Switched Multimegabit Data Services Physical Cut-Through
Mechanism
This document
describes a mechanism to boost the performances
of a connectionless Switched Multimegabit Data Services (SMDS) server
by minimizing the amount of traffic and processing that the server
function has to perform.
ITU-T
Recommendation I.364, "Support of broadband
connectionless data service on B-ISDN", describes the support of
connection-less data services as an overlay network on top of
Asynchronous Transfer Mode (ATM). This
specification is the ITU-T
version of Bellcore Switched Multimegabit Data Services.
In this
recommendation, two implementation options are
discussed, option a) where the Connection-Less Server function is
implemented in the same equipment together with the ATM switching
equipment and option b) where the Connection-Less Server function is
implemented as a separate equipment, directly connected to the ATM
switch via a standard interface (an ATM UNI for example). These two
options are both depicted in the Figure.
For both
options, all the ATM cells composing a given datagram
follow the same physical path, from the input I/O (input/output)
adapter (A) to the server to the output I/O adapter (B).
Connection-less end-users exchange variable length frames (called
datagrams in this document since each frame contains the address of
the destination end-user). These
variable length datagrams are
segmented (using AAL 3/4) to be
transported via ATM virtual circuits
between end-users and their server or
between servers. This
segmentation process can either be performed by the end-station
directly (if an ATM interface capability is available), or via an
external box called an ATM DXI. For
option b), there is also
another I/O adapter (C) for the communication between the ATM
switching equipment and the Connection-Less Server. This flow is
illustrated as Path 1 in the Figure.
A
Connection-Less Server is inherently a shared resource into
which the sum of all communication from all the directly attached
resources(00) converges. It is obvious that this immediately limits
the overall service that the network can provide to the maximum cell
rate that the server can process or to the maximum throughput of the
physical connectivity leading to the server.
This physical
connectivity can be the switching fabric (internal server) or the
switching fabric and the physical media to the external server. It
is important to note that adding more servers to the network as an
attempt to remove this bottleneck introduces many other problems as
the difficulty to operate the service is greatly increased and leads
to inefficiencies (wasted bandwidth and increased end-to-end delays)
and complexity as a server to server protocol is required. It is
currently anticipated that most SMDS services offered as an overlay
network over ATM will contain a limited number of servers (usually
one, at least for the f...
