In a network, communication happens between devices or computers through electrical, optical or radio-wave signals. Methods of signalling are widely categorized into baseband and broadband.
Baseband: Data is sent as digital signals by using entire bandwidth of the media (Single Channel), supporting single communication at a time. Signals are sent over co-axial, twisted pair or fiber optic cables. Baseband supports higher transfer rates as compared to broadband; however, baseband is limited with distance. Baseband uses TDM (Time Division Multiplexing) to send multiple signals over a single cable. Example: Ethernet, Token Ring & FDDI.
Broadband: Data is send as analog signals by using portion of a bandwidth. Broadband supports use of multiple signals at different frequencies (multiple channels). Signals are split into channels by using FDM (Frequency Division Multiplexing). Example: xDSL, where telephone lines are used for both voice (telephone) calls and data (Internet connectivity).
Channels operation refers to the mode of communication between connected devices or computers. Channel operation can be simplex, half-duplex or full duplex. Simplex is a one way communication, similar to that of a radio. Half duplex is a two way communication but only one way at a time, similar to that of a walkie-talkie. Full-duplex is two way simultaneous communication (data can be received and sent at the same time), similar to that of a telephone.Channel Operation
Multiple Signaling Methods
When multiple devices or computers are connected in a network, they use multiple signals that are combined at the source and separated at the destination by use of a technique called multiplexing. For multiplexing, a device called the multiplexer is used for multiplexing / demultiplexing signals. Types of multiplexing include:
- TDM (Time Division Multiplexing) is a method in which multiple signals are combined and send over a single transmission media such as wires or radio waves. This is achieved by use of time sharing; multiple signals are transmitted for a defined amount of time in cycles. For example, a device sends and receives signals every alternate second.
- FDM (Frequency Division Multiplexing) is a method in which multiple signals are transmitted at different frequencies. Multiple signals can be sent at the same time over a single channel using this technique. For example, a device sends multiple signals at the same time using different frequencies similar to that of a radio (FM) or cable TV and the end device receives by tuning in to a particular channel.
Data Transmission methods
Data can be transferred over a network using the following techniques:
Circuit Switching: In this method, a dedicated path is established between the endpoints before the data is transferred. Once a dedicated path is established, no other devices can use the circuit. Example: Dial-Up, ISDN.
Packet Switching: In this method, data is divided into blocks referred to as packets. Multiple packets can be sent via different paths allowing more than two devices to communicate at the same time. Modes of operation can be connectionless or connection-oriented.
- In connectionless mode, packets have source & destination address for routing that may take different paths. Example: Ethernet, IP, UDP.
- In connection-oriented mode, connection is defined (a virtual circuit is created) before a packet is transferred. Packet switching supports variable packet sizes. Example: X.25, Frame Relay. TCP.
Cell Switching: Cell switching method is similar to that of packet switching but has a fixed size for the cells transmitting data. Cell switching is efficient when large amounts of data need to transferred. Example: ATM.
Channel Access Methods
Channel access methods refer to how devices communicate using a shared medium such as bus networks, star networks, ring networks, hub networks i wireless networks. When multiple devices or computers are used in a shared medium, a pre-defined method of transmission needs to be defined. Channel access methods in circuit switching networks include FDM, TDM, etc and in packet switching networks include CSMA/CD, CSMA/CA, Token passing, etc.
Ethernet is a family of computer networking technologies for local area networks (LANS) and has largely replaced competing wired LAN technologies. Ethemet standard defines how communication happens between network interface cards, hub, switches, repeaters, etc. Devices on Ethernet networks use frames or Ethernet frames for communication. IEEE 802.3 standard defines the Media Access Control (MAC) portion of the data link layer and the physical layer of the OSI model. Ethernet protocols are covered by this standard.
CSMA/CD (Carrier Sense Multiple Access/Collision Detect) as per IEEE 802.3 standard is a mechanism that defines how transmission takes place in a network. Only one device in the collision domain may transmit at any one time, and the other devices in the domain listen to the network before sending any packets in order to avoid data collisions. Collisions also decrease network efficiency on a collision domain. If two devices transmit simultaneously, a collision occurs, and both devices device will wait for a random amount of time before attempting to transmit again.
Collision domains are found in a hub environment where each host segment connects to a hub that represents only one collision domain and only one broadcast domain. Modern wired networks use a network switch to eliminate collisions. By connecting each device directly to a port on the switch, either each port on a switch becomes its own collision domain (in the case of half duplex links) or the possibility of collisions is eliminated entirely in the case of full – duplex links. Collision domains are also found in wireless networks such as Wi-Fi.; CSMA/CA is used in wireless networks.
Carrier sense multiple access with collision avoidance (CSMA/CA) is a network multiple access method in which nodes attempt to avoid collisions by transmitting only when the channel is sensed to be “idle”. It is particularly important for wireless networks, where the collision detection of the alternative CSMA/CD is unreliable due to the hidden node problem.
Token passing is a channel access method where a signal called a token is passed between nodes that authorize the node to communicate. The most well-known examples are token ring and ARCNET.
When multiple computers or devices are connected in a network, signals can be addressed as unicast, multicast’s or broadcasts. Unicast refers to one-to-one communication, for example signal is sent from one computer to another. Multicast refers to one-to-may communication, for example signal from one computer or device is sent to selective set of computers or devices. Broadcast refers to one-to-all communication, for example single from one computer or device is sent to all devices and computers in a network.