Media Access Control (MAC) Techniques for Network Communication
Media Access Control (MAC) Techniques
Media Access Control (MAC) techniques manage access to the transmission medium, ensuring fair allocation of sending/receiving capabilities among terminals. There are two main types:
Static MAC
In static MAC, the channel capacity is permanently divided among users. Examples include:
- Time Division Multiplexing (TDM): Each terminal is assigned a specific time slot during which it can transmit information. Users transmit in a predefined sequence.
- Frequency Division Multiplexing (FDM): The channel bandwidth is divided into different frequency ranges, each allocated to a specific user. Users transmit simultaneously using their designated frequency range.
Dynamic MAC
Dynamic MAC allows users to access the channel on demand. Some common techniques include:
ALOHA
Developed in the 1970s for radio environments, ALOHA allows stations to communicate with a central node using a specific frequency (f1). The central node receives packets and forwards them to the target station using a different frequency (f2). Collisions occur when multiple stations transmit simultaneously, resulting in packet loss. To ensure successful transmission, stations wait for an acknowledgment (ACK) from the central node.
S-ALOHA (Slotted ALOHA)
S-ALOHA is a synchronized version of ALOHA where time is divided into slots. Stations can only transmit at the beginning of a slot, reducing the probability of partial collisions.
CSMA (Carrier Sense Multiple Access)
CSMA requires stations to listen to the channel before transmitting. If the channel is busy, the station waits until it becomes free. There are different variations of CSMA:
- 1-persistent CSMA: The station continuously monitors the channel and transmits as soon as it becomes free.
- Non-persistent CSMA: The station listens to the channel but waits a random time before checking again.
- P-persistent CSMA: The station checks the channel with probability “p” and waits a random time with probability (1-p).
CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
CSMA/CD extends CSMA by monitoring the channel during transmission. If a collision is detected, the station stops transmitting, allowing the channel to clear. The station then waits a random time before attempting retransmission.
Interface Devices
Interface devices allow for expanding LANs by adding more segments or stations. Two common types are:
- Hub: A hub blindly forwards received frames to all its ports. There are two types: bus and star.
- Bridge: A bridge analyzes received frames to extract the sender’s MAC address and selectively forwards them to the appropriate segment.
Error Control Techniques
Error control techniques ensure data integrity during transmission. Some common methods include:
Parity Techniques
- Parity Word: An additional bit is added to the bit string to be transmitted. Parity can be even or odd. This technique can detect errors when the number of bit errors is odd.
- Parity Block: A parity bit is added to the vertical positions of the bits of each character. The number of characters for which parity is calculated needs to be defined.
Polynomial Codes
Polynomial codes use polynomials to generate redundant estimates and detect bit errors.
CRC (Cyclic Redundancy Check)
CRC uses polynomial operations to calculate the Frame Check Sequence (FCS), which is appended to the bit string. Both the transmitter and receiver agree on a generator polynomial (G(x)) beforehand. The FCS is used to detect errors during transmission.
Domain Name System (DNS)
DNS is a system responsible for translating domain names into IP addresses. It allows users to access machines using their names instead of IP addresses.
DNS Operation
DNS consists of a hierarchical network of servers dedicated to address translation. The system must meet three requirements:
- Transparency: The translation process should be transparent to the user.
- Scalability: The system must handle the growing number of internet requests.
- Reliability: The system must provide accurate and consistent responses.
DNS Servers
DNS servers maintain a database of domain names and their corresponding IP addresses. They respond to client inquiries within their designated area. There are different types of DNS servers:
- Root DNS Servers: The highest level in the hierarchy, responsible for managing top-level domains (TLDs).
- Parent Servers: Manage information for a specific geographical area or TLD.
- Child Servers: Manage information for a smaller geographical area.
- Caching-Only Servers: Initially have no information but cache responses from higher-level servers to speed up future queries.
Top-Level Domains (TLDs)
TLDs are the most significant part of a domain name. There are two main types:
- Country-code TLDs (ccTLDs): Represent specific countries (e.g., .es, .dk, .br).
- Generic TLDs (gTLDs): Represent different activities or organizations (e.g., .com, .org, .net).