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Last updated: September 21, 2019
Digital Assignment-1 NAME: KARTIK SINDHU REGISTRATION No.
: 16BCE0919COURSE: Network and Communication FACULTY: SALEEM DURAI (CSE1004) ALOHA· Historyof AlohaDevelopment of the ALOHA network wasstarted in the year 1968 at the University of Hawaii.The goal of the ALOHA network wasto use low-cost commercial radio equipment to connect users on Oahu (an islandon Hawaii) to the other Hawaiian Islands with a central time-sharing computeron the main Oahu campus.There are two types of ALOHA:o Pure ALOHAo Slotted ALOHAPure ALOHA and Slotted ALOHA bothare the Random-Access Protocols, that are implemented on the Medium AccessControl (MAC) layer, which is a sublayer of Data Link Layer. · BasicIdea Behind ALOHA ü Use of two distinct frequencies in a hub/starconfiguration. The hub machine broadcasting packets to everyone on the”outbound” channel.ü Various client machines sending data packets tothe hub on the “inbound” channel.ü If data was received correctly at the hub, ashort acknowledgment packet was sent to the client.ü If an acknowledgment was not received by aclient machine after a short wait time, it would automatically retransmit thedata packet after waiting a randomly selected time interval.
ü This acknowledgmentmechanism was used to detect and correct for “collisions” createdwhen two client machines both attempted to send a packet at the same time. · PureALOHAThe original ALOHA protocol iscalled pure ALOHA. The idea behindthis ALOHA is that each station send frame whenever it has a frame to send. Senderfinds out whether transmission was successful or experienced a collision bylistening to the broadcast from the destination station. Sender retransmitsafter some random time if there is a collision.Fig: Frames in a pure ALOHA Fig: Procedure for pure ALOHA protocol Fig: vulnerable time for pure ALOHAprotocol.
Pure ALOHA vulnerable time = 2*Tfr Throughput: If a frame is damaged, then the stations wait for arandom amount of type and retransmits the frame till it transmits the framesuccessfully. The waiting time of each station must be random and it must notbe same just to avoid the collision of the frames again and again.The throughput of the Pure ALOHA is maximized when the frames are of uniformlength. The formula to calculate the throughput of the Pure ALOHA isS-=G*e^-2G, the throughput is maximum when G=1/2 which is 18% of the totaltransmitted data frames.Fig: Pure ALOHA Offered Load vs. Throughput · Slotted ALOHA Pure ALOHA has a vulnerable time 2* Tfr.This because there is no rule that defines when the station can send.
SlottedALOHA was invented to improve the efficiency of pure ALOHA as the chances forthe collisions in the pure ALOHA are very high. In slotted ALOHA are divide the time intoslots of Tfr s and force the station to send only at the beginningof the time slot. In slotted ALOHA, there is still apossibility of collision if two stations try to send at the beginning of thesame time slot. But then also slotted ALOHA is better than pure ALOHA as thechances of the collisions in slotted ALOHA are reduced to the half as comparedto the pore ALOHA. Fig: Frames in a slotted ALOHA network.
Fig: vulnerable time for slotted ALOHAprotocol Pure ALOHA vulnerable time = Tfr Throughput:Synchronization can be achieved in Slotted ALOHA with the help of a specialstation that emits a pip at the beginning of every time slot as a clockdoes. The formula to calculate the throughput ofthe Slotted ALOHA is S=G*e^-G, the throughput is maximum when G=1 which is 37%of the total transmitted data frames. In Slotted ALOHA, 37% of the time slot isempty, 37% successes and 26% collision.
Fig: Throughput versus offered traffic forALOHA systems · Differencebetween pure ALOHA and slotted ALOHA BASIS FOR COMPARISON PURE ALOHA SLOTTED ALOHA Frame Transmission The user can transmit the data frame whenever the station has the data to be transmitted. The user has to wait until the next time slot start, to transmit the data frame. Time In Pure ALOHA the time is continuous. In Slotted ALOHA the time is discrete. Throughput The maximum throughput occurs at G = 1/2 which is 18%. The maximum throughput occurs at G = 1 which is 37%. Successful Transmission The probability of successful transmission of the data frame is: S= G* e^-2G The probability of successful transmission of the data frame is: S= G*e^-G Synchronization The time is not globally synchronized. The time here is globally synchronized.
IEEE 802 StandardsIEEE-802 standards are a set ofnetwork standards developed by the IEEE. The 802 family of standards is onesdeveloped for computer networking. Standards Name Explanation 802.1 Internetworking Covers routing, bridging and internetwork communications 802.2 Logical Link Control Covers error control and flow control over data frames 802.3 Ethernet LAN Covers all forms of Ethernet media and interfaces, from 10 Mbps to 10 Gbps 802.
4 Token Bus LAN Covers all forms of token bus media and interfaces 802.5 TOKEN Ring LAN Covers all forms of token ring media and interfaces 802.6 Metropolitan Area Network Covers MAN technologies, addressing and services 802.7 Broadband Technical Advisory Group Covers broadband networking media interfaces and other equipment 802.8 Fiber-Optical Technical Covers use of fiber-optic media 802.9 Integrated Voice/Data Networks Covers integration of voice and data traffic over a single network medium 802.
10 Network Security Covers network access controls, encryption certification and other security topics 802.11 Wireless Networks Sets standards for wireless networking for many different broadcast frequencies and technologies 802.12 High-Speed Networking Covers a variety of 100 Mbps-plus technologies, including 100VG-AnyLAN 802.13 Unused standard number 802.14 Defunct Working Group Specifies data transports over cable TV 802.15 Wireless PAN Covers the standards for wireless personal area network 802.
16 Wireless MAN Covers wireless metropolitan area network 802.17 Resilient Packet Ring Covers emerging standards for very high-speed, ring-based LANs and MANs 802.18 Wireless Advisory Group A technical advisory group that monitors radio-based wireless standards 802.19 Coexistence Advisory Group A group that addresses issues of coexistence with current and developing standards 802.
20 Mobile Broadband Wireless A group working to enable always-on multivendor mobile broadband wireless access · IEEE802.1 StandardIt is concerned with:ü 802 LAN/MAN architectureü internetworking among 802 LANs, MANs and widearea networksü 802 Link Securityü 802 overall network managementü protocol layers above the MAC & LLC layers · IEEE802.2 StandardIEEE 802.
2 is the original name of theISO/IEC 8802-2 standard which defines logical link control (LLC) as the upperportion of the data link layer of the OSI Model. · IEEE802.3 Standard802.3 is the standard which Ethernetoperates by.
It is the standard for CSMA/CD (Carrier Sense Multiple Access withCollision Detection). This standard encompasses both the MAC and Physical Layerstandards. · IEEE802.4 Standardü Token Busü Implementing a token ring protocol on a coaxialcableü Used for industrial applicationsü Used by General Motors in their MAP(Manufacturing Automation Protocol)ü Not reliable · IEEE802.11 StandardIEEE 802.11 standard is defined forwireless LAN, which covers the physical and data link layers. There are mainly three popularstandards: 802.
11a, 802.11b, 802.11g and latest one is 802.11n.
Each standard uses a frequency toconnect to the network and has a defined upper limit for data transfer speeds.802.11a was one of the first wireless standards. 802.11a operatesin the 5Ghz radio band and can achieve a maximum of 54Mbps.
Wasn’t as popularas the 802.11b standard due to higher prices and lower range. 802.11b operates in the 2.
4Ghz band andsupports up to 11 Mbps. Range of up to several hundred feet in theory. Thefirst real consumer option for wireless and very popular. 802.11g is a standard in the 2.4Ghz band operating at 54Mbps. Sinceit operates in the same band as 802.11b, 802.
11g is compatible with 802.11bequipment. 802.11a is not directly compatible with 802.
11b or 802.11g since itoperates in a different band.Wireless LANs primarily use -Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). It has a”listen before talk” method of minimizing collisions on the wirelessnetwork. This results in less need for retransmitting data.Fig: MAC layers in IEEE 802.11 standards