Carrier Sense Multiple Access Overview

Carrier Sense Multiple Access

• If the channel is sensed as "idle", a device is permitted to transmit data.
• If the channel is sensed as "busy", the transmitting device must defer its transmission.
• This is the essence of both CSMA/CA and CSMA/CD

CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance)

• Used in wireless LANs, where it is not possible to listen while sending.
• This means collision detection is not possible.
• After a channel is cleared, a station sends a signal to other stations to tell them to defer transmission before sending its packet.
• Collision avoidance aims to reduce the probability of collisions, making CSMA more efficient.
• If a channel is sensed as busy before transmission, the transmission is deferred for a random interval.

Ethernet

• IEEE 802.3 supports a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation, Intel Corporation and Xerox.
• This was called Ethernet.
• The evolution of Ethernet is broken down into 4 different standards: 10Base5, 10Base2, 10Base-T and 10Base-F.
• The first number (10) indicates data rate in Mbps.
• The last number or letter (5, 2, T and F) indicates maximum cable length or the type of cable.

Backbone of Wired Ethernet

• The backbone of wired Ethernet is coaxial, UTP, fiber in 10Base5, 10Base-T, 10 Base- F respectively.
• Ethernet uses Manchester digital encoding for signaling.
• Ethernet LANs can support data rates between 1 Mbps and 100 Mbps.

IEEE 802.3 Frame Format

• IEEE 802.3 specifies one type of frame containing seven fields: preamble, SFD, DA, SA, length/type of PDU, 802.3 frames, and the CRC.

IEEE 802.3 MAC Layer Versions

• The most popular type of IEEE 802.3 Media Access Control protocol is Ethernet protocol.
• It uses CSMA/CD (Carrier Sense Multiple Access/Collision Detect) to determine Bus Arbitration.
• The MAC layer is responsible for:
  • The order of the bits.
  • Converting the Datagram from the Network Layer into Packets/Frames.

Ethernet Frame Format

• The frame format consists of:
  • Preamble - 7 bytes of 10101010, used to synchronize the receiving station's clock.
  • Start Frame Delimiter (SFD) - 1 byte of 10101011, indicates the start of the frame.
  • Destination Address (DA) - 6 octets long (48 bits), indicates the destination of the frame.
  • Source Address (SA) - 6 octets long (48 bits), indicates the source of the frame.
  • Length - indicates the length of the data field.
  • Data - contains the actual data being transmitted.
  • FCS - Frame Check Sequence, used for error detection.

Destination Address (DA) Field

• The DA field consists of:
  • I/G (Individual/Group) - indicates whether the destination is for an individual or for a multicast broadcast.
    • 0 = Individual
    • 1 = Group
  • U/L (Universal/Local) - allows for unique addresses.

Multicast Broadcast

• Multicast to all stations: Destination Address = FFFFFFFFFFFFh (h - hexadecimal notation).
• To multicast to a specific group, the Network Administrator must assign unique addresses to each station.

Guided Transmission Media

• There are 4 basic types of Guided Media:
  • Open Wire
  • Coaxial Cable
  • Optical Fiber
  • Twisted Pair

Media Bandwidth

• Comparison of usable bandwidth between the different guided transmission media:
  • Open Wire 0 - 5 MHz
  • Coaxial Cable 0 - 600 MHz
  • Optical Fiber Cable 0 - 10 GHz
  • Twisted Pair Cable 0 - 100 MHz

Wired Ethernet Media Types

• IEEE 802.3 defines 5 media types of IEEE 802.3 Ethernet variants:
  • 10Base5 (Thick Coax) - used for new installations in the 1990s and was replaced by 10BaseT in the mid 1990s.
  • 10Base2 (Thin Coax) - commonly used for new installations in the 1990s and was replaced by 10BaseT in the mid 1990s.
  • 10Broad36 - rarely used, combined analog and digital signals on the same medium. Broadband means multiple streams of data can be sent on the same medium.
  • 1Base5 - Star LAN
  • 10BaseT - Cat5 Twisted Pair
  • 100BaseT - Cat5 Twisted Pair
  • 1GbaseT - Cat5e Twisted Pair

Baseband vs Broadband

• Baseband - only a single stream of intelligence or data is transmitted. Example: a television station broadcasts one television channel from its transmitter.
• Broadband - multiple streams of intelligence or data is transmitted. Example: a cable company broadcasts many television channels on its cable system.

IEEE 802.3 Media Types - Summary

• IEEE 802.3 defines 5 media types of IEEE 802.3 Ethernet variants:

Standard Organizations

• ISO (International Organization for Standards):
  • A nongovernmental organization based in Geneva.
  • Develops standards for quality assurance across a wide range of computer-related topics.
  • Focuses on "open systems", defining communication protocols that allow any two computers to communicate independently of their architecture.
• ITUT (International Telecommunication Union):
  • Formerly known as CCITT.
  • An agency of the United Nations.
  • Sets standards for:
    • Modems (V-Series): data transmission over phone lines.
    • Switching networks (X-Series): data transmission over switching digital networks.
  • Coordinates international communications and recommends standard interfaces and policies for the interconnection of national networks.
• ANSI (American National Standard Institute):
  • Represents several US standards organizations.
  • A private agency that sets standards for:
    • FDDI (Fiber Distributed Data Interface): a LAN interface.
    • ASCII (American Standard Code for Information Interchange): used by many computers for storing information.

Data Transmission on LAN

• Ethernet is a common type of network that uses IEEE 802.3 standards.
• IEEE 802.3 defines five media types for Ethernet, each with different cabling, transfer rates, distances, and methodologies:

  • 10Base5 (thick coax): Original Ethernet, 10 Mbps, 500m distance, Baseband

  • 10Base2 (thin coax): Replaced 10Base5, 10Mbps,185m distance, Baseband

  • 10Broad36 (broadband): Rarely used, 10Mbps, 3600m distance, Broadband

  • 1Base5 (star LAN): 1 Mbps, 500m distance, Baseband

  • 10BaseT (Cat5 twisted pair): Commonly used, 10 Mbps, 100m distance, Baseband

  • 100BaseT (Cat5 twisted pair): Commonly used, 100 Mbps, 100m distance, Baseband (also known as Fast Ethernet)

  • 1GBaseT (Cat5e twisted pair): Commonly used, 1 Gbps, 100m distance, Baseband (also known as Gigabit Ethernet)

  • Baseband allows only one stream of data to be transmitted at a time.

  • Broadband allows multiple streams of data to be transmitted simultaneously. -Error detection methods are used to ensure data accuracy during transmission. Popular methods include:

  • Parity checking

  • Block coding

  • CRC (Cyclic Redundancy Check)

  • Checksum

• Block coding divides data into blocks of 'k' bits (data words) and adds redundant bits to create 'n' bits (code words).
• 4b/5b coding is an example of block coding, using 4 bits of data and 5 bits for encoding, with 16 data words and 32 code words.
• Error-detecting codes are designed to detect specific error types, but may not detect all errors.
• Error correction allows the receiver to correct errors without needing to know the original data.
• Redundant bits are added to data to enable error detection and correction.

Data Transmission Basics

• User data is sent in packets with a header, data, and trailer.
• The header contains information like addresses of sender and receiver, packet type, and control data.
• The data field holds the user’s data.
• The trailer field contains a checksum used for error detection during transmission.

Networks

• A network is a collection of devices (nodes) connected by communication links.
• Nodes can be computers, printers, or any device that can send and receive data.
• The main advantage of networks is resource sharing.
• Most networks use distributed processing, where tasks are divided among multiple computers.
• Distributed processing provides advantages like security, database distribution, and faster problem solving.

Network Criteria

• Network criteria include performance, reliability, and security.
• Performance is measured by factors like transit time (time for a message to travel between devices) and response time.
• Network reliability is measured by the frequency of failures, recovery time from failures, and network robustness in case of a catastrophe.

Intranets

• An intranet is a private network limited to an organization.
• It is accessible only to members/employees of the organization.
• Each computer in the intranet has a unique IP address within the local network.

Internet

• The internet is a network of networks, connecting thousands of computer networks worldwide.
• It enables information transfer across geographical boundaries at high speed.
• It serves as a vast repository of information and is accessible to users globally.

Intranet vs. Internet

• Similarities: Both use protocols like TCP/IP and FTP. Both can be accessed via web browsers, but intranets are limited to members.
• Differences: Internet access is open to all, while intranet access is restricted to the organization. The internet provides wider access to websites than intranets. Intranets are generally considered more secure than the public internet.

Digital to Analog Conversion

• Digital-to-analog conversion converts digital data into analog signals for transmission over long distances.
• A modem is a device that performs digital-to-analog and analog-to-digital conversion for data transmission.

Analog to Analog Conversion

• Analog-to-analog conversion represents analog information using an analog signal.
• Modulation is used when a band pass channel is available.

Analog to Digital Conversion

• Analog-to-digital conversion converts analog signals into digital data for transmission.
• Pulse code modulation (PCM) is a technique used for analog-to-digital conversion.

Digital to Digital Conversion

• Digital-to-digital conversion represents digital data using digital signals.
• This conversion includes techniques like line coding, block coding, and scrambling.

Transmission Media Categories

• Transmission media are categorized as guided media (using physical cables) and unguided media (wireless).
• Unguided media includes wireless communication, which transmits electromagnetic waves without a physical conductor.

Error Detection and Correction

• Error detection methods add redundant bits to data for identifying potential errors in transmission.
• Error correction methods aim to correct errors without requiring retransmission.
• Parity checking, block coding, CRC, and checksum are common error detection methods.
• Block coding divides data into blocks, adds redundant bits, and converts them into code words.
• 4b/5b coding is an example of block coding, where 4-bit data words are encoded into 5-bit code words.

MAC Frame Structure (802.3)

• MAC frame is the data format used in the media access control (MAC) layer of the Ethernet network.
• It consists of the following fields:
  • Destination Address (DA): The address of the receiving device.
  • Source Address (SA): The address of the transmitting device.
  • Length (L): Indicates the length of the information field.
  • Information Field (Data): Contains the data from the upper layer.
  • Pad: Used to fill the information field to the minimum length when necessary.
  • Frame Check Sequence (FCS): A 32-bit CRC used for error checking.
• The minimum frame length is 64 bytes, and the maximum is 1518 bytes.
• The FCS is calculated based on the content of the DA, SA, L, Data, and Pad fields.