2.1 Communication at the Physical Layer:
At the physical layer of the OSI model, communication occurs through the transmission of signals over a communication medium, such as copper wire, optical fiber, or radio waves. The physical layer is responsible for converting digital data into analog signals that can be transmitted over the medium, and then converting the received analog signals back into digital data.
The data rate limits on a communication channel depend on whether the channel is noiseless or noisy. For a noiseless channel, the maximum bit rate is given by Nyquist's theorem, which states that the maximum bit rate is equal to twice the bandwidth of the channel.
For a noisy channel, the maximum bit rate is given by Shannon's capacity theorem, which states that the maximum bit rate is equal to the channel bandwidth multiplied by the logarithm of the signal-to-noise ratio (SNR). The SNR is a measure of the ratio of the power of the transmitted signal to the power of the noise in the channel.
Performance metrics for a communication system include bandwidth, throughput, latency, bandwidth-delay product, and jitter. Bandwidth is the range of frequencies that a communication channel can transmit. Throughput is the actual data rate achieved by a communication system. Latency is the time delay between the transmission of a packet and its reception. The bandwidth-delay product is the amount of data that can be in transit on a communication channel at any given time. Jitter is the variation in latency over time.
2.2 Design issues of Data Link Layer:
The data link layer is responsible for the reliable transmission of data over a communication channel. Design issues of the data link layer include framing, flow control, error control, congestion control, and link layer addressing.
Framing involves dividing the data stream into frames, which are then transmitted over the channel. Flow control is used to ensure that the sender does not overwhelm the receiver with data. Error control is used to detect and correct errors that occur during transmission. Congestion control is used to manage the amount of traffic on the channel to prevent congestion. Link layer addressing is used to identify the source and destination of the data being transmitted.
2.3 Framing Methods:
There are several methods for framing data at the data link layer, including character count, flag bytes with byte stuffing, flag bits with bit stuffing, and physical layer coding violations.
The character count method involves placing a count of the number of characters in each frame at the beginning or end of the frame. The flag bytes with byte stuffing method involves using special characters, called flags, to delimit the beginning and end of a frame. If a flag character appears within the data, it is replaced with a special escape character, followed by the flag character. The flag bits with bit stuffing method involves using a special bit pattern, called a flag sequence, to delimit the beginning and end of a frame. If the flag sequence appears within the data, an extra bit is inserted into the data stream to prevent it from being interpreted as a flag sequence. The physical layer coding violations method involves violating the encoding rules of the physical layer to create a special bit pattern that indicates the beginning and end of a frame.
2.4 The Channel Allocation Problem:
The channel allocation problem refers to the problem of allocating a shared communication channel among multiple users. This can be done using static or dynamic allocation.
In static allocation, the channel is divided into fixed time slots, and each user is assigned a fixed time slot for transmission. In dynamic allocation, users compete for access to the channel, and the channel is allocated on a first-come, first-served basis.
Multiple-access protocols are used to manage access to the channel in dynamic allocation. These protocols can be classified into three categories: contention-based, controlled access, and demand-assigned.
2.5 Switching and TCP/IP layers:
Switching refers to the process of forwarding data packets between network devices. There are three main types of switching: circuit switching, packet switching, and message switching.
Circuit switching establishes a dedicated communication path between two devices before transferring any data. This type of switching is commonly used in telephone networks.
Packet switching breaks data into smaller packets and sends them individually across a network. Each packet contains a destination address and is forwarded independently to its destination. This type of switching is commonly used in computer networks, including the Internet.
Message switching involves storing entire messages at each switching node and forwarding them based on destination addresses. This type of switching was used in early computer networks and is now mostly obsolete.
The TCP/IP protocol stack is a set of communication protocols used on the Internet and other computer networks. It consists of four layers: the application layer, transport layer, network layer, and data link layer.
2.6 Wired LANs:
A wired local area network (LAN) uses physical cables to connect devices within a limited geographical area. The most common type of wired LAN is Ethernet.
Ethernet is a standard protocol for wired LANs. It uses a physical addressing system called Media Access Control (MAC) to identify devices on the network. Ethernet also uses a carrier-sense multiple access with collision detection (CSMA/CD) access method to ensure that devices do not transmit data at the same time, which would cause a collision.
Ethernet implementations can vary in terms of speed. Fast Ethernet, for example, operates at a speed of 100 megabits per second (Mbps), while Gigabit Ethernet operates at a speed of 1 gigabit per second (Gbps).
2.7 Wireless LANs:
A wireless LAN (WLAN) uses wireless signals to connect devices within a limited geographical area. There are two main architectural models for wireless LANs: ad-hoc and infrastructure.
Ad-hoc wireless LANs do not require a central access point. Instead, devices communicate directly with each other. Infrastructure wireless LANs, on the other hand, use a central access point (AP) to connect devices to the network.
The IEEE 802.11 standard is the most common standard for wireless LANs. It specifies the physical layer and media access control (MAC) sublayer protocols for WLANs.
The physical layer defines how wireless signals are transmitted and received. The MAC sublayer defines how devices access the wireless medium and avoid collisions.
Bluetooth is another wireless technology that is commonly used for short-range wireless communication between devices, such as smartphones and wireless headphones. Bluetooth operates at a lower power level than WLANs and has a shorter range. Bluetooth also has a layered architecture, with a physical layer, link layer, and application layer.
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