to Error Checking

7. Cyclic Redundancy Check (CRC) A calculation is done on the data to be sent. Advantages of CRC: Works with whole packets, so bit redundancy is less. Should trap “even” and “odd” numbers of errors. Back to Error Checking This calculation is similar to a checksum, but involves dividing the data by a number, and taking the remainder. This remainder is sent with the data. When received, the same calculation is done. If the remainder is the same, the data is probably correct, otherwise, it’s retransmitted.

8. CSMA/CD (Page 86) CSMA/CD is a protocol for making it less likely that collisions occur in a network, and dealing with them when they do. CSMA/CD is mainly used in bus LANs (like Ethernet). CS (Carrier Sense) The system checks to see if there is any traffic on the bus channel. This is known as “listen before talking”.

9. CSMA/CD (Page 86) CSMA/CD is a protocol for making it less likely that collisions occur in a network, and dealing with them when they do. CSMA/CD is mainly used in bus LANs (like Ethernet). CS (Carrier Sense) The system checks to see if there is any traffic on the bus channel. This is known as “listen before talking”. If there is traffic already on the channel, the message “backs off” either for a random amount of time, or for a time determined by the node’s unique MAC address.

10. CSMA/CD (Page 86) CSMA/CD is a protocol for making it less likely that collisions occur in a network, and dealing with them when they do. CSMA/CD is mainly used in bus LANs (like Ethernet). CS (Carrier Sense) The system checks to see if there is any traffic on the bus channel. This is known as “listen before talking”. If there is traffic already on the channel, the message “backs off” either for a random amount of time, or for a time determined by the node’s unique MAC address. When the channel is clear,

11. CSMA/CD (Page 86) CSMA/CD is a protocol for making it less likely that collisions occur in a network, and dealing with them when they do. CSMA/CD is mainly used in bus LANs (like Ethernet). CS (Carrier Sense) The system checks to see if there is any traffic on the bus channel. This is known as “listen before talking”. If there is traffic already on the channel, the message “backs off” either for a random amount of time, or for a time determined by the node’s unique MAC address. When the channel is clear, the node can transmit its packet, although it continues to “listen while talking”.

12. CSMA/CD (Page 86) CSMA/CD is a protocol for making it less likely that collisions occur in a network, and dealing with them when they do. CSMA/CD is mainly used in bus LANs (like Ethernet). CS (Carrier Sense) The system checks to see if there is any traffic on the bus channel. This is known as “listen before talking”. If there is traffic already on the channel, the message “backs off” either for a random amount of time, or for a time determined by the node’s unique MAC address. When the channel is clear, the node can transmit its packet, although it continues to “listen while talking”.

13. CSMA/CD (Page 86) CSMA/CD is a protocol for making it less likely that collisions occur in a network, and dealing with them when they do. CSMA/CD is mainly used in bus LANs (like Ethernet). CS (Carrier Sense) The system checks to see if there is any traffic on the bus channel. This is known as “listen before talking”. If there is traffic already on the channel, the message “backs off” either for a random amount of time, or for a time determined by the node’s unique MAC address. When the channel is clear, the node can transmit its packet, although it continues to “listen while talking”. More than one node could try to transmit - perhaps at the same time. MA (Multiple Access) Even if both listen before talking, they could both transmit at the same time.

14. CSMA/CD (Page 86) CSMA/CD is a protocol for making it less likely that collisions occur in a network, and dealing with them when they do. CSMA/CD is mainly used in bus LANs (like Ethernet). CS (Carrier Sense) The system checks to see if there is any traffic on the bus channel. This is known as “listen before talking”. If there is traffic already on the channel, the message “backs off” either for a random amount of time, or for a time determined by the node’s unique MAC address. When the channel is clear, the node can transmit its packet, although it continues to “listen while talking”. More than one node could try to transmit - perhaps at the same time. MA (Multiple Access) Even if both listen before talking, they could both transmit at the same time. /CD (Collision Detection) If a collision happens, it makes a jamming signal, which makes both nodes back off for a random amount of time, or time determined by the MAC address.

15. CSMA/CD (Page 86) CSMA/CD is a protocol for making it less likely that collisions occur in a network, and dealing with them when they do. CSMA/CD is mainly used in bus LANs (like Ethernet). CS (Carrier Sense) The system checks to see if there is any traffic on the bus channel. This is known as “listen before talking”. If there is traffic already on the channel, the message “backs off” either for a random amount of time, or for a time determined by the node’s unique MAC address. When the channel is clear, the node can transmit its packet, although it continues to “listen while talking”. More than one node could try to transmit - perhaps at the same time. MA (Multiple Access) Even if both listen before talking, they could both transmit at the same time. /CD (Collision Detection) If a collision happens, it makes a jamming signal, which makes both nodes back off for a random amount of time, or time determined by the MAC address.

16. CSMA/CD (Page 86) Collisions are more likely if: There are lot of nodes. The nodes are far apart - (greater than 150 metres - because there will be a greater delay in detecting collisions) Obviously, collisions reduce the network performance.

17. Wireless WPAN (Pages 87 - 89) Wireless Personal Area Network Allows communication of mobile devices such as: mobile ‘phones PDAs pagers etc An international standard is with a range of 10metres at 780 kbps. Bluetooth with: laptops embedded processors etc

18. Wireless Wireless LAN (Pages 87 - 89) Good for: temporary networks places where cabling would be ugly or cause damage domestic networks Problems: interference hacking piggy backing Hardware: Access points which receive and transmit to & from the network. (transceivers) Netcards which have aerials (antennae) and are fitted to each node within 350metres of an access point.

19. Wireless Wireless WAN (Pages 87 - 89) Two Main types: 1Broadband Receiver/transmitter using fixed dish Useful for businesses because: based on normal cellular technology, like mobile ‘phones simple use of fixed dish quite secure as it uses many frequencies Problem: The high frequencies used makes it prone to interference from the weather etc. 2Satellite based broadband Useful for remote locations that are far from the ISP Problem: Built-in delay (“latency”) in communications.

20. Other Points About Wireless Internet Access (Page 68) WAP Wireless Application Protocol For allowing portable devices to use the Internet Problem: Quite low speed (9.6kbps) means it’s not suitable for HTML pages, or TCP/IP… …so… WML Wireless Markup Language Overcomes the problems caused by small screens, low speeds etc.

21. Connections Dialup Cable Modem Leased Line ISDN ADSL Ordinary modem & ‘phone line Uses coax cable TV network & analogue modem Max 56kbps bandwidth Up to 36 Mbps (but shared) Always on, but low security. Specialised telecommunications line from one fixed point to another. T-1 = 1.5Mbps, T-4 =275Mbps Very expensive. Integrated Services Digital Network, Digital dialup, & uses ordinary ‘phone lines 128kbps to 1.5 Mbps, depending on cost. Asymmetric Digital Subscriber Line. READ THE BOOK! (Pages 90 - 91)

22. Network Interface Cards (Pages 32 & 91-92) Next, make sure you know the functions of an interface (from the Systems unit). 5 Page 32 gives the basic definition of a Network Interface card (NIC): “…slotted onto the motherboard… provides the physical connection and electronics needed to convert a computer to a LAN.” For a NIC the main ones could be summarised as: Data Storage (buffering): during the Data Format Conversion stage so that the packet can be re-transmitted after error detection so that the packet can be re-transmitted after collision detection Data Format Conversion: parallel to serial & vice-versa to suitable media – eg electrical pulses, light pulses, radio waves Protocol Conversion: making up packets (revise TCP) Status Signals (Auto Sensing) “sees” the bandwidth of the network and automatically adjusts accordingly MAC addresses

23. Network Interface Cards (Pages 32 & 91-92) The NIC also permanently stores the unique (MAC) address of the node. MAC address structure: 6 bytes first 3 bytes indicate the NIC manufacturer last 3 bytes hold the unique address Having MAC addresses: ensures that messages are only received by the correct machines allows routers and some switched hubs to make up routing tables Media Access Control