home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
QRZ! Ham Radio 8
/
QRZ Ham Radio Callsign Database - Volume 8.iso
/
pc
/
files
/
p_misc
/
netconf.arc
/
TEXNET.002
< prev
next >
Wrap
Text File
|
1988-12-17
|
7KB
|
128 lines
[ Note: This series of articles was found on Compuserve and downloaded
from HAMNET there on 21 July 1985 by Dwight Ernest KA2CNN 70210,523. ]
An Introduction to Networks
part 2
by T.C. McDermott, N5EG
networks SIG, TPRS
In the last article the distinction between a LAN and long- haul
network was made. Also the performance problems associated with
unreliable radio circuits and the End-to-end ACK was discussed. This
article will present some other methods of information transfer
possible.
Another method of information transfer possible is "HOP-TO- HOP"
acknowledge. In this method, each packet, or small group of packets, is
acknowledged by every receiving station along the path from the sender
to the receiver. For example, using the same terminology as the last
article, S=sender, R=receiver, D1,D2, ... Dn = digipeaters.
S : send packet
D1 : ACK to S, repeat frame
D2 : ACK to D1, repeat frame
R : ACK to D2.
Why does this method improve the throughput of the system ? Because
now that the sender, S does not have to wait for the ACK to return from
R, S may send another packet after the ACK from D1. That is, it may
OVERLAP traffic.
S : send packet 1
D1 : ACK S-1, repeat 1
D2 : ACK D1-1, repeat 1 S : send packet 2
R : ACK D2-1 D1 : ACK S-2, repeat 2
D2 : ACK D1-2, repeat 2
R : ACK D2-2
In other words, once that S has received it's acknowledge, it may
transmit the next packet almost immediately (if D1 is on the same
channel, it should wait for the D2 --> D1 ack first, if the D2 --> D1
link is on a different frequency [as in a network] then S could transmit
the next packet immediately upon receiving the D1 ACK). What happens to
the flow of information in the presence of errors in the transmission ?
Lets look at an example:
S : send packet
D1 : ACK S, repeat packet
D2 : gets garbled packet from D1
D1 : waiting for ACK from D2
D1 : retransmits packet to D2 after time-out
D2 : ACK D1, repeat packet
R : ACK D2
In article one, with this same exact scenaro, it took 25 packet
times to accomplish the transfer of one packet from S to R through 2
digipeaters. In this example it took 6 packet times, a 417 %
performance improvement in the transmission time. This performance
improvement actually increases with more digipeaters, or worse RF paths.
In fact with 8 digipeaters, and a 70 % probability of a sucessful
packet-hop, this approach offers about a 10,000 % performance advantage
!
Additional to the transit time advantage (time delay per packet in
seconds-from-S-to-R), there is the advantage in throughput (bytes/sec.).
The throughput in the HOP-TO-HOP ack method is NOT dependent upon the
number of digipeaters. This is because as soon as the first digipeater
has acknowledged the reception of the senders' packet, the sender is
free to send the next packet, regardless of the number of hops in the
path. Contrast this to the end-to-end hop method, where the throughput
is very dramatically dependent upon the number of hops in the path.
What is required of the digipeaters in the network to handle this
type of repeating function, i.e. HOP-TO-HOP digipeating? Each
repeating station is required to contain a fair amount of memory, enough
to buffer every packet that it digipeats until that packet is
acknowledged by the next repeater. Since the repeater may receive
packets from several different stations at nearly the same time, and
perhaps some of them are occaisonally garbled in transmission to the
next repeater, then they must be stored in repeater memory until they
are sucessfully passed to the next repeater.
The repeaters must also implement some sort of flow control. If
packets arrive faster than they can be sent, then the buffer memory
could overflow. Thus the repeater must be able to tell the previous
repeater, or sender, that the packet is rejected, and to stop sending.
When the repeater clears the messages, and thus frees up some memory,
then it re-initiates packet transfer from the previous station. This
finite memory size limitation actually causes the end-to-end performance
of the network to become more heavily dependent upon the quality of the
RF links. Thus performance of the HOP-TO-HOP system is dependent upon
the probability of RF path-hop success, but is not heavily dependent
upon the number of repeaters in the path, unlike the END-TO-END scheme.
This was taken into account when I stated that the performance of the
HOP-TO-HOP ack with 8 digipeaters, and 70% path-hop probability of
success was about 10000% better than the END-TO-END method.
There is one intetersting dis-advantage to the HOP-TO-HOP scheme,
although it is not a strong disadvantage, and that is the issue of data
integrity. In the END-TO-END ack ( EEA ) scheme when the receiver
acquired the data, the ACK was sent. Thus when the sender receives the
ACK, there is certainty that the data was in fact received. In the
HOP-TO-HOP ack ( HHA ) all that is known when the sender receives the
ack, is that the first digipeater received the ack. A failure in the
network could still block the receiver from receiving the data - thus
the sender was ACK'ed even though the receiver had not received the
data. This is not as serious a problem as it sounds at first, however,
since there is still a method to determine whether the data is received
at the final destination correctly.
This is handled by layer 4 of the OSI model - the transport layer.
It is responsible for data integrity in the real world of unreliable
networks. One protocol for doing this is familiar to those of us with
AX.25 units, and this is the Virtual Circuit protocol. Each of us is
intimately familiar with virtual circuits. Any time that you connect to
another station, you have generated a virtual circuit. You and the
receiver communicate on a common channel with everybody else. But your
traffic only goes to your desired destination, not all destinations on
the channel. Thus there is a circuit between you and the connected
receiver on a channel with a (theoretically) unlimited number of
circuits. This is called a virtual circuit. The only reason there is a
circuit is because you and the connected receiver previously agreed to a
connection. The circuit is dissolved when you and the receiver agree to
this (disconnect). The next article in this series will deal with the
virtual circuit protocol on the network, as opposed to the virtual
circuit between the sender and the network, or between the receiver and
the network, which is something different.