Instead of existing independently, multiple BSSs may form a network.
The BSSs could be physically disjoint. Logically there is no limit to the distance between BSSs. This portal provides logical integration between the IEEE The Big Picture. Services The services provided by the Association Disassociation Distribution Integration Reassociation. Asynchronous data services Provided by the following three primitives. The format is as show below. The frame control field has the following format.
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Collision Avoidance - Avoidance scheme is used because it is difficult to detect collisions in the RF media. Hence is interoperates with the physical layer by sampling the transmitted energy over the medium transmitting data. The physical layer uses an algorithm for clear channel assessment CCA to determine if the channel is clear.
If the received signal is below a certain threshold the channel is declared clear. Used because of the Hidden terminal problem. It occurs when there is a station in a service set that cannot detect the transmission of another station, and thus cannot detect that the media is busy.
PPT - TCP Over Wireless Ad-hoc Networks PowerPoint Presentation - ID
Communications is established when one of the wireless nodes sends a short message RTS frame. The message duration is known as the network allocation vector NAV. This alerts to back off during the transmission time. Difference between General wireless networks and Ad-Hoc Networks? Absence of a central base station Frequent route re-computation Network partitions Multi-path routing algorithms. What is an Ad-Hoc Network?
Mobile Ad hoc Network ppt
AD-HOC network is a collection of mobile nodes with wireless network infrastructure capable of organizing themselves into a temporary network without the aid of any centralized network manager. According to the IETF definition, a mobile Ad Hoc network is an autonomous system of mobile routers and associated hosts connected by wireless links--the union of which form an arbitrary graph. Why retransmit when there is no route Retransmission wastes power and bandwidth Low throughput as a result of slow start recovery after route restoration this is actually desirable.
Why use TCP at all in such cases? For seamless portability to applications like file transfer, e-mail and browsers which use standard TCP. Chandran, S. Raghunathan, S. Venkatesan and R.
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Introduction ECN is not used. Invalidates the particular route If it knows of an alternate route, that route is used for further communication and RFN is discarded Else, RFN is propagated towards source. Protocol contd. Stays in this state till it receives an RRN packet. Conclusion Communication resumes at the same rate as before the route failure occurred There is no unnecessary loss of throughput Is this ok?
Use of an additional packet RRN. Is it really needed? Conclusion Overhead on routers Detect route failures and reestablishments Provide feedback to the source Store the source id after forwarding an RFN so that it can send an RRN on finding a route The paper does not discuss about multiple flows Enhancements Buffering at intermediate nodes. Holland and N. Introduction DSR complicates the situation — stale routes may be cached and propagated Throughput is degraded Turning of caching works only with single TCP connection What if there are more sources?
Variations Time interval between probe packets greater it is, slower is the route discovery smaller it is, causes congestion best would be to have it as a function of RTT.
Variations Choice of probe packet First packet in congestion window Lowest sequence numbered packet that was lost — indicated by ELFN If forward and reverse routes are different , both approaches are same. What if smaller network buffers are used? Limitations Large number of routing packets force TCP packets to reside in network buffers for long intervals Data packets can get delayed or dropped due to change in network configuration. Queue management is thus very critical. Limitation — Fairness Issues One flow reduces its CWND, allowing another to maintain a larger window and send more packets Short range flows have fewer timeouts and a larger CWND causing them to contend more aggressively.
Rate control mechanisms are more suited for ad hoc networks. Rate Control Methods Hop-by-hop rate control each node individually controls the outgoing rates of all flows prevents buffers from getting large congestion causes only the immediate hop to reduce sending rate more responsive to changes in connectivity , congestion or number of contending nodes can reduce number of packets stored at a disconnected node in a stale route. Liu and S. Some Issues ECNs may not reach the sender Sender keeps re-transmitting, congesting the network further When would this happen?
Possible solution. References IEEE Balakrishnan, V. Padmanabhan, S. Seshan and R. Brown and S. Hierarchal routing: Each node is divided into clusters and each cluster has node. Flat routing: There is no need for choosing the cluster to recognize the network. They always execute shortest path or flooding type algorithm. Proactive routing: By this routing between a source and destination are always available.
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