Prognosemetoder – en oversikt - Telenor

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Remote
router/
Bridge
hand, the data communications area is now an area of competition,
and, needless to say, cost effective solutions always have
to be found as one proceeds.
2 LAN interconnections
During the last few years we have witnessed a dramatic transition
from centralized, proprietary main frame solutions to distributed
network solutions with an ever increasing number of
interworking PCs or UNIX workstations. The workstations have
access to powerful servers over local area networks of the type
Ethernet (10 Mbit/s), Token Ring (4–16 Mbit/s) and more
recently FDDI (100 Mbit/s).
The number of LANs has thus increased and the traffic has
exploded. The number of LANs is about 4 millions world-wide
and is growing at a very high rate: according to (1) the annual
growth rate in Europe is about 40 %. The growth in traffic is,
however, not only due to the increasing number of users, but
also to a great extent a result of new applications with their need
for transport of large amount of data and short response times.
The shift from main frames to network servers enlarges this
trend even further. This evolution creates a need for effective
infrastructures that, in addition
to handling the
transport of data,
satisfy the necessary
requirement of
reliability and
security. In this
multitude of needs
and demands proprietary
solutions
like SNA will be
faded out; open
standards become a
must.
PUBLIC
NETWORK
Remote
router/
Bridge
Figure 1 Two LANs interconnected via routers/bridges and leased lines
Variable length
1 byte 2 bytes 2 bytes 1 byte
Flag DLCI Data
FCS Flag
Figure 2 The frame format (HDLC) used for Frame Relay
DTE
Frame Relay
Interface
DTE
Frame Relay
We see two trends concerning LANs: Within
larger companies and institutions there is a need
for splitting up large LANs into smaller segments,
the reason being both of traffic and security origin.
However, certain services, like administrative
routines, will still require interworking between
the segments. The other trend is to interconnect
different LANs between various geographically
separated departments or branches of a company,
so as to build one large transparent network.
Hence, in both cases the result is an ever increasing
need for LAN interconnections.
The most common means of interconnecting
LANs today is by bridges and routers. Bridges, being the simpler
of the two, work on the OSI link layer (2nd layer) and are
limited in routing and load sharing capabilities. Routers, on the
other hand, operate on the network layer (3rd layer) and work
between LANs with the same network layer. They support various
parallel paths in the network and have better security and
load sharing facilities. There is a continuous dialogue between
the routers in a network. Information of the traffic load in the
network will thus be known by all the routers and an optimal
routing can be chosen. Needless to say, routers are more expensive
than bridges. Figure 1 depicts a typical network with
bridges or routers.
Bridges as well as routers have been on the market for several
years and represent today an important product segment for the
data dealers. In (2) is made a survey of sold equipment in Norway
up to 1992 and it thus gives an indication of the potential
market for the PNOs to attack. The study indicates, however,
that the need for capacity for the LAN interconnection is, at the
moment, rather modest. This might change as soon as FDDI
LANs penetrate the market.
The traffic out of LANs is typically bursty, i.e. outgoing line
might be idle for a certain period followed by a period of peak
traffic. Hence, leased lines might not be the most cost effective
solution. As a result network solutions like Frame Relay, MAN
and ATM are being considered. In designing these networks,
however, attention has to be paid to delay and throughput. The
delay should be of the same order as in the connecting LANs.
Usually the requirement is set to be of the same order as the
delay between two bridged LANs; this delay admits an absolute
maximum of one second and a
mean value of 200 ms.
DTE
DTE
Server
Figure 3 Local area networks interconnected with Frame Relay
Server
3 Network
structures
In this chapter we
will describe some of
the network candidates
for interconnecting
LANs.