📄 http:^^www.cs.nmsu.edu^~arthur^renweisman^renweisnetwork.html
字号:
Date: Wednesday, 15-Jan-97 00:19:51 GMTServer: NCSA/1.1MIME-version: 1.0Content-type: text/htmlLast-modified: Monday, 27-Mar-95 17:11:58 GMT<html><head><!-- This document was created from RTF source by rtftohtml version
2.7.5 --></head><body><b><B>A CATV Based Metropolitan Area Network</b>
<b>Using Distributed Switching</B></b><p>
<b></b><p>
<b></b><i>Arthur I. Karshmer and Renwei Yan</i><p>
<i></i><p>
<i></i>Computer Science Department<br>
Computing Research Laboratory<br>
New Mexico State University<br>
Las Cruces, NM 88003 USA<br>
<p>
<p>
<b>ABSTRACT</b><p>
<b></b><p>
<b></b>A number of protocols have been proposed to allow the use of standard
cable television plants to support metropolitan area networking. In the present
work we describe a new approach to solving the problem that has some
interesting properties in its ability to support large numbers of users at
fixed transmission rates at a relatively low cost.<p>
<p>
<b> Introduction</b><p>
<b></b> A major goal of all computer network designs is the distribution of
information in a fast, error free manor at the lowest possible cost. One method
of accomplishing this goal is to use already existing technologies in new and
imaginative ways. Although designed for different purposes, the cable
television systems (CATV) already in place in much of the U.S. provide an
interesting and potentially useful platform for the distribution of digital
data on a metropolitan area scale. As the CATV systems in use were initially
designed for one way communications, any plan to use them must include either
upgrading currently installed upstream amplifiers or the actual implementation
of them.<p>
The problems associated with using these types of facilities were studied and
it has been demonstrated that using such facilities is technically and
economically feasible [Karshmer, Thomas and Phelan - 1991; Karshmer and Thomas
- 1989]. These studies along with others [Hatamain - 1985; Hafez - 1985] have
demonstrated potential strengths and weaknesses in using such systems. Possibly
the greatest strength of using CATV based networks is their almost universal
existence in the U.S. Approximately 60% of all homes are currently connected to
a cable plant, and virtually every building in most cities is "passed" by a
cable.<p>
One of the key problems associated with using the branching tree
topology of a cable plant is access control. Due to the separation of upstream
and downstream channels, any message from a source node (a leaf) must be sent
upstream to the head end and then downstream to the destination node (another
leaf). This round-trip time can cause a large delay message transmission and
reception which can render an access control technique such as CSMA/CD useless.
<p>
In the current work, we present results from the analysis of a system that
reduces message transmission time on a two-way CATV network while not
decreasing network throughput. We develop a model of a two-level CATV
networking scheme which is then tested using well know simulation techniques.
Finally, the simulation results are analyzed and are compared to best
throughput and expected delay times.<p>
<b>Distributed switching over a single-trunk CATV network</b><p>
<b></b>Figure 1 shows a simplified view of our two-level, single-trunk CATV
network with <b>M</b> switches installed on the trunk. Each switch is in turn
connected to a branch cable that supports <b>K</b> branch nodes (network
subscribers). Each <b>trunk</b> node (switch) has three inputs (trunk upstream,
trunk downstream and branch upstream) as well as three outputs. Each
<b>branch</b> node has one output for branch upstream and one input for branch
downstreamSince all trunk nodes have a certain level of intelligence, the
traffic flow on the trunk is totally controlled. The goal of the intelligent
trunk nodes is to turn-around messages onto the appropriate branch cable at the
earliest possible time. The head end is therefore not the only node to that can
redirect messages. If our system is able to redirect messages at the earliest
possible time, we believe the system can reduce message travel time, reduce
message delay and increase system throughput.<p>
Figure 2 shows the simulation results for the 512 (<b>M</b> * <b>K</b>) node
case with different node distribution on the trunk and branch cables. The
distributions are 1 trunk node and 512 branch nodes, 5 trunk nodes and103 nodes
per branch, 10 trunk nodes and 51 nodes per branch, 20 trunk nodes and 25 nodes
per branch and 40 trunk nodes and 13 nodes per branch. Figure 5 shows the
DELAYs related to these different distributions. The following figure shows a
packet length of 512 or 2048-bits and a baud rate of 1Mbps.<p>
Figure 2 shows that with a fixed baud rate, (1Mbps) and a fixed packet size
(512 or 2048 bits), the throughput (total number of packets) on each branch
cable is almost identical. Therefore, the total throughput of the system within
a given time is expressed in the following equation.<p>
As the value, Num_Packets_per_branch is almost identical to the same values
for<b> G</b> and <b>S</b> on each branch cable, it seems clear that putting the
control node(s) on the trunk cable will linearly increase the throughput for
the entire network. In this case, the implementation of an intelligent node on
the trunk cable resulted in the desired performance. Since Slotted Aloha can
double the throughput on each branch, we can see that the total throughput is
also doubled. This implies that the better the protocol on the branch level,
the more the total system throughput increases.<p>
Figure 3 shows that the DELAY time may be reduced even if the number of trunk
control nodes increases. The following equation shows that the de
<!WA0><iMG SRC="http://www.cs.nmsu.edu/~arthur/RenweisMAN/Network1.GIF">lay
is mainly caused by two factors: the size of waiting queue and the
retransmission rate (<b>G</b>), which is what queuing theory would predict. If
there are too many messages at the trunk control nodes waiting to be
transmitted, the DELAY will increase. This means that if the traffic is greater
than trunk capacity, the store and forward protocol on truck level will cause a
long DELAY and deteriorate overall system performance
<!WA1><iMG SRC="http://www.cs.nmsu.edu/~arthur/RenweisMAN/RenweisNetwork2.GIF">.<p>
<p>
<b>Conclusions</b><p>
<b></b>The results from the current study lead us to a number of conclusions
regarding the use of a two-level distributed intelligence approach to using
CATV equipment to support metropolitan area networking. Briefly stated, they
are:<p>
Adding control nodes on the trunk cable (increasing<b> M</b>) will reduce the
DELAY with the number of uses (subscribers) in the system being fixed.<p>
Proper values for the average transmission success rate (<b>S</b>) and average
transmission attempts (<b>G</b>) will also tend to increase the throughput.<p>
Increasing the Baud rate, tends to reduce the Packet_time, and therefore has a
positive effect in reducing the Waiting_queue_SIZE, which in turn reduces the
DELAY (see Figure 5).<p>
Interestingly, increasing the Baud rate beyond a certain limit tends to cause
the Waiting_queue_SIZE to increase. Consequently the Packet_time is reduced and
the DELAY time increases (see Figure 5). <p>
Essentially, our ability to reduce the DELAY time is a function of the trunk
cable's transmit rate (bit rate) and the Packet_Length. Increasing the value of
<b>M</b> will not cause the DELAY time to be increased because the throughput
on each branch is effectively fixed and has a normal distribution probability.
Therefore, the more nodes (users) on a branch, the less frequently each branch
node can send a Packet onto branch cable as the throughput on each branch is
fixed. As a result, to the extent that a trunk node can handle the traffic, the
waiting queue size will not increase and the DELAY time will not increase.<p>
The model clearly demonstrates that our design is extendable: we can increase
the number of users by simply adding trunk control nodes to increase the system
throughput without increasing the average system DELAY. This is an important
result as it tells us that a small number of trunk nodes, which are inexpensive
and simple to install, have a dramatic impact on overall network performance.
Our results were obtained using a Baud rate of 1Mbps - using a more realistic
figure in the range of 4Mbps could only enhance our findings.<p>
</body></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -