ALOHA PROTOCOL : ALOHA PROTOCOL MEDIUM ACCESS SUBLAYER Contributed by :
R.No.- 1252 Transmission Technology : Transmission Technology Transmission technology can be
categorized into two categories :
Point-to point networks
Broadcast networks Point-to-point networks : Point-to-point networks Point-to-point networks are those in which when a message is
sent from one computer to another, it usually has to be sent via
other computers in the network. A point-to-point network consists
of many connections between individual pairs of computers. Broadcast networks : Broadcast networks Broadcast networks have a single communication channel that is shared by all the
machines on the network. A packet sent by one computer is received by all the
other computers on the network. The packets that are sent contain the address of
the receiving computer; each computer checks this field to see if it matches its
own address. If it does not then it is usually ignored; if it does then it is read.
Broadcast channels are sometimes known as multi-access channel. Shared channel Need of protocols inBroadcast channel : Need of protocols inBroadcast channel Issues in multi-access channel :
WHO is going to use the channel ?
WHEN the channel is going to be used ?
For HOW much time the channel is used ?
Due to shared channel and unregulated traffic over the
network collisions and data loss occur. Some protocol must
be followed for regulated and safe transmission over the
network. MEDIUM ACCESS SUBLAYER : MEDIUM ACCESS SUBLAYER MAC (Medium access control sub layer) is a sub layer of Data link
layer. MAC is the bottom part of the Data link layer. The protocols used
to determine who goes next on a multi-access channel belongs to this
layer. Some of the algorithms for allocating multi-access channel are as
Carrier Sense Multiple Access Protocols(CSMA)
Collision-free protocols :
Limited contention protocol
Wireless LAN protocol
Digital Cellular radio : Aloha Protocols Slide 8: In 1970’s ,Norman Abramson and his colleagues at the University of Hawaii devised a new and elegant method to solve the channel allocation problem and this method is known as ALOHA SYSTEM
The basic idea is applicable to any system in which uncoordinated
users are competing for the use of single shared channel.
There are two versions of Aloha system which differ with respect to whether or not time is divided up into discrete slots into which all
frames must fit. :
SLOTTED ALOHA PURE ALOHA : PURE ALOHA The basic idea on which pure aloha is based upon is : It let users transmit whenever they have data to be sent.
So there will be collisions ,of course, and the colliding frames will be damaged. However due to feed back property of broadcasting a sender can always find out whether its frame was destroyed by listening to the channel. All the receivers acknowledge the packets which means that the packet is received and if there is no acknowledgement from the receiver then the transmission is assumed to be unsuccessful and the station then retransmits the packet after random amount of time.
The systems in which multiple users share a common channel in a way that can lead to conflict are widely known as contention systems. Pure Aloha : Pure Aloha If the first bit of a new frame overlaps with just the last bit of a frame almost finished, both the frames will be totally destroyed. It does not distinguish between a total loss or a near miss. : If the first bit of a new frame overlaps with just the last bit of a frame almost finished, both the frames will be totally destroyed. It does not distinguish between a total loss or a near miss. Slotted Aloha : Slotted Aloha In 1987, Robert’s publish a method called slotted aloha which doubled the capacity of pure aloha.
Divide time up into discrete intervals, each corresponding to one packet.
The stations can only transmit data in one of the time slots only.
The vulnerable period is now reduced in half. If the frames collide they will overlap completely instead of partially. Slotted : Slotted Summary : Summary Aloha Protocol:
Whenever a station has data, it transmits immediately
Receivers ACK all packets
No ACK = collision. Wait a random time and retransmit Slide 15: The Slotted Aloha Protocol
Slotted Aloha - Aloha with an additional constraint
Time is divided into discrete time intervals (=slot)
A station can transmit only at the beginning of a frame