INTRODUCTION:

MEDIA ACCESS CONTROL (MAC)

TYPES OF MAC:

CARRIER SENSE MULTIPLE ACCESS

/ COLLISION AVOIDANCE (CSMA/CA)

CARRIER SENSE MULTIPLE ACCESS

/ COLLISION DETECTION (CSMA/CD)

TOKEN PASSING

MEDIA ACCESS CONTROL (MAC)

LOCAL AREA NETWORK (LAN) part 2

MEDIA ACCESS CONTROL (MAC) 1) Provides a unique identification and access control for computers using Internet Protocol (IP) network. The main task of MAC protocol is to minimize collisions in order to utilize the bandwidth by determining when a station can use the link (medium). 2) MAC sub layer provides addressing and channel access control The hardware that implements MAC is referred to as a medium access controller. The MAC sub layer acts as an interface between the logical link control (LLC) sub layer and the network's physical layer. 3) In LAN or other network, the MAC address is your computer's unique hardware number. When you are connected to Internet from your computer (or host), a correspondence table relates your IP address to your computer's physical (MAC) address of LAN.

Local Area Networks differ, depending on which MAC technique is used, which describes how devices share access to the LAN.

MAC technique is necessary, because every LAN uses broadcasting, which means that independent devices can start sending data at the same time.

Networking tasks becomes critical especially main responsibility of LAN that allows users from different types of data processing equipments to enter the network without having problem of compatibility.

IMPORTANCE OF MAC

The protocol access can be done only if all equipments follow the same set of protocol which control communications between each other on bus network.

TYPES OF MAC

CARRIER SENSE MULTIPLE ACCESS/ COLLISION AVOIDANCE (CSMA/CA)

CARRIER SENSE MULTIPLE ACCESS/ COLLISION DETECTION (CSMA/CD)

TOKEN PASSING

Stands for Carrier Sense

Multiple Access with Collision Avoidance.

Each device listens to media for transmissions.

When media is clear, device sends an intent to

transmit signal. As this signal is small, chances of

collision are minimized.

Used often in wireless networking.

CSMA/CA

CSMA/CA

The following is configuration of CSMA/CA:

CSMA/CA cont

CSMA/CA cont

CSMA/CA PROCESS

In this process, station will counts start of sending time for a data to minimize the probability of two stations receives (or use) network at one time.

Every station must hear (check) the carrier (at channel) before executing a transmission.

If the channel is not used, thus station can send data. If the channel is used, then station will wait until the channel is free.

CSMA/CA cont

CSMA/CA CHARACTERISTICS

1) Instead of detecting data collisions, CSMA/CA method attempts to avoid them altogether.

2) On a network that uses the CSMA/CA access method, when a computer has data to transmit, its Network Interface Card (NIC) first checks cable to determine if there is already data on wire.

3) So far, the process is identical to CSMA/CD. However, if the NIC senses that cable is not in use, it still does not send its data packet.

4) Instead, it send a signal of intent indicating that it is about to transmit data out of the cable.

CSMA/CA cont

Used to prevent collisions at the moment they are most likely to occur, for example when the bus is released.

All clients are forced to wait for a random number of timeslots and then sense the medium again, before starting a transmission.

If the medium is sensed to be busy, then the client freezes its timer until it becomes free again.

Thus, the chance of two clients starting to send simultaneously is reduced.

Inappropriate for large or active networks. Distance limitations since it must listen for signals of intent. Slow in speed because intent signal must be sent everytime a computer wants to transmit data.

DISADVANTAGES ADVANTAGES

Cheaper than CSMA/CD Effective in avoiding data collisions and can be operated in high data rate A reliable data transmission service because intent signals are sent until cable is clear.

CSMA/CA cont

Stands for Carrier Sense Multiple Access with

Collision Detection.

Each device listens to media for transmissions. When

media is clear, initiates transmission and listens for

collision.

If collision occurs, device waits for random amount of

time before attempting transmission again.

Commonly used on physical networks.

CSMA/CD

CSMA/CD

The following is configuration of CSMA/CD:

CSMA/CD cont

Used to allow device attached to medium to transmit on medium (multiple access), if it senses that medium is free (carrier sense).

This means that collisions may occur if two computers send data at exactly the same time, however NICs of a sending computers will detects that a collision has occurred so they can resend their data.

In CSMA/CD, station detects line to make sure the line is not used in sending data. Station also detects line to ensure no collision after the data is send.

CSMA/CD cont

CSMA/CD PROCESS

CSMA/CD cont

At first level where station A acts as a Sender station:-

Station A will detects line before transmission to determines the line is not used by other station at that time.

If another station use the line for transmission, then station A has to wait for a while and detects the line again.

When another station has finished using the line, then station A can send its message.

CSMA/CD PROCESS

At second level is where message is heading to destination:-

Station A will detects message and make sure there is no collision. Collision occurs when 2 stations send data at similar time.

This situation happens because both stations detect line is not used and both stations send message.

When collision happens, both stations have to send message again after waiting for a while. Waiting time for both stations are different to avoid another collision from occurs.

CSMA/CD cont

Inappropriate for large or active networks because collisions slower down the network. Distance limitations because the collision detection mechanism will restricts the length of cable segment that can be used.

DISADVANTAGES ADVANTAGES

Relatively fast because computer does not have to wait its turn to transmit data. A reliable data transmission service because collisions are detected and packets are resent, so no data lost.

CSMA/CD cont

TOKEN PASSING

The following is configuration of Token Passing:

TOKEN PASSING

TOKEN PASSING cont

Method of controlling access to multiple equipments to media which used a bit pattern known as token.

Token is send around the network and a station or equipment can only send data after it owns the token.

Token passing is usually used for ring topology. By this method, data collision can be eliminated in the network.

Thus, the chance of two clients starting to send simultaneously is reduced.

TOKEN PASSING PROCESS

TOKEN PASSING cont

With token passing, a specific packet of data, called a token, is circulated around the ring from station to station, always in the same direction.

Before a station is allowed to transmit, it must first posses the token, then fills it with address of destination station and data and marks the token as being used.

Each station, in turn, acquires the token and examines the data frame to determine if it is carrying a packet addressed to it.

When the destination station reads the message, it sends the message back to the sender station with an acknowledgement to say that it has received the data. The sender station unmarks the token and passes the token along to the next station in the topology. In this process, when a station receives the token and has no data to send, it just passes the data to the next station.

TOKEN PASSING CHARACTERISTICS

TOKEN PASSING cont

In this process, the token must always moves. For example, if computer C receives the token but the system is down before it can send the data; then the token will lost.

When the data sent received back by computer A (after full round), then it will marks the token as unused and send it to the next station.

To overcome this problem, more than one control station must be placed in the network to sense the token transmission.

If the token does not arrive in time or during maximum duration, thus control station must generate a new token.

Not suitable for network where few stations generate traffic burst while other station idle all the time. Damages at any station in the network disturbs transmission and reception of token.

DISADVANTAGES ADVANTAGES

Ensure about token arrivals, where maximum (round trip time). If each station has message to send every time it owns the token, the real round trip time is less than the maximum value (very small).

TOKEN PASSING cont

CSMA/CD CSMA/CA TOKEN PASSING

Carrier Sense Multiple Access/ Collision Detection.

Carrier Sense Multiple Access/ Collision Avoiding

-

Suitable in BUS and STAR Topology

Suitable in BUS and STAR Topology

Suitable in RING Topology

Have THREE steps: 1) Each station freely to send

the data. 2) While sending the data,

each station will examine and detect any collision of data between the other data sending.

3) IF the collision occurred, each station will terminate their data sending for a while and send back the previous data.

***Each station will be given a

different time waiting to avoid the same collision.

Have TWO steps only: 1) Each station will ensure

the line is free from any transmitting process before sending the data.

2) IF the line is surely FREE then only one station can do the sending process. While another station have to wait after previous transmitting process is done.

Each station have to use a TOKEN to send the data. Only ONE token used in ONE network . IF no data to be sent, an EMPTY token will round the ring. IF stations need to send data, WAIT for the token, GRAB the token, SENT the data and RELEASE an empty token.

Standards are required so that different manufacturers can create equipment that will interoperate without special configuration.

Standards will allow different computers to communicate.

Standards will increase the market for products adhering (mematuhi) to the standard, resulting in mass production and cheaper prices . Standards groups include: ISO. International Organization for Standardization establishes standards for networking operation. ANSI. American National Standards Institute is the US representative to ISO. EIA/TIA. Electronics Industries Alliance/Telecommunications Industry Association is an industry based standards group. IEEE. Institute of Electrical and Electronics Engineers is an international professional organization that sets communications standards. IEEE Project 802 sets standards for cabling and data transmission on local area networks.

LAN Standards

OSI Model

OSI Reference Model: 7 Layers

Internet Layers (TCP/IP)

IEEE 802 refers to a family of networking standard, developed by the Institute of Electrical and Electronic Engineers (IEEE).

IEEE 802 is a Standard for Local Area Network (LAN) and Metropolitan Area Networks (MAN).

IEEE 802 is subdivided into 22 parts ( 802.1 - 802.22) that cover or map to the two layers of ISO/OSI model:

Data link layer Logical link control (LLC) sublayer Media Access Control (MAC) sublayer

Physical layer In general, IEEE 802 standard define physical network interfaces such

as network interface cards, bridges, routers, connectors, cables, and all signaling and access methods associated with physical network connections.

The most widely used standards : The Ethernet family (802.3), Token Ring (802.5), Wireless LAN (802.11), Wireless PAN (802.15) , WIRELESS MAN (802.16)

LAN standard : IEEE 802

Application

Application Presentation

Session

Transport Host-to-Host

Network Internet

Data Link Network Access

Physical

IEEE 802

OSI Model TCP/IP Model

Network

Link

Transport

Application

Presentation

Session

Transport

Network

Link

Physical

The 7-layer OSI Model The 4-layer Internet model

Application FTP

ASCII/Binary

IP

TCP

Ethernet

OSI vs. TCP/IP Stack

OSI Model

Data unit Layer Function

Host

layers

Data

7. Application Network process to application

6. Presentation Data representation, encryption

and decryption

5. Session Interhost communication

Segments 4. Transport End-to-end connections and

reliability, Flow control

Media

layers

Packet 3. Network Path determination and logical

addressing

Frame 2. Data Link Physical addressing

Bit 1. Physical Media, signal and binary

transmission

OSI Layers

Going from layer 1 to 7: Please Do Not Throw Sausage Pizza Away

Going from layer 7 to 1: All People Seem To Need Data Processing

OSI TCP/IP

Application Layer Application Layer

TELNET, FTP, SMTP, POP3, SNMP, NNTP, DNS,NIS, NFS, HTTP, ...

Presentation Layer

Session Layer

Transport Layer Transport Layer TCP , UDP , ...

Network Layer Internet Layer IP , ICMP, ARP, RARP, ...

Data Link Layer Link Layer

FDDI, Ethernet, ISDN, X.25,... Physical Layer

TCP/IP Layers

Position of the data-link layer

Data link layer duties

OSI Protocols and IEEE 802

Data Link Layer is divided into two sublayers:

a. Logical Link Control (LLC)

b. Media Access Control (MAC)

Logical Link Control (LLC) :

This sublayer is responsible to manage and ensure the integrity of data transmissions ; provides data transmission method in different network.

LLC provide multiplexing mechanisms that make it possible for several network protocol to be transported over the same network medium, and optionally provides flow control, acknowledgment, and error notification.

The LLC provides addressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the originator and recipient machines.

LLC and MAC

LLC and MAC

Media Access Control (MAC):

MAC may refer to the sublayer that determines who is allowed to access the media at any one time. Other times it refers to a frame structure with MAC addresses inside.

The Media Access Control sublayer also determines where one frame of data ends and the next one starts frame synchronization.

802.1: Covering architecture, addressing, internetworking and management.

802.2: Logical Link Control (LLC)

802.3: CSMA/CD

802.4: Token Bus LAN

802.5: Token Ring LAN

802.6: Metropolitan Area Network

802.7: Broadband Technical Advisory Group

802.8: Fiber-Optic Technical Advisory Group

802.9: Integrated Voice/Data Networks

802.10: Network Security

Variation of LAN standard (IEEE 802)

Variation of LAN standard (IEEE 802) cont

802.11: Wireless Networks

802.12: Demand Priority Access LANs

Ex: 100BaseVG-AnyLAN

802.15: Wireless Personal Area Network

802.16: Wireless Mertopolitan Area Network

802.20: Mobile Broadband Wireless Access

The basic frame consists of seven elements split between three main areas:-

Header

Payload

Trailer

Header Payload Trailer

MAC Frame Formats

Header Preamble (PRE) - This is seven bytes long and it consists of a pattern

of alternating ones and zeros, and this informs the receiving stations that a frame is starting as well as enabling synchronisation.

Start Of Frame delimiter (pemisah) (SOF) - This consists of one byte and contains an alternating pattern of ones and zeros but ending in two ones.

Destination Address (DA) - This field contains the address of station for which the data is intended.

The left most bit indicates whether the destination is an individual address or a group address. An individual address is denoted by a zero, while a one indicates a group address. The next bit into the DA indicates whether the address is globally administered, or local. If the address is globally administered the bit is a zero, and a one of it is locally administered. There are then 46 remaining bits. These are used for the destination address itself.

MAC frame formats cont

MAC frame formats cont

Header cont

Source Address (SA) - The source address consists of six bytes, and it is used to identify the sending station.

As it is always an individual address the left most bit is always a zero. Length / Type - This field is two bytes in length. It

provides MAC information and indicates the number of client data types that are contained in the data field of the frame. It may also indicate the frame ID type if the frame is assembled using an optional format.(IEEE 802.3 only).

MAC frame formats cont

Payload

Data - This block contains the payload data and it may be up to 1500 bytes long. If the length of the field is less than 46 bytes, then padding data is added to bring its length up to the required minimum of 46 bytes.

Trailer

Frame Check Sequence (FCS) - This field is four bytes long. It contains a 32 bit Cyclic Redundancy Check (CRC). A 32-bit CRC provides error detection in the case where line errors (or transmission collisions in Ethernet) result in corruption of the MAC frame. Any frame with an invalid CRC is discarded by the MAC receiver without further processing.

MAC frame field

Field Description

Preamble Contains 7 octet field that is used to allow the PLS circuitry to reach its

steady state synchronization with the received frames timing.

Start Frame

Delimiter (SFD)

Sequence of 10101011. It immediately follows the preamble pattern and

indicates the start of a frame.

Destination

Address

Specify the destination addressee(s) for which the frame is intended. It

may be an individual or multicast (including broadcast) address.

Source

Address

Identify the station from which the frame was initiated. The Source

Address field is not interpreted by the CSMA/CD MAC sublayer.

Length/Type This two-octet field takes one of two meanings, depending on its numeric

value. For numerical evaluation, the first octet is the most significant octet

of this field.

MAC Client

Data

Contains the original Length/Type field from the MAC frame prior to

insertion of the QTag Prefix. The QTag Prefix offsets this field exactly 4

octets from its position in an untagged MAC frame.

PAD Data field is extended by appending extra bits (refers to pad) in units of

octets after data field but prior to calculating and appending FCS.

Frame Check

Sequence (FCS)

Contains a 4-octet (32-bit) of CRC value. This value is computed as a

function of contents of source/destination address, length/type, LLC data

and pad.

Extension Follows the FCS field and is made up of a sequence of extension bits,

which are readily distinguished from data bits.

Nine fields of MAC frame format

MAN is collection of LANs with the same geographical area, for instance a city between 5 and 50 km2.

Is a network of computers located at different sites within a large physical area, such as a city.

MAN often acts as a high speed network (although not as fast as LAN) to allow sharing

of regional resources.

MAN can defined as a group

of computers and network

devices connected together

within a large physical area.

Metropolitan Area Network (MAN)

Is the largest network of all network types.

The internet is the largest WAN in the world.

WAN generally covers large distances such as states, countries or continents.

WAN is group of MANs or LANs or the mixture of both network.

WAN often uses

transmission facilities

provided by the

common carriers,

such as telephone companies.

An example in the society using WAN is the banking organization.

Wide Area Network (WAN)

Differences between Types of Computer Networks

Characteristic MAN WAN

Geographical

spread

Covers an area of between 5 and 50 km

diameter. Many MANs cover an area size of a

city, a group of buildings or as large as the North

of Scotland.

Large geographical range generally

spreading across boundaries and

need leased telecommunication.

Speed High speed network to allow sharing of regional

resources.

Less speed (150 Mbps).

Data Transfer

Rate

High data transfer rate Lower data transfer rate

Technology Some legacy technologies used for MAN are

ATM, FDDI, DQDB and SMDS. Use technology like MPLS, ATM,

Frame Relay and X.25 for

connectivity over longer distances.

Connection The role is similar to Internet Service Provider

(ISP), but for corporate users with large LANs. It

typically uses wireless infrastructure or optical

fiber connections to link their sites.

Computers are often connected

through public networks, such as

the telephone system. They can also

be connected through leased lines or

satellites.

Ownership Not owned by a single organization. Its

communications links and equipment generally

owned by either a consortium of users or by a

single network provider who sells the service to

the users.

Not owned by any one

organization but rather exist under

collective or distributed ownership

and management over long

distances.

Differences between LAN, WAN and MAN

REFERENCES:

Main: Forouzan, B.A. (2012). Data Communications and Networking (5th edision). Mc Graw Hill. (ISBN: 978-0-07-131586-9) Additional: William Stallings. (2011). Data And Computer Communication (9th edition). Prentice Hall. (ISBN-10: 0131392050)