Hardware and Wi-Fi

Wired Connections

Copper Cables

Coaxial

• Cables need to be replaced from time to time as the insulation may degrade.

Unshielded twisted pair

• Copper cables are twisted around each other.

Fibre-optic cable

• Made of glass
• The digital data is transmitted in the form of light signals using the principle of total internal reflection.

Ethernet

• 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

Ethernet Network

• 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.

CSMA/CD

• 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.

Wireless Connectivity

• A wireless network uses radio signals to connect devices

Radio Transmission

• 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.

Wi-Fi

• 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.

2.4GHz waveband

• 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.

WPA 2

• 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.

Circuit Switching

• 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

Packet Switching

• 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.

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