Prognosemetoder – en oversikt - Telenor

3.1 Frame Relay
Frame Relay is a new standardized connection-oriented data
service where the data is transported in variable length, HDLC
based frames between end users. The service is intended to be a
cost effective alternative to leased line for LAN interconnection,
and might in the short term be the most attractive way of LAN
interconnections. The service was first introduced in the US and
was followed by several European countries by the end of 1992.
Norwegian Telecom will provide Frame Relay services from
January 1994.
The HDLC (High-level Data Link Control) carries, in addition
to the variable data field, Data Link Connection Identifier
(DLCI) that identifies the call and Frame Checking Sequence
(FCS) that checks for bit errors. Since there is a dynamic allocation
of bandwidth, the service is well suited for bursty traffic
which is the typical traffic pattern for LAN interconnections.
A Frame Relay network is made up of network nodes and user
terminals, DTE (Data Terminal Equipment). The latter connects
the user to the network. DTE can be a PC, bridge, router or host
computer that is provided with interface defined for Frame
Relay. The standard, which is described in both CCITT and
ANSI recommendations, defines signal and data transmission at
link level, OSI level 2, in the interface between user equipment
and network.
The first versions of Frame Relay work in a Permanent Virtual
Connections (PVC) mode which indicates that the paths through
the network are predefined by the network operator. When the
DTE transmits frames to the network, the nodes will read the
identifier, DLCI, look up in tables to find the right outgoing
channel and send the frame to the next node (or end terminal).
Use of several identification codes permit several parallel sessions
in different directions to coexist on the same physical link.
In this way a DTE can communicate simultaneously with different
destinations over the same access to the network, a prerequisite
for LAN users.
FCS is used to check that the frame has been transferred correctly.
Frames indicated as erroneous by the FCS are discarded;
means of ensuring an error-free DTE-DTE transmission has to
be implemented in the
higher-level proto-
col. If errors occur
and are corrected,
data must thus
be retransmitted
through the
entire network.
This obviously
takes longer
time than
User
networks
DQDB
DQDB
DQDB
MSS
DQDB : Distributed Queue Dual Bus
MSS : MAN Switching System
Transport
network
Figure 4 MAN architecture
retransmission only over the link where the error occurred.
Hence, the quality of the transmission medium plays an important
role for Frame Relay. An approximate limit where Frame
Relay is effective is set to a BER < 10 -6 on individual links.
This error rate can be met with speeds of several Mbit/s over
most transmission links today; a maximum link transmission
speed of 2 Mbit/s is, though, at present defined. Higher speed
(2–34 Mbit/s) is considered, but it is a question whether other
broadband services, both connectionless and connection oriented,
might be more suitable for this part of the market.
3.2 Metropolitan Area Network (MAN)
MAN is in this context understood as a network consisting of
subnetworks that use the IEEE standard 802.6, Distributed
Queue Dual Bus (DQDB), as access protocol on a dual bus (see
figure 4) which also uses principles defined by ETSI.
The DQDB protocol will eventually support both connectionless
and connection oriented (included isochronous) services,
at the moment only the connectionless one is specified.
The interest in connectionless service stems from the need to
provide cost effective services with high throughput and low
delay for bit rates higher than the ones provided by Frame
Relay. Bellcore thus defined the Switched Multi-megabit Data
Service (SMDS) that operates on US standards for bit rates.
ETSI has specified a European counterpart which is named
CBDS (Connectionless Broadband Data Service). DBDS offers
bit rates of 2 Mbit/s, 34 Mbit/s, 139 Mbit/s and 155 Mbit/s, i.e.
bit rates compatible with existing transmission hierarchy in the
public network. The connectionless service features make
DBDS particularly suitable for LAN interconnections and might
be attractive in a medium and long term perspective. As FDDI,
with its nominal rate of 100 Mbit/s, is becoming widespread, the
bandwidth requirement for LAN interconnection might come up
in the region 30–60 Mbit/s which can easily be handled by
CBDS.
CBDS being a service in MAN may also be offered over ATM.
Offered in MAN it gives regional coverage, offered over ATM
it gives unlimited coverage depending on the penetration of
ATM. The service offers transmission of data units of variable
length that tolerate variable delay.
MSS
DQDB
DQDB
DQDB
User
networks
User
networks
Broadband connectionless
services are
offered in a number
of field exper-
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