- Cables need to be replaced from time to time as the insulation may degrade.
Unshielded twisted pair
- Copper cables are twisted around each other.
- Made of glass
- The digital data is transmitted in the form of light signals using the principle of total internal reflection.
- Ethernet is one of the most used technologies in LANs since the 1980s.
- LAN is used to interconnect devices using a wired medium such as UTP copper cables or fibre-optic cables, hubs and switches.
- This technology supports transmission rates of up to 100Gb/s
- An ethernet network is divided into various segments
- Network interface card and routers segment data into frames
- An error-checking mechanism is applied to the frames
- The receiver checks for errors in the frames, and in case of error, the sender is requested to resend the data packet.
Disadvantages of Ethernet
- Length of cabling is limited
- Limited size of segments results in more data packets, hence more collisions
- CSMA/CD is not optimal when a large number of devices are present in a network. Therefore, a network is segmented and more switches are used.
In an Ethernet network, if two data frames are transmitted at the same time, both frames may collide with each other resulting in errors
CSMA/CD is a technology incorporated into LANs to overcome this issue
In CSMA/CD the sender senses the channel before sending data frames. The data frames are only sent if the channel is sensed to be silent.
In case two senders transmit at the same time, collision occurs and the data corrupts.
In CSMA, the problem with the scenario is solved as the senders receive a corrupted form of data frame sent by them.
Then the senders wait for a random amount of time and resend the data
As the waiting time is different for both senders, data collision is avoided
Cyclic Redundancy Check (CRC)
A mechanism to detect errors in a data packet
Extra information is generated from the data using an algorithm such as a checksum
This checksum is added to the packet before transmission by the sender
When this packet is received by the receiver, it calculates the checksum using the same algorithm.
If the expected number is returned, there are no errors. If different numbers are returned, the packet will be resent.
- A wireless network uses radio signals to connect devices
- Radio signals are identified using their frequency
- Wi-Fi uses 2.4GHz and 5GHz
- The range of this signal is up to 20 metres. This range can vary with the thickness of a wall as the signal needs to pass through it.
- A computer requires a wireless network adaptor to connect to a wireless network
- A computer a long with a network interface controller is referred to as a station.
- All stations within a WAP share the same channel and are tuned to the frequency of a channel to receive transmissions.
- This waveband can be separated into different channels
- A channel is a communication link to send and receive data
- Many channels are adjacent to each other
- Adjacent channels may cause interference which leads to data corruption.
- There are typically only 4 “clear” bands on 2.4GHz
Performance of Wi-Fi systems
- A Wi-Fi access point shares its bandwidth among several devices. This might lead to poor performance.
- Performance of this system depends on the number of users and usage data.
CSMA/CA in Wireless Networks
Wireless networks use CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) to allow nodes to transmit data at high speed and avoiding the collision of data frames at the same time.
Wireless stations are not capable of transmitting and receiving at the same time and hence, if a collision occurs during a transmission, it cannot be detected.
When a wireless node tries to send a data frame, it checks whether the channel is idle or not using CSMA/CA.
If the channel is idle, the data is sent. If the channel is busy, the node waits a random amount of time before checking the channel again. The mechanism is called a back-off mechanism and reduces the chances of collision.
The data frame is sent when the channel is sensed to be free.
After receiving the data frame, an acknowledgement is sent back to confirm that the data is received without any errors.
If the acknowledgement is not received within a specified amount of time, the data frame is sent again by the user.
Hidden Nodes Problem
- Hidden nodes problem is a scenario in which a node can communicate with the wireless access point, but cannot communicate directly with other nodes that are communicating with the access point.
- As a result, multiple nodes may send data to the access point at the same time leading to interference
- In the example given, nodes A and C communicate with B (WAP) but are unable to communicate with each other.
Security in Wi-Fi systems
- Wireless networks are preferred for their ease of installation
- Security of data cannot be assured in this network
- Any device with a Wi-Fi facility will be able to receive data packets in that range of 20m.
- Therefore, it is important to secure data using encryption techniques.
- Uses AES to secure data.
- Password protection is enabled on the network.
- An SSID (Service Set Identifier) is used to identify an individual network (not WPA specific), disabling broadcasting can improve security.
This type of connection is used in traditional telephone networks. This type of switching does not allow other data packets to be transmitted during a connection session.
In case of circuit failure, the communication is disrupted. These disadvantages are overcome in packet switching.
An advantage of this is that data packets are sent serially.
Bandwidth is wasted if it isn’t being used
Sender and transmitter must talk at the same rate
If it breaks, a whole line needs replacing
- A routing table contains information about the topology of the network. Using this information, a router forwards the packets to the next router using certain algorithms.
- In case a router is not available, the data packet is sent through another router.