Datacom basics - Physical layer and corresponding protocol

The physical layer is the lowest layer in the OSI model of a computer network. Physical layer: provides mechanical, electronic, functional, and normative features for creating, maintaining, and tearing down physical links required for data transmission. Simply put, the physical layer ensures that the original data can be transmitted over a variety of physical media. LANs and WANs are both Layer 1 and Layer 2.

The physical layer is the first layer of OSI. Although it is at the bottom layer, it is the foundation of the whole open system. The physical layer provides transmission media and interconnection devices for data communication between devices, providing a reliable environment for data transmission. If you want to remember this first layer with as few words as possible, it's "signal and medium."

OSI incorporates a variety of off-the-shelf protocols including RS-232, RS-449, X.21, V.35, ISDN, and physical layer protocols for FDDI, IEEE802.3, IEEE802.4, and IEEE802.5.

In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer. The implementation of this layer is often termed PHY.

The physical layer consists of the basic networking hardware transmission technologies of a network. It is a fundamental layer underlying the logical data structures of the higher-level functions in a network. Due to the plethora of available hardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSI architecture.

The physical layer defines the means of transmitting raw bits rather than logical data packets over a physical link connecting network nodes. The bitstream may be grouped into code words or symbols and converted to a physical signal that is transmitted over a hardware transmission medium. The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium. The shapes and properties of the electrical connectors, the frequencies to broadcast on, the modulation scheme to use, and similar low-level parameters, are specified here.

Within the semantics of the OSI network architecture, the physical layer translates logical communications requests from the data link layer into hardware-specific operations to affect the transmission or reception of electronic signals.

Popular Physical Layer Technologies

OSI Layer 1

To transmit electrical signals, a transmission medium is required. Common transmission mediums are

① Coaxial cable

With a diameter of more than 1 cm, there are four layers inside and outside: external insulation layer, mesh conductor, internal insulation layer, and data line. I remember when I was a kid that the TV was closed in my home, and it's now largely obsolete.

② twisted pair cable

Currently, the most widely used network cable is connected to a device through an RJ45 interface. The RJ45 interface consists of a male connector and a female connector. The end of the network cable is called a male connector, and the interface of the network adapter is called a female connector.

A twisted pair consists of four pairs of wires twisted together. There are eight wires in total. Currently, only two pairs of wires are used. One pair is used to receive data and the other pair is used to transmit data.

③ optical fiber

An optical fiber is a fiber made of quartz or plastic that transmits signals through light: an electrical signal is converted into an optical signal by an optical transmitter, and the optical signal is transmitted along with the optical fiber to a destination, which is then converted into an electrical signal by an optical receiver.

Optical fibers are classified into single-mode optical fibers and multi-mode optical fibers. Single-mode fibers use lasers with a single wavelength. Multimode fibers use white light and contain multiple wavelengths. In terms of transmission distance and efficiency, single-mode fibers have better performance.

The loss of light transmitted in an optical fiber is lower than that of electricity transmitted in a wire. However, twisted pairs are generally used because all interfaces must be replaced with optical bayonets, and optical fibers of the same length are more expensive than twisted pairs.

④ Hub

Hubs are used to connect multiple devices, such as computers, printers, routers, etc.

Differences Between Hubs and Switches

Similarities:

Both are used to connect multiple devices and forward data.

Differences:

a. The number of ports varies with the number of ports. The number of ports on a hub is small, usually, four ports and the number of ports on a switch is large, usually 24 or 48 ports.

b. The data forwarding mode is different. The hub uses the broadcast mode. The hub forwards the information to all ports except the sending port. All connected devices can receive the information, which is not conducive to information confidentiality. The switch forwards data based on the MAC address of the device and is associated with only the sending source and receiving source. This prevents data loss and improves the throughput.

c. Hubs belong to OSI layer 1 and switches belong to OSI layer 2. Switches outperform hubs, which have slowly been withdrawn from the market as technology advances.

Network topology and CSMA/CD protocol

The network topology represents the way machines are connected to each other. There are three ways

① Bus Topology

All machines are connected to the same cable, and only one machine can send messages at a time, so the more machines, the less chance each machine will be able to send messages.

When multiple machines send messages at the same time, a conflict occurs. In this case, the conflict needs to be resolved by using the CSMA/CD protocol. If two machines send messages at the same time, the conflicting machines stop sending messages, wait for a random duration, and then resend messages. If the conflict occurs again, then waits for a random duration.

② Ring topology

All the machines are connected to the same cable, but the cable forms a loop, all the machines share a token, and only the machine with the token can speak. The larger the loop, the longer it takes to pass the token.

③ Star topology

This is the main network topology currently used. All machines are connected to a central machine, such as a switch, which is responsible for receiving and sending information.

Function

Functions of Physical Layer

1. Following are the various functions performed by the Physical layer of the OSI model.

2. Representation of Bits: Data in this layer consists of a stream of bits. The bits must be encoded into signals for transmission. It defines the type of encoding i.e. how 0's and 1's are changed to signal.

3. Data Rate: This layer defines the rate of transmission which is the number of bits per second.

4. Synchronization: It deals with the synchronization of the transmitter and receiver. The sender and receiver are synchronized at bit level.

5. Interface: The physical layer defines the transmission interface between devices and the transmission medium.

6. Line Configuration: This layer connects devices with the medium: Point to Point configuration and Multipoint configuration.

Topologies: Devices must be connected using the following topologies: Mesh, Star, Ring, and Bus.

7. Transmission Modes: Physical Layer defines the direction of transmission between two devices: Simplex, Half Duplex, Full Duplex.

Deals with baseband and broadband transmission.

Design Issues with Physical Layer

• The Physical Layer is concerned with transmitting raw bits over a communication channel.

• The design issue has to do with making sure that when one side sends a 1 bit, it is received by the other side as a 1 bit and not as a 0 bit.

• Typical questions here are:

  • How many volts should be used to represent a 1 bit and how many for a 0?

  • How many nanoseconds does bit last?

  • Whether transmission may proceed simultaneously in both directions?

  • Whether transmission may proceed simultaneously in both directions?

  • How many pins the network connector has and what each pin is used for?

• The design issues here largely deal with mechanical, electrical, and timing interfaces, and the physical transmission medium, which lies below the physical layer.

Major problems to be solved at the physical layer:

(1) The physical layer should shield the physical devices and transmission media as much as possible so that the data link layer does not perceive the differences and only considers the protocols and services of the layer.

(2) Provide the service user (data link layer) with the capability of transmitting and receiving bitstreams (usually serially transmitted bitstreams) over a physical transmission medium. Therefore, the physical layer should solve the problems of establishing, maintaining, and releasing physical connections. (3) Uniquely identifies a data circuit between two adjacent systems.

[2] Main functions of the physical layer: providing data transmission paths and data transmission for data-end devices.

1. Provide a data transmission path for the data end device. The data path may be one physical medium or may be formed by connecting multiple physical media. A complete data transfer, including activating a physical connection, transferring data, and terminating a physical connection. Activation means that no matter how many physical media are involved, two data terminal devices are connected to form a path.

2. Data transmission: The physical layer must form an entity suitable for data transmission to serve data transmission. One is to ensure that data can pass through correctly, and the other is to provide sufficient bandwidth. (Bandwidth refers to the number of bits that can be passed per second.) to reduce congestion on the channel. The data transmission mode can meet the requirements of point-to-point, point-to-multipoint, serial or parallel, half-duplex or full-duplex, synchronous or asynchronous transmission.

3. Complete some management of the physical layer.

What are the protocols at the physical layer?

· Telephone network modems-V.92

· EIARS-232, EIA-422, EIA-423, RS-449, RS-485

· Ethernet physical layer including 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, and 100BASE-FX. 100BASE-T, 1000BASE-T, 1000BASE-SX, and others

· Varieties of 802.11 Wi-Fi Physical Layer

· DSL

· ISDN

· T1 and otherT-carrierlinks, and E1 and otherE-carrierlinks

· SONET/SDH

· Optical Transport Network (OTN)

· GSMUm air interface physical layer

· Bluetooth physical layer

· IEEE 1394 interface

· TransferJet physical layer

· Etherloop

· ARINC 818 Avionics Digital Video Bus

· G.hn/G.9960 physical layer

· CAN bus (controller area network) physical layer

Data transmission type at the physical layer

The signals transmitted in the transmission medium of the physical layer are classified into analog signals and digital signals. The analog signals are used for telephones and cable TV at home, and the digital signals are used for computers to access the Internet.

When it comes to analog signals, you need to talk about modems. Because the earliest home networks were connected to the Internet through telephone lines, you need to use modems to convert digital signals and analog signals.

Modulation: converts digital signals to analog signals

· Demodulation: converts analog signals to digital signals

One might ask, what signal is that fiber? An optical fiber is only a transmission medium. In an optical fiber network, both analog and digital signals can be transmitted. The physical layer devices at both ends determine the signals be transmitted.

Physical layer data transmission mode

·Physical layer data transmission can be classified into serial communication and parallel communication based on the number of channels used.

Serial communication: The binary code of a character is transmitted in order from the lower bit to the upper bit. A channel is established during the transmission.

Parallel communication: The binary code of a character is transmitted through eight channels at the same time. Eight channels need to be established for each transmission of a character, which is costly.

Simplex, half-duplex, and full-duplex are based on the signal transmission direction and time.

Simplex communication: messages can only be sent in one direction, e.g. radio, radio, pager

Half-duplex communication: Messages can be sent in only one direction at a time, for example, radios.

Full-duplex communication: can send and receive messages at the same time, e.g. cell phone, phone

Common Physical Layer Devices

· NIC

· Fiber optics

· CAT-5 cable

   RJ-45 connector

· The hub has a full-wave effect.

· Repeater strengthens the signal.

· Serial port

· Parallel port