These study notes cover essential concepts related to computer networks subject especially for students of Maharshi Dayanand University and its affiliated colleges. Topics include network architectures, protocols, routing algorithms, network security, and more. The material is organized in a concis...
Unit 2 : Network Protocols and Devices
Syllabus :
Medium Access Control: MAC layer functions, Random access, Controlled Access
and channelization protocols.
Network Layer: Network layer functions and services, Logical addressing, IPv4
classful and classless addressing, subnetting, NAT, IPv4, ICMPv4, ARP, RARP and
BOOTP, IPv6, IPv6 addressing, DHCP.
Network Devices: Repeater, hub, switch, router and gateway.
Section 1 : Medium Access Control
1.1 Functions of the MAC Layer
The data link layer in a computer network is responsible for the transmission of data
between two devices or nodes. It is divided into two sublayers:
1. Data Link Control: Ensures reliable data transfer over a dedicated link using
techniques such as framing, error control, and flow control.
2. Medium Access Control (MAC): Manages the control of data frame access to
a shared channel, minimizing collisions and ensuring efficient data
transmission.
The Medium Access Control (MAC) layer is a sublayer of the Data Link Layer in the
OSI model. It is also known as Media Access Control. Its primary function is to
control how data packets are transmitted to and received from the physical layer
across a shared communication channel. It governs how networks transmit data
between two devices. Essentially, it prevents data collisions when multiple networks
transmit information simultaneously.
MAC Layer Functions
1. Frame Delimiting: Identifies the start and end of frames, ensuring data
integrity.
2. Addressing: Provides unique hardware addresses (MAC addresses) to
devices on the network.
3. Error Detection: Uses checksums or cyclic redundancy checks (CRC) to
detect errors in transmitted frames.
4. Access Control: Manages the access to the shared medium to avoid collisions
and ensure fair use of the channel.
5. Flow Control: Regulates the flow of data to prevent overwhelming the
receiver.
1.2 Multiple Access Protocols
Multiple Access Protocols (or Access Control Protocols) are mechanisms used by
the Medium Access Control (MAC) layer to manage how multiple devices share a
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communication channel. They are essential in scenarios where multiple device share
the same communication channel and need to transmit data simultaneously. These
protocols manage the way devices access the shared medium to minimize collisions
and ensure efficient data transmission.
Example Scenario
Imagine a classroom where a teacher asks a question, and all students
(representing different stations) try to answer at the same time. Without a protocol
to manage this, their responses overlap, causing chaos. Here, the teacher acts as
the multiple access protocol, managing who speaks and when, ensuring clear and
orderly communication.
The primary categories of access control protocols are Random Access, Controlled
Access, and Channelization.
A. Random Access Protocols
Random Access Protocols are used in network communication to manage how
multiple devices access a shared communication medium. These protocols allow
devices to transmit data whenever they have information to send, potentially
leading to collisions. The primary types of Random Access Protocols are Aloha and
Carrier Sense Multiple Access (CSMA).
1. Aloha Protocol
The Aloha Protocol was one of the earliest random access protocols developed for
wireless networks. It has two main variations:
Pure Aloha:
● Operation: A simple protocol where devices transmit whenever they have
data to send. After sending, they wait for an acknowledgment. If no
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acknowledgment is received (indicating a collision occurred), the device
waits a random period before retransmitting.
● Collision Management: Collisions are detected by the lack of an
acknowledgment. Devices use a random backoff period to minimize the
chance of repeated collisions.
● Efficiency: Pure Aloha is simple but has a low channel utilization efficiency,
approximately 18.4%, due to the high probability of collisions.
Slotted Aloha:
● Operation: An improved version of Aloha, where time is divided into equal-
sized slots. Devices can only begin transmission at the start of a time slot,
reducing the chance of collisions.
● Collision Management: Similar to Pure Aloha, devices wait for an
acknowledgment and retransmit after a random backoff if a collision is
detected.
● Efficiency: Slotted Aloha improves efficiency to around 36.8% because
synchronizing transmissions to time slots reduces the collision probability.
2. Carrier Sense Multiple Access (CSMA)
CSMA improves upon Aloha by sensing the channel before transmitting data to
avoid collisions. There are several variations of CSMA including CSMA/CD and
CSMA/CA.
Basic Principle: Nodes (devices) listen to the channel before transmitting to avoid
collisions.
Operation:
● Nodes check if the channel is idle.
● If idle, nodes transmit their data.
● If busy, nodes wait until the channel becomes free.
Advantages: Simple implementation, suitable for wired networks.
Disadvantages: Susceptible to collisions if multiple nodes transmit simultaneously
after sensing the channel is idle.
CSMA Access Modes
CSMA protocols operate in different access modes to manage channel access:
1-Persistent CSMA:
● Nodes continuously sense the channel.
● If the channel is idle, the node transmits immediately.
● If busy, the node keeps sensing until the channel is free.
● Increases the chance of collision due to immediate transmission upon idle
detection.
Non-Persistent CSMA:
● Nodes sense the channel.
● If idle, the node transmits immediately.
● If busy, the node waits for a random period before retrying.
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