Bandwidth Efficient Router Design WithIntroduction of Buffering to ReduceRetransmission Cost of Corrupt DataAbbad Ur Rehman

School of AutomationBeijing Institute of Technology

Beijing, Chinaabbadurrehman@gmail.com

Shaheer Hussain, Asad Abbas, Arslan AkbarDepartment of Electronics EngineeringThe Islamia University of Bahawalpur

Bahawalpur, Pakistanshaheerhusssain, asadabas57, arslan.rao35 @gmail.com

AbstractA new design of router is proposed in thisresearch paper to reduce the excessive wastage of bandwidthdue to retransmission of corrupt packets. A buffer is attachedto each router. All the packets that pass through a routerare saved in the buffer for a very small time. In case if apacket is found corrupt at any router, it can be requestedfrom the previous router which has a correct copy of thepacket in its buffer. In this way we save a lot of bandwidthas a retransmission from sender to receiver is not required.In case of any corrupt packet, the extra bandwidth requiredfor retransmission is limited to a single hop only. The paperanalyzes the new design with the help of computer simulationand general discussion. Behavior of extra buffer attachedto each router is analyzed as well as the extra load inmanaging of buffer is discussed. The simulation results showthat proposed system reduces a lot of retransmission costand a clear increase in throughput of system is observed.keywords: Buffering, Router Buffer, Throughput

I. INTRODUCTIONThe need for buffering is a fundamental fact of life for

packet-switching networks[1]. Router is a device whichpasses data between multiple networks. It works at theNetwork Link Layer. Router must be able to recognizeand understand data packets so it can send them to theirdestination. In a uni cast connection a source sends to asingle destination and in a multicast connection a sourcesends to multiple destinations [2]. Packets are transmittedthrough a host to other host, and in between, if few packetsare corrupted in any router than it demands the packetsfrom transmitting host which results in loss of bandwidthand time. Retransmission request from a receiver canlead to sender and network overload [3]. This will wastenetwork bandwidth and degrade overall performance [3].In this paper we introduce a new scheme that saves bothBandwidth and time which in turn increases our through-put very efficiently. We associate buffer for each router.Now when router sends or transmits packets it saves a copyof that packet in its Buffer memory until the very nextrouter transmits that packet. Now if somewhere packetcorrupts then it demands from the previous router andnot from the transmitting host. This saves lot of time andbandwidth. This paper is organized as follows: Section IIpresents the original or normal behavior of Router. SectionIII presents our modified behavior of router to overcomethe deficiency in normal behavior and it also verifies ourproposed scheme.Section IV presents conclusion remarks.

II. BEHAVIOR OF EXISTING ROUTER SYSTEM IN CASEOF CORRUPT PACKETS

During the transmission of data, packets get corruptdue to various factors. These factors include noise, slowlink speed, link overload, and also due to packet size isgreater than the link capacity. The number of packets thatgets corrupt is different for different links because eachlink is working in a different environment. Probabilityof a packet getting corrupt is less for a low noisy link,but for a case of high noise environment the chanceof packet getting corrupt increases significantly. When apacket gets corrupts and does not reach its destination inrequired time, a retransmission is required. Therefore thepacket is transmitted again from sender to the receiver.It consumes both bandwidth and time, as the result thenetwork is overload and did not work efficiently. Packetconsists of data that is transmitted across a packet switchednetwork. Packet route is specified by header informationthat contains a destination address. If a Host need to senda packet to another Host, it specifies the destination hostaddress in packet header. Such a packet starts travelling bypassing through different routers to reach true destination.Each router contains link layer as well as network layercapability therefore a packet is verified for errors at everyrouter it passes through. A corrupt packet is discardedat router. When a packet is discarded at a router anddoes not reach its destination, the receiving host sendsa NACK to the sending Host. A discarded packet is thenretransmitted by sender. Such retransmissions consume alot of bandwidth especially where ratio of corrupt packet isvery high. It is observed that a packet does not always getcorrupt on first hop, it actually covers a lot of distanceand passes through multiple routers without any errorbefore getting corrupt. Therefore when such a packetis corrupt, all the bandwidth it has consumed so far iswasted and bandwidth required for retransmission furtheraffects the efficiency of system. In figure 1 a packet getscorrupt at router Y so the router discards corrupt packet.After the time out the Host A retransmits that packet tothe Host B. Such retransmission of packet will consumemore bandwidth and time which makes the transmissioninefficient.

Fig. 1. Packet getting Corrupt at a router and retransmission scenario

III. MODIFIED ROUTER DESIGN WITH INTRODUCTIONOF BUFFER

A:Introducing Buffer at each Router to reduceBandwidth requirement for retransmission of corruptpackets

As highlighted above there are few very clear problemsthat decrease the performance of a network in transmittingthe packets from the sending device to the receiving de-vice. We have proposed a solution to the above highlightedproblems. Our system reduces a significant amount ofbandwidth that was wasted on retransmission of corruptpackets. We have applied some modifications at routers.These modifications give a significant improvement tosave bandwidth and time, which in turn also improves thethroughputs. A small memory buffer is attached to eachrouter. A router stores every packet that arrives for somesmall time time store. In case if a packet is found corruptat a router,the router can request for correct version ofcorrupted packet from previous router. The correct copyof packet is stored in the buffer of previous router forsome time. The previous router can therefore providewith the error free packet from its buffer. A packet issaved in a router for a time until it passes through thevery next router. So in case if a packet get corruptssomewhere on the link, it can be taken from the buffer ofprevious router. This scheme if applied at all the routerswill result in significant improvement in throughput ofa system. We achieve two advantages as compared toprevious system. The maximum bandwidth that is wasteddue to corrupt packet is limited to a single hop only.Secondly a retransmission of a corrupt packet from senderto receiver is not required as an error free copy of a corruptpacket can be requested from previous router. Figure 2explains the proposed system. In the figure Host A issending packets to Host B through router A and routerB. A buffer is associated with each router which savesa copy of each packet that passes through the router. Asshown packet 2 is found corrupt at router B, thereforerouter B requests packet 2 from the previous router that isrouter A because router A has a correct copy of corruptedpacket in its buffer. Router A provides the correct copyof packet to router B. Unlike the original system it doesnot involve Host A to wait for a time out and then send aNACK to Host A and request for a retransmission. In thisway it saves both bandwidth and time.

A router memory is not required to be very large asa packet is stored only for very short time time store. Arouter needs to wait for a small time so that a packetsuccessfully passes the next router. After waiting for time

Fig. 2. Retransmission of Corrupt Packet when buffer is associated witheach Router

store, a router discards the packet from its buffer assumingthat packet has successfully passed the next router anda copy is no more required. New packets can now besaved in empty space in buffer. Storage time time storeis adjusted considering worst scenario where a maximumwait is required. Worst scenario is when a packet travelfrom one router to another router and is found corrupt atreceiving router. The receiving router then sends a requestpacket back to the sending router for a correct copy ofpacket. Therefore the time store must be adjusted so thata packet is stored in the buffer for at least as long as achance of a request to retransmit packet is present.Time store = One Rtt + time proc + time margin wheretime proc is the time required to process a packet andcheck for error and margin time is set if due to somecongestion on link a transmitted packet or request messagemay suffer a delay. We consider another scenario with fourrouters (Ra,Rb, Rc, Rd) which provide the path for datato be transmitted from sender (host A) to receiver (hostB). In this scenario we are sending packets (1 to N) fromhost A to host B and the packet 2 corrupts on RB, packet3 corrupts on Rc, packet 5 corrupts on Rd.We analyze thescenario for existing system where the concept of bufferwith each router is not present, these three packets arenot received by the host B. Host B send the NACK tothe host A and after receiving all the packets send theACK. Now the packets which are corrupted on each routerdemand from host A and then Host A resend the packetsto host B. If in resending, another packet corrupts on anyrouter then it again demand from the host A and sendto the host B. In this process of resending the corruptpackets or data from Host A, lot of time is consumedso the, error is increasing, link speed is down and mostof all the bandwidth is increasing.Now we improve thescenario and save the bandwidth by associating a memorywith each router. As shown in figure 3, each router storesthe packets for some time. The storage time of packetdepends on the link speed. It varies inversely with thelink speed. Now in case of a corrupt packet each routerdemands it from the previous router and sender or hostA does not need to retransmit. In the figure host A sendsN packets to host B. Packet 2 is found corrupt at Rb,

packet 3 is corrupt at Rc and packet 5 is found corrupt atRd. Now each router demands its corrupt packet from theprevious router. Rb demands the packet 2 from the Ra, Rcdemands the packet 3 from Rb and Rd demands the packet5 from Rc. On receiving a request for a correct copy ofcorrupt packet a router provides the error free version ofrequested packet from its buffer. In this way wastage ofbandwidth is limited to a single hop only. This modifyingscheme saves the time, increases the speed, which makesthe system efficient and most of all saving the bandwidth,consequently improving the throughput which is the needof present time.

Fig. 3. Retransmission scenario of a complete network with bufferassociated with each router

B. Computer Simulation

For our simulation we take a scenario with two hostsand 10 routers that connect the hosts. A buffer is associatedwith each router as discussed above. Figure 4 shows thebuffer status of each router during transmission. It is clearfrom figure that all the packets first pass through router1 therefore router 1 buffer is filled before others. As thetime passes and number of packet increases all the 10buffers reach their maximum value. The maximum numberof packets stored at different routers varied depending onthe link speed. Similarly at the end of transmission router1s buffer is emptied first followed by other routers.

Fig. 4. Behavior of Buffer memory associated with each router

Buffering at each router reduces the bandwidth requiredfor retransmission but it increases the processing costassociated with each router. Each router now needs togenerate a request for corrupt packet and in case a requestis received it has to retransmit the packet from its buffer.

This whole procedure puts extra load on router. Figure5 shows the distribution of extra load for each router. Itshows the number of packets each router transmits from itsbuffer in response to corrupt packets request. In existingsystem a router does not transmit any extra packets asthere is no concept of buffer.

Fig. 5. Extra packets retransmitted by each router using buffer memory

Figure 6 shows throughput of the system with respectto packet loss. When the number of packet lost are less,the throughput or the overall efficiency of normal systemis better as there is no extra processing cost involved inbuffering of packets. But as the number of packet lostincreases the throughput of the proposed system becomesmuch better than the normal system as a lot of bandwidthdue to corrupt packets is saved.

Fig. 6. Throughput of System for different ratio of Packet Loss

Similarly figure 7 shows the results of throughputversus total number of packets. For less number ofpackets, the throughput of the normal and the modifiedsystem is very close to each other but as the numberof transmitted packets increases the throughput of thesystem with router buffering is better than throughputwithout router buffering.

Fig. 7. Throughput of System for different number of transmitted packets

Figure 8 shows throughput of normal and modifiedsystem for different size of packets. An improved resultin throughput is obtained by the new system.

Fig. 8. Throughput of System for different Packet Size

IV. CONCLUSIONS

This research paper provides a new and more reliabletechnique for data transmission through routers. Prob-lems in data transmission for normal case are highlightedand discussed in detail. A solution to these problems ispresented by introducing buffer memory in routers. Thistechnique saves both time and bandwidth which improvesthe overall efficiency and throughput of the system. Themodified system costs much cheaper than the normalsystem, because main problem of todays technology ishigh cost of bandwidth.

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