DTJ Number 3 September 1987 - Digital Technical Journals

More documents

Recommendations

Info

New Productsstudies of extended LANs are discussed in thisissue of the Digital Technical ]ournal.1 5•16The case study described here uses a very simpleanalytical model to assist in designing anEthernet adapter.Three common approaches used for interfacingmachines to a LAN are depicted in Figure 1 .Case A represents the approach used in theDigital UNIBUS Ethernet Adapter (DEUNA)product. Case B represents the approach used inthe Ethernet adapters made by 3COM Corporationfor UNIBUS and Q-bus systems. Case C representsthe approach used in adapters like theDigital Q-bus Ethernet Adapter (DEQ NA) product.Each approach has certain advantages anddisadvantages.In Case A the packets received are first bufferedon the adapter, then moved via direct memoryaccess to buffers in the host's memory. Packetsto be transmitted follow the same path,except in reverse. The throughput limit of thedevice is limited by how quickly it can movepackets between the adapter and the host' sbuffers.In Case B the packets received are also bufferedon the adapter. In this case, however, thepacket buffer memory is dual ported, with thehost side being mapped into the address space ofthe host. This scheme allows the host to examineABBACKPLANE HOSTLAN ADAPTER BUS MEMORY• • L _r-;;lCOMMON ADDRESS SPACEpackets in the buffers on the adapter, withoutusing any backplane bus bandwidth to receivethem. However, to receive the packets, the hostmust copy them to buffers elsewhere in memorywith a programmed move:In Case C the packets flow in real-time intobuffers in the host memory. The backplane is isolatedfrom the channel with a first-in, first-out(FIFO) control point. This approach reduces theoverhead on the host, as well as that on theadapter processors. In this case, however, excessiveDMA-request latencies may cause overflowin the FIFO control; hence a packet may be lostwhen received. When packets are transmitted,these latencies may cause an underflow in theFIFO control; hence a packet may be aborted.The performance of the DEUNA adapter is sensitiveto two factors: the number of receivebuffers in the adapter, and the number of wordsto be transferred per UNIBUS capture. The numberof receive buffers chosen affects the packetloss rate in the adapter. The number of wordstransferred affects the response to disks and otherdevices on the UNIBUS system. Transferring toomany words per UNIBUS capture may cause adisk to experience "data lates, " indicating thatthe disk could not get the bus for data transferwithin the required time.We wanted to know if the number of bufferschosen by the designers of the DEUNA adapterwould cause these problems to occur.We used a simple analytical model to determinethe packet loss rate an d a simulation modelto determine the words per UNIBUS capture. Thesimulations showed that the DEUNA adaptershould transfer only on e word per UNIBUS capture.The an alytical model is described here.The packet-arrival process is assumed to followa "bulk Poisson" distribution in whichbursts of packets arrive at a rate of A. bursts persecond. The number of packets per burst isassumed to be geometrically distributed with amean B. The burst size is thus described by theformulaFigure 1Three Ways of Organizing BuffersPr (k packets in burst) =(1- 1/B) X B (k - I) , k 1For mathematical convenience we assume thatall service times are exponentially distributed:this assumes that packet lengths are exponentiallydistributed, with a mean of L words perpacket. The UNIBUS bandwidth is approximately800,000 words per second. A fraction of thatbandwidth, J.L words per second, is available forDigiUU TecbnicalJournalNo. 3 September 198627
---Performance Analysis and Modeling of Digital's Networking Architecturethe transfers between the DEUNA buffer andmemory. The rest of the bandwidth is used up bytransfers betWeen the disk and memory, and byother devices on the UNIBUS system.If the DEUNA buffer has a capacity for notmore than N packets, then any packet arriving ata full buffer will be lost. Let us first calculate theprobability P ( n), 0 < n < N, that there are npackets (including the one, if any, in servie)queued at the DEUNA adapter. The distributionof the number of packets in the queue has a relativelysimple form:P(n) = (1 - P)/(1 - p X a N ) for n = 00.04i:: 0.03::::;ii5a? 0.0211.(/)(/)0...J 0.01B =BURST SIZEandP(n) = P(O) X (pjB) X a
Page 1 and 2: Netwo;king ProductsDigital Technica
Page 3 and 4: Editor's IntroductionRichard W. Bea
Page 5 and 6: BiographiesRaj Jain Raj Jain gradua
Page 7: BiographiesDavid R. Oran Dave Oran
Page 10 and 11: increase the sizes of networks, we
Page 12: New Productsmust be able to coexist
Page 15 and 16: A Digital Network Architecture Over
Page 21 and 22: ---A Digital Network Architecture O
Page 27: Performance Analysis and Modeling o
Page 31 and 32: Performance Analysis and Modeling o
Page 33 and 34: ---Performance Analysis and Modelin
Page 35 and 36: --Performance Analysis and Modeling
Page 37 and 38: The DECnet/SNA Gateway Product• H
Page 39 and 40: The DECnetjSNA Gateway ProductFrom
Page 41 and 42: The DECnetjSNA Gateway Productcommu
Page 43 and 44: The DECnetjSNA Gateway ProductUSER
Page 45 and 46: The DECnetjSNA Gateway ProductThe u
Page 47 and 48: The DECnetjSNA Gateway ProductDECne
Page 49 and 50: The DECnetjSNA Gateway Productopera
Page 51 and 52: The DECnetjSNA Gateway ProductSegme
Page 53 and 54: The DECnetjSNA Gateway Product• O
Page 55 and 56: WiUiam R. HaweMark F. KempfAlanJ. K
Page 57 and 58: The Extended Local Area Network Arc
Page 75 and 76: Terminal Servers on Ethernet Local
Page 77 and 78: Terminal Servers on Ethernet Local
Page 79 and 80:
Terminal Servers on Ethernet Local
Page 81 and 82:
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Paul R. Beckjames A. KryckaThe DECn
Page 91 and 92:
The DEC net- VA X Product - An Inte
Page 93 and 94:
The DECnet- VA X Product - An Integ
Page 95 and 96:
The DECnet- VAX Product - An Integr
Page 97 and 98:
The DEC net- VA X Product - An Inte
Page 99 and 100:
The DECnet- VAX Product - An Integr
Page 101 and 102:
john Forecastjames L. jacksonjeffre
Page 103 and 104:
The DECnet,ULTRIX SoftwareComponent
Page 105 and 106:
The DECnet- ULTRIX Softwarehave pro
Page 107 and 108:
The DECnet- ULTRIX Softwaresystems.
Page 109 and 110:
Peter 0. MierswaIDavid J. MittonMar
Page 111 and 112:
The DECnet-DOS Systemthat CCB, whic
Page 113 and 114:
The DECnet-DOS Systemdata link laye
Page 115 and 116:
Tbe DECnet-DOS SystemNDU and the vi
Page 117 and 118:
Th e DECnet-DOS Systemconsole can a
Page 119 and 120:
The Evolution of Network Management
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Page 129 and 130:
Page 131 and 132:
The NMCCjDECnet Monitor Design5. Op
Page 133 and 134:
Th e NMCCjDECnet Monitor DesignFigu
Page 135 and 136:
The NMCCjDECnet Monitor DesignKERNE
Page 137 and 138:
The NMCCjDECnet Monitor Designinto
Page 139 and 140:
The NMCCj/JECnet Monitor DesignThe
Page 141 and 142:
The NMCCjDECnet Monitor Designcan s
Page 143 and 144:
Editorial StaffEditor - Richard W.
show all

DTJ Number 3 September 1987 - Digital Technical Journals

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?