Data Transmission and Error Detection Techniques in Networking
Data Transfer Phase and Release Phase of Virtual Circuits
Data Transfer Phase
To transfer a frame from a source to its destination, all switches in a virtual circuit network require a table entry. This table, in its simplest form, has four columns, meaning that the switch stores four items of information for each established virtual circuit. We will explore how switches update these entries later, but for now, assume each switch has a table with entries for all active virtual circuits.
Release Phase
In this phase, after sending all frames to the destination (B), the source (A) sends a special frame called a “request for release.” The destination (B) responds with a “release acknowledgment” frame. Subsequently, all switches delete the corresponding entry from their tables.
V.90 and V.92 Modems
V.90 Modems
V.90 modems have a data transmission speed limit of 33.6 kbps, as determined by the Shannon capacity. However, some V.90 modems, known as 56K modems, can achieve speeds of 56,000 bits per second. This speed is derived from the telephone companies’ sampling rate of 8000 times per second with eight bits per sample. Since one bit is used for control purposes, each sample effectively has seven bits, resulting in a speed of 8000 x 7 = 56,000 bps or 56 kbps.
V.92 Modems
V.92 modems offer adjustable speeds and can reach data rates of 48 kbps if noise conditions permit. The download speed remains at 56 kbps. Additionally, V.92 modems have features like call waiting, allowing them to interrupt the internet connection when an incoming call is detected.
ADSL and DSL
DSL stands for Digital Subscriber Line, while ADSL stands for Asymmetric Digital Subscriber Line. ADSL is an asymmetric communication technology designed for residential users and is not suitable for businesses. It utilizes existing local loops, which can handle bandwidths of up to 1.1 MHz. Factors such as distance from the local exchange, cable size, and signaling methods can affect the actual bandwidth.
Discrete Multitone (DMT)
DMT is the standard modulation technique for ADSL. It combines Quadrature Amplitude Modulation (QAM) and Frequency Division Multiplexing (FDM). The available bandwidth of 1.104 MHz is typically divided into 256 channels, each using a bandwidth of 4312 kHz.
Hybrid Fiber-Coaxial (HFC) Network
An HFC network is a hybrid network that uses both fiber optic and coaxial cables. The backbone of the network consists of fiber optic cables, while coaxial cables connect to individual users. Data downloads are modulated using 64-QAM, with a theoretical download speed of 30 Mbps and a theoretical upload speed of 12 Mbps.
Cable Network for Data Transmission
To use a cable network for data transmission, two key devices are required: a cable modem (CM) at the subscriber’s premises and a cable modem termination system (CMTS) at the cable company’s distribution center. The CM is similar to an ADSL modem, while the CMTS receives data from the internet and passes it to the combiner, which then sends it to the subscriber. The CMTS also receives data from the subscriber and transmits it to the internet.
Hamming Distance and Error Detection
Hamming Distance
The Hamming distance between two words is the number of differences between corresponding bits. The minimum Hamming distance is the smallest Hamming distance among all possible pairs in a set of words. To ensure the detection of up to ‘s’ errors, the minimum Hamming distance should be dmin = s + 1. To ensure the correction of up to ‘t’ errors, the minimum Hamming distance should be dmin = 2t + 1.
Linear Parity and Block Parity
Linear parity involves adding a parity bit to each word, making the total number of 1s either even or odd. Block parity extends this concept by taking multiple blocks of code words and adding a new parity word in addition to the linear parity.
Cyclic Redundancy Check (CRC)
CRC involves treating data as a polynomial and dividing it by a generator polynomial using modulo-2 division. The resulting residue, known as the Frame Check Sequence (FCS), is appended to the data and transmitted. The receiver performs the same division, and if the residue matches, it indicates no errors occurred during transmission.
Checksums for Error Detection
Checksums involve adding extra bits to the transmitted information. The destination checks if the sum of the bits is correct and rejects the information if it is not. For example, if the data is A089, B8AF, and 3219, additional bits would be sent to allow the destination to verify the integrity of the data.