Multiple Access, Sub Layer of Data Link Layer

Data Link layer divided into functionality-oriented sublayers:

Logical Link Control (LLC)	®   Responsible for flow and error control.
Media Access Control (MAC)	®   Responsible for Multiple Access resolution.

The upper sub-layer that is responsible for flow and error control is called Logical Link Control (LLC) layer; and the lower sub-layer that is mostly responsible for multiple access resolution is called the Media Access Control (MAC) layer.

When nodes or stations are connected and use a common link, called multi point or broadcast link, we need a multiple access protocol to co-ordinate access to the link.

Multiple Access Protocols
®   Random Access Protocol	®   Controlled Access Protocol	®   Channelization Access Protocol
1. ALOHA 2. CSMA 3. CSMA/CD 4. CSMA/CA	1. Reservation 2. Polling 3. Token Passing	1. FDMA 2. TDMA 3. CDMA

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Error Detection and Correction

It is one of the most important works handled by the data link layer. Data can be corrupted during transmission, so some applications are required that errors be detected and corrected.

Types of error:

1.  Single bit error: Means that only one bit of a given data unit is changed from 1 to 0 or from 0 to 1.

2.  Burst error: Means that 2 or more bits in the data unit have changed.

Redundancy: To detect or correct errors, we need to send extra (redundant) bits with data.

Detection and correction: the correction of errors is more difficult than the detection.

Method of error correction:

There are two methods of error correction:

1.  Forward error correction, and

2.  Correction by retransmission.

• Forward error correction: Forward error correction is the process in which the receiver tries to guess the message by using redundant bits. This is possible if the number of errors is small.

• Correction by retransmission:  It is a technique in which the receiver detects the occurrence of an error and asks the sender to resend the message. Resending is repeated until a message arrives that the receiver believes its error free.

Coding:

Redundancy is achieved through various coding schemes.
The sender adds redundant bits through a process that creates a relationships between the bits and the actual data. The receiver checks the relationships to detect and correct the errors accordingly.

Coding scheme are divided into two broad categories:

1.  Block coding

2.  Convolution coding

Block coding: in block coding, we divide our message in to blocks, each of k bits, called Datawords and add r redundant bits to each block to make the length n=k+r. The resulting n bit blocks are called Codewords.

From the moment, it is important to know that we have a set of Datawords each of size k and a set of Codewords each of size n.

With k bits we can create a combination of (2)^k Datawords;

With n bits we can create a combination of (2)ⁿ Codewords.

Since n>k, the number of possible Codewords is larger than the number of possible Datawords.

The block coding process is one to one, the same Dataword is always encoded as the same Codeword. This means that we have (2)ⁿ-(2)^k Codewords that are not used. We call this Codewords invalid or illegal.

Hamming distance: 
The hamming distance between two words is the number of differences between corresponding bits.

Ex. D(000, 011) is 2

TCP/IP Protocol Suit

TCP/IP is the acronym that is commonly used for the set of network protocols that compose the Internet Protocol suite. Many texts use the term “Internet” to describe both the protocol suite and the global wide area network.

TCP/IP refers specifically to the Internet protocol suite. “Internet” refers to the wide area network and the bodies that govern the Internet.

TCP/IP consists four layers :-

-          Host-to-Network (Physical + Datalink layers of OSI) or Network Interface Layer.

-          Internet layer

-          Transport layer

-          Application layer (Session + Presentation + Application layers of OSI)

Host-to-Network layer (or Network Interface Layer):

It does not define any specific protocols.  It support all the standard and proprietary protocol.

Network layer: 

TCP/IP support Internetworking  Protocol (IP) in turn uses four supporting protocols ARP, RARP, ICMP and IGMP.

IP (Internetworking  Protocol): it is the transmission mechanism used by TCP/IP. It is unreliable & connectionless protocol, a best effort delivery service.

The term best effort means that IP provides no error checking or tracking. IP assumes the unreliability of the underlying and does it best to get transmission through to its destination, but with no guarantee.

ARP (Address resolution protocol): It maps logical address (IP address) to the corresponding physical address (MAC address).

RARP (Reverse ARP): Maps physical tp logical address.

ICMP (Internet Control Message Protocol) : It is a mechanism used by hosts and gateways to send notification of datagram problems back to the server. ICMP sends query and error reporting message.

IGMP (Internet Group Message Protocol): It is used to facilitate the simultaneous transmission of a message to a group of recipients.

Transport Layer: 

Traditionally the transport layer was represented in TCP/IP by two protocols TCP & UDP, but a new transport layer protocol SCTP has been devised to meet the needs of some newer applications.

 -          IP is a host-to-host protocol, meaning that it can deliver a package from one physical device to another.

 -          UDP and TCP are transport layer protocols responsible for delivery of a message from process (running program) to another process.

-          TCP (Transmission Control Protocol): It is Connection oriented reliable protocol.

-          UDP (User Datagram Protocol): It is Connectionless unreliable protocol.

-          SCTP (Stream Control Transmission Protocol): Provides support for newer applications such as voice over the internet. It combines the best features of UDP & TCP.

Must Read This Also......OSI Model

What are the 7 layers of the OSI model and their function ?

ISO - OSI Model in Communication Networks

ISO stands for International organization of Standardization. This is called a model for Open System Interconnection (OSI) and is normally called as OSI model.

The ISO-OSI model consists of seven layer architecture. It defines seven layers or levels in a complete communication system.

Lets know the features of OSI Model :

1.  OSI model enables us to easily understand the big picture of network.

2.  Through OSI models we can understand how hardware and software work together.

3.  Through OSI models we can understand new technologies as they are developed.

4. Through OSI models troubleshooting is easier by separate networks.

5.Through OSI models can be used to compare basic functional relationships on different networks.

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Each of all 7 layers of OSI model have distinct functions responsible to complete the communication.

Functions of Different Layers are described as follows :-

Layer 1: The Physical Layer :

This layer activates, maintain and deactivate the physical connection. This layer is responsible for :-

1.  Movements of individual bits from one node to next.

2.  Physical characteristics of interface and medium.

3.  Representation of bits.

4.  Data rate.

5.  Synchronization of bits.

6.  Line configuration.

7.  Physical topology.

8.  Transmission mode.

Layer 2: Data Link Layer :

Data link layer synchronizes the information which is to be transmitted over the data. This layer is responsible for :-

1.  Moving frames from one node to next (node to node delivery).

2.  Framing

3.  Physical addressing

4.  Flow and error control

5.  Access control : when two or more devices are connected to the same link, this layer protocols are necessary to determine which device has control over the link at any given time.

Layer 3: The Network Layer :

This layer routes the signal through different channels to the other end. It acts as a network controller. This layer is responsible for :-

1.  Source to destination delivery of individual packet.

2.  Logical addressing.

3.  Routing : by which route data should take.

4.  Congestion control : controls the traffic of the network

5.  Billing.

This layer divides the outgoing messages into packets and to assemble incoming packets into messages for higher levels.

Layer 4: Transport Layer :

This layer decides if data transmission should be on parallel path or single path. Functions such as multiplexing, segmenting or splitting on the data done by layer four that is transport layer.

1.  This layer is responsible for Process to Process delivery of entire message.

2.  Ensures that whole message arrives intact and in-order.

3.  Overseeing both Error Control and Flow Control.

4.  This layer is responsible for Segmentation and Reassembly.

Transport layer breaks the message (data) into small units so that they are handled more efficiently by the network layer.

     The transport layer can be either Connection Less or Connection Oriented.

     A connection less transport layer treats each segment as in independent packet and delivers it to the transport layer at the destination machine.

     A connection oriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. After all data are transmitted, the connection is terminated.

Layer 5: The Session Layer :

Session layer manages and synchronize the conversation between two different applications.

1.  Responsible for dialog control and synchronization.

2. It establishes, maintains and synchronizes the interaction amongst communication systems.

Transfer of data from one destination to another session layer streams of data are marked and are resynchronized properly, so that the ends of the messages are not cut prematurely and data loss is avoided.

Layer 6: The presentation Layer :

Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data.

1.  Responsible for translation, compression and encryption.

Languages (syntax) can be different of the two communicating systems. Under this condition presentation layer plays a role translator.

Layer 7: Application Layer :

It is the top layer and responsible for providing services to the users. 

1. Enables the users (whether human or software) to access the network. 

2.  It provides user interface and supports for services such as e-mail, remote file access & transfer, shared database management etc.

3.  Manipulation of data (information) in various ways is done in this layer. 

4.  Transferring of files disturbing the results to the user is also done in this layer.

Merits of OSI reference model:

1.  OSI model distinguish between the services, interfaces and protocols.

2.  Protocols of OSI model are very well hidden.

3.  They can be replaced by new protocols as technology changes.

4.  Supports connection oriented as well as connectionless service.

Demerits of OSI reference model:

1.  Model was devised before the invention of protocols.

2.  Fitting of protocols is tedious task.

Must Read This Also…

TCP/ IP Model (TCP/ IP Protocol Suite)

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Wireless LAN 802.11 Standards

802.11 refers to a family of specifications developed by the IEEE for wireless LAN (WLAN) technology. 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients.

The IEEE accepted the specification in 1997.

There are several specifications in the 802.11 family:

802.11 —

• Applies to wireless LANs.
• Provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).

802.11a —

• An extension to 802.11 that applies to wireless LANs
• Provides up to 54-Mbps in the 5GHz band.
• 802.11a uses an orthogonal frequency division multiplexing encoding scheme rather than FHSS or DSSS.

802.11b (also referred to as 802.11 High Rate or Wi-Fi) —

• An extension to 802.11 that applies to wireless LANS and
• Provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1-Mbps) in the 2.4 GHz band.
• 802.11b uses only DSSS.
• 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.

802.11e —

• A wireless draft standard that defines the Quality of Service (QoS) support for LANs, and is an enhancement to the 802.11a and 802.11b wireless LAN (WLAN) specifications.
• 802.11e adds QoS features and multimedia support to the existing IEEE 802.11b and IEEE 802.11a wireless standards, while maintaining full backward compatibility with these standards.

802.11g —

• Applies to wireless LANs and is used for transmission over short distances at up to 54-Mbps in the 2.4 GHz bands.

802.11n —

• 802.11n builds upon previous 802.11 standards by adding multiple-input multiple-output (MIMO).
• The additional transmitter and receiver antennas allow for increased data throughput through spatial multiplexing and increased range by exploiting the spatial diversity through coding schemes like Alamouti coding.
• The real speed would be 100 Mbit/s (even 250 Mbit/s in PHY level), and so up to 4-5 times faster than 802.11g.

802.11ac —

• 802.11ac builds upon previous 802.11 standards particularly the 802.11n standard, to deliver data rates of 433Mbps per spatial stream, or 1.3Gbps in a three-antenna (three stream) design.
• The 802.11ac specification operates only in the 5 GHz frequency range and features support for wider channels (80MHz and 160MHz) and beamforming capabilities by default to help achieve its higher wireless speeds.

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