Internet Flashcards
What is serial transmission
In serial transmission, data bits are sent in a sequence, one after the other, over a single wire. It can operate either in synchronous mode or asynchronous mode.
Classifications of serial transmission
Simplex method, Half-duplex methos , Duplex method
What is simplex method?
Simplex method allows communication in one direction only. This only requires one channel.
What is half-duplex method?
Half-duplex method allows communication is both directions, but not at the same time. This means there is a single channel and the direction is switched after the completion of transmission in the other direction.
What is duplex method?
Duplex method allows communication in both directions at the same time by having two channels permanently available. This is a necessity in interactive systems (where two-way communications are continuously required).
What is parallel transmission?
With parallel transmission more than one series of data bits can be transmitted at the same time by using several cables in place of the single cable found in serial communication systems. Parallel transmission is considered to be synchronous since there is a central clock that controls the timing of data sent on different wires.
What is crosstalk?
This occurs when electromagnetic interference between wires that are in proximity such as parallel links results in transmitting corrupted data that will need to be re-sent.
What is skew?
Skew happens when the bits that are transmitted across parallel links travel at different speeds. In synchronous data transmission, this can result in data falling out of sync with the clock signal and therefore not being read correctly.
Advantages of serial over parallel transmission
- Parallel transmission can only work efficiently over shorter distance and with lower bit rates than serial transmission. This is mainly due to the issues of skew and crosstalk, which become more prominent when using high bit rates and longer wires
- Serial communication medium, which use just one line, are cheaper to install than parallel mediums which use more than one line
What is synchronous data transmission?
In synchronous transmission, streams of bits are transferred over a communication channel at a constant rate. The transmitter and the receiver are synchronised using a common clock signal.
What is asynchronous data transmission?
In asynchronous transmission, there is no clock signal, so additional data (start and stop bits) is used to control the communication. Data is transmitted when it is available, rather than at specific intervals. This means that there can be periods of time when the transmission channel is idle. The sender and receiver must use the same baud rate and need only synchronise their clocks for the duration of data transmission.
What is the purpose of start and stop bits in asynchronous data transmission
In asynchronous transmission, the beginning and end of a transmission are communicated using start and stop signals: a start bit is sent at the beginning of the transmission so that the receiver can prepare for the incoming data, and a stop bit marks the end of the transmission. The stop bit needs to be opposite to the start bit so that the receiver can recognise the next set of bits. Also, the stop signal is typically longer than one bit so that the receiver has time to get ready for the arrival of the next set of data. Making sure that the receiver has time to ‘reset’ before the transmitter sends the next sequence of data is what ensures that the sender and receiver are in sync.
In synchronous transmission, there is no need for a start or stop bit, so more information can be sent per unit of time than in asynchronous transmission. Synchronous transmission is commonly used with parallel transmission. For example, inside a computer, the clock signal is used to synchronise the signals of the control bus and of the data transmitted over the address and data buses.
Define symbol
A symbol is a particular pattern of bits represented by a signal.
Define baud rate
The baud rate is the number of times that the signal changes per second. A symbol corresponds to a set of bits that are transmitted with every signal change. Therefore, baud rate represents the symbol rate of a channel, i.e. how many symbols are transmitted per second.
Define bit rate
A communication medium’s bit rate refers to the number of bits that are transmitted over the medium per second. This is often measured in bits per second. Therefore, a communication system’s bit rate is equal to its baud rate multiplied by the number of bits per signal in the communication medium. Bit rate = baud rate * number of bits per signal. A communication medium’s bit rate will be higher than its baud rate if there is more than one bit sent per signal.
Define bandwidth
Bandwidth expressed in Hertz relates to the range of frequencies that a communication medium is capable of transmitting. There is a direct relationship between bandwidth and bit rate. Higher bandwidth results in a higher bit rate.
Define latency
Latency in a communication medium, often measured in milliseconds, is the difference in a time between an action being initiated and its effect being noticed. Latency usually increases with distance.
Define protocol
A protocol is a set of rules relating to communication between devices. International organisation decide upon and publish protocols which allow devices made by different manufacturers in opposite ends of the world to communicate seamlessly.
Differentiate between baud rate and bit rate
Baud rate measures the number of symbol changes per second, whereas bit rate measures the number of bits transmitted per seconds. While baud rate is relevant in contexts where symbols represent multiple bits, bit rate directly measures the amount of data transmitted regardless of encoding
What is the relationship between bit rate and bandwidth?
The relationship between bit rate and bandwidth is directly proportional as increasing the bandwidth of the channel increases the maximum achievable bit rate. A wider bandwidth allows for more data to be transmitted simultaneously, leading to higher bit rates.
What are the types of topologies?
Topology refers to the structure of a network. The two types are physical and logical.
Physical: This refers to the actual architecture of a network
Logical: This refers to the flow of data packets within a network. A logical bus network delivers packets to all clients on the network whereas a logical start network delivers packets only to their recipient.
What is a physical star topology?
In a physical star network, each client (that is, a device connected to the hub) has its own direct connection to the central hub. The hub receives packets for all the clients connected to it and is responsible for delivering them to the correct recipient. A server can be added to the network in the same way that clients are connected to the central hub.
Advantages of star topology
Packets are sent directly to their recipient, over a cable that is connected only to the recipient. Other clients on the network cannot see packets that aren’t intended for them.
It is easy to add and remove clients to and from the network
Each cable has just one device communicating over it, eliminating the possibility of collisions
The failure of one cable does not affect the performance of the rest of the network.
Disadvantages of star topology
Should the central hub fail, all communication over the network is stopped.
Expensive to install thanks to the amount of cable required.
