Vankeirsbilck, Bert

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Ghent University Academic Bibliography

Graphical Update Cachingfor Mobile Thin ClientsBert VankeirsbilckSupervisor(s): Bart Dhoedt, Filip De TurckI. INTRODUCTIONBecause of form factor constraints, requiredportability and battery lifetime, mobile deviceshave limited resources. Despite the rapid devel-opment of smaller and more efficient hardware,software requirements on CPU, memory anddisk space cause significant hardware needsand battery drains. The general idea behind thisarticle is to extend the mature thin client tech-nology for use on mobile devices, offloadingheavy computations to a distant server andleaving the applications unaltered. User inputis sent over a network towards a server that ex-ecutes the application, the generated screen up-dates are returned to the user’s device. Thefunctionality of the mobile device is reducedto presenting graphical output, capturing userevents such as key strokes and pointer move-ments, and transmitting data over the network.The overall goal of this doctoral study is toenable demanding applications on mobile thinclients. This will be reached by optimizingthin client computing concepts on the protocollevel, as well as on the infrastructural level andthe application level. The need for multi-layeroptimizations is thoroughly explained in [1].In this paper we propose graphical updatecaching as a method to reduce long term re-dundancies in thin client sessions. When an-alyzing the sequence of graphical updates gen-erated by a typical user session on a desktopcomputer, we found that a lot of frames re-—————————————————–B. Vankeirsbilck is with the Depart-ment of Information Technology of GhentUniversity (UGent), Belgium. E-mail:Bert.Vankeirsbilck@intec.UGent.be.semble others that have already been transmit-ted earlier in that session. Since we operate ina thin client environment where all graphicalupdates have to be sent over a wireless, lim-ited bandwidth network, benefit can be foundin this phenomenon by storing well-chosen keyframes both at client and server side, and trans-mitting only the differences with respect to thatframe. Since less data is to be received fromthe network at the client, the battery autonomyof the device decreases slower. This will con-tribute to the user satisfaction and will lessenthe load on the environment.We assess the bandwidth optimization poten-tial of a static cache. This is a cache for graph-ical updates, that is filled before the thin clientcomputing session starts. When the user logsin, the cache is loaded both at client and serverside, and does not get altered during this ses-sion.II. STATICALLY CHOOSING CACHE FRAMESWe have taken a thin client usage session of-fline by storing the sequence of graphical up-dates and computed the byte-per-byte differ-ences between subsequent full screen frames(in uncompressed format). We have taken thenumber of different bytes as a measure for theresemblance between frames. Through a ma-trix of mutual distances, we were able to iden-tify the optimal combination of a predefinednumber of cache frames. Optimal cache framesare those that, combined, result in the smallestdistance to the complete sequence. After vi-sual inspection we found that the optimal cacheframes represented the applications that wereexecuting, i.e. the desktop background, the of-fice program, the Google startup page and thehomepage of the local newspaper.III. EXPERIMENTAL RESULTSFigure 1. The caching architectureThe architecture for integrating a static cachein a thin client computing system is shown inFigure 1. At the server side, the executing ap-plications of the user write their graphical out-put to the server’s framebuffer. This frame-buffer is analyzed in order to choose an op-timal encoding method. If no suited cacheframes are found for this graphical update, itwill be directly encoded using a native encod-ing scheme from the used thin client proto-col and sent to the client. If there is a wellmatching cache frame, this cache frame is sub-tracted from the graphical update and the dif-ference is encoded using the classic encodingscheme and sent over to the client. In addi-tion, a cache header containing necessary pa-rameters such as the used cache frame has tobe sent over to the client. At the client side,the received data is decoded, and depending onthe presence of a cache header, the indicatedcache frame is added to the decoded frame anddelivered to the client framebuffer which even-tually is presented on the screen of the clientdevice. Figure 2 shows that optimally encod-ing with respect to one cache frame, the desk-top background, already yields 20.56% band-width reduction. Adding an extra cache frame,the browser with the homepage of the localnewspaper, brings the total bandwidth gain to29.94%. Eventually, with five cache frames weachieved a bandwidth requirements decrease of34.40% over classic encoding all updates.100015002000250030003500r at ed  n et wo rk  t ra ff i c  ( kB )Reduced network traffic, multiple cache framesclassic tight encoding1 cache frame2 cache frames3 cache frames4 cache frames5 cache frames05000 50 100 150 200 250 300 350 400Ge neFrame numberFigure 2. Influence of cache size on generated net-work trafficThe cache will be efficient in the case that abig update is requested. These big updates areexpected when the user starts an application orwhen switching between applications, typicallycausing a full screen refresh. A video file, caus-ing very fast successive screen refreshes willnot be handled well by a cache though, sincethey will not often map to the cache frames thatare to be predicted before the session starts.A solution for this, as suggested in [2], is tostream these video files.IV. CONCLUSIONIn thin client computing systems, benefit canbe found in caching certain graphical updates.Other updates that resemble one of these cacheframes can be efficiently coded by computingthe difference to the cache frame, to be encodedand transported over the network to the thinclient.ACKNOWLEDGMENTSB. Vankeirsbilck is funded by the Institutefor the Promotion of Innovation through Sci-ence and Technology in Flanders (IWT).REFERENCES[1] B. Vankeirsbilck, et al, Bringing Thin Clientsto the Mobile World, 2008 NEM-SUMMIT, To-wards Future Media Internet, Saint-Malo, France,2008.[2] P. Simoens, B. Vankeirsbilck, et al, Designand implementation of a hybrid remote displayprotocol to optimize multimedia experience onthin client devices, Proceedings of AustralasianTelecommunications and Network ApplicationsConference, Adelaide, Australia, 2008.

Graphical update caching for mobile thin clients

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Graphical update caching for mobile thin clients

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Ghent University Academic Bibliography