An Introduction to the Ericsson Transport Network Architecture ...

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well as the number of simultaneous broadcast
connections - is unlimited. This
means that an AXD 4/1 Switch is suitable
for services on leased or switched circuits
from 1.5 Mbit/s up to 140/155 Mbit/s.
SNI-4 column, the basic switching
entity
An SNI-4 column, 9 x 64 kbit/s, is the smallest
possible switchable entity. The bandwidth
of one SNI-4 column is regarded as
sufficiently small for internal applications
too.
Each external signal terminated by the
AXD 4/1 and each standard tributary of that
signal is mapped into an integer number
of columns. As a maximum, a whole
STM-1 channel (155 Mbit/s) including
overhead (OH) can be switched.
To be able to provide connections that use
a number of columns, the integrity of the
frame of columns is guaranteed, i.e. the
time sequence integrity.
Non-blocking, full connectivity Switch
matrix
The Switch matrix is designed to provide
circuit-switched connections without any
internal congestion and to ensure full connectivity
between any input and any output
port, independently of the hierarchical
level of the cross-connected signals and
of the mix of hierarchies (CEPT, NAS,
SDH, simplex, broadcast).
The Switch is a Time-Space-switch (TS)
consisting of a number of Speech Stores
(SS), arranged in rows and columns: one
row for each input and one column for each
output, Fig. 3.
Every time a signal is to be fed to a certain
output, it can be chosen from among the
signals stored in one of the SS (n) associated
exclusively with that output. The
choice is made through the use of Control
Stores (CS) associated with that same output.
No switch action will affect connections already
established.
Characteristics of the TS structure
Since no internal routing is necessary and
only one T-stage is involved, the TS switch
has the following characteristics:
- No rearrangement of established connections
is needed to permit the set-up
of a new connection, capable of being
used for broadcast also
- The delay introduced by the switch is
minimised.
This results in a switch with short set-up
time and very low transmission delay,
which is necessary in a large network with
stringent service requirements.
Multicolumn switching
Multiconnection switching is characterised
by the connection of a number of columns
executed at the same time. The number of
columns that can be connected in a multiconnect
switching operation has no upper
limit. As an extreme, the whole switch can
be reconfigured at the same time.
Internal communication
The AXD 4/1 Switch is capable of handling
packet-oriented signals. This function
is used both for internal control communication
and to handle Embedded Communication
Channels (ECC) in SDH signals.
A special communication protocol is developed
to handle the internal communication
- the Basic Communication Protocol
(BCP). BCP is a self-addressing
protocol that uses the tree-structure of the
integrated control paths to give reliable internal
communication. Information is carried
in 'packet format'. The packet-handling
equipment and the circuit switch are
triplicated, so as to fulfil general requirements
for failure immunity and fault detection
capability.
The supervision of internal communication
is handled by BCP functionality and by the
triplication.
Devices
All types of equipment connected to the
Switch are called Devices. The most common
types are Termination Access Units
(TAU), which terminate transmission signals.
The Central Processor, too, is connected
as a Device.
In addition to the termination of signals, i.e.
line signal access and multiplexing, a Device
terminates the switching network. Majority
vote is used to terminate the signals
from the triplicated Switch. This is the basic
function that forms a single fault-tolerant
system.