This feature allows basic calls to be made from the system to a DASS II protocol Central Office (CO), using E1 Digital trunks and DASS II signaling. Digital Access Signaling System Number 2 (DASS II) is a signaling system which allows access to the British Telecommunications plc (BT) network over a digital line from a system to the local exchange. From the local exchange, the system user has voice and data access to the Integrated Services Digital Network.
Traffic between the local exchange and the system travels over a 2.048 Mbps digital link. Thirty voice or data channels are multiplexed into the link. Each channel can handle data at baud rate of 64kb/s.
The 3300 ICP uses the Universal Network Services Unit (NSU) which supports E1 CEPT links, as the interface with the Multiple Integrated Digital Access (Multi IDA) public network. This unit is also used as the interface with a DPNSS private network and the two circuits on a card can be arranged with one connected to DASS II and the other connected to DPNSS.
The information sent in a DASS II call can include the identities of the caller and the destination. This information can be displayed on a telephone or console and can include the location directory number and the system extension number. This information can be suppressed in customer data.
DASS II circuits can be used to provide:
Two-way voice and data traffic
DISA lines
DDI lines
Incoming lines to the operator's console or to other answer points.
By virtue of its digital operation, DASS II allows fast call set-up and loss-free working between the system and the local exchange.
DASS II recognizes three types of call, these are:
Category 1: A call that must take a fully digital route that is, either a DASS II route, a DPNSS route or a combination of both DASS II and DPNSS routes. Supervision tones or operator announcements can be connected to the call path only if the network fails and the call record has been lost. The call can carry voice or data traffic. The complete route must be capable of supporting digital transmissions at 64 kbit/s.
Category 2: A call where a digital route is preferred but which will accept an analog route or a mixed analog and digital route. Tones and announcements can be connected to the circuit at any time. The call can carry voice and data traffic. The Category 2 route is not suitable for data traffic.
Telephony Call: This call will accept any route. The call can carry only voice traffic. The local exchange does not inform the user that this type of call is unsuitable for data traffic.
Assuming that both digital and analog routes are available to the local exchange, calls are routed according to the following rules:
Data calls at 64 kbit/s which originate on the system connected to the DASS II link are treated as Category 1 calls if the data device does not use a modem. The calls are treated as Category 2 calls if the data device uses a modem to connect to the system.
Category 1 or Category 2 data calls which originate on another node in a private network are given a digital route to the local exchange, if one is available. The system connected to the DASS II link drops an incoming Category 1 call if a fully digital route to the local exchange is not available. Incoming Category 2 calls are routed to the local exchange via an analog circuit if a fully digital route is not available.
Telephony calls originating in the system connected to the DASS II link or incoming to that system, are routed to the local exchange via the first available route in the route list. The route can be analog or digital.
There is no customer data or user action required for route selection by category. Route category is selected on the basis of the type of device originating the call.
If the destination of a DASS II call clears down first, the source will continue to hold a DASS II circuit through the local exchange for up to three minutes, collecting call charge units. The line is cleared immediately when the source clears down.
If the destination does not answer, the source will hold the circuit for a period dependent on time settings in the source and destination system, and the intermediate local exchanges. The source system does not collect call charges during this time.
This section provides background information on the design rules for network synchronization. Knowledge of these design rules could be valuable when tracing an obscure fault.
In order to find network synchronization faults, you need to know the quality of the clocks provided by the various nodes. Since there are, in theory, as many clock qualities as there are clocks, it will be necessary to establish some form of categorization. The following categories are recommended:
Category
A:
Basic frequency tolerance is less than or equal to 1 part per million
(ppm)
Short term stability (24-hour) is less than or equal to 0.4ppm.
Long term stability (20-year) is less than or equal to 5ppm.
Control range sufficient to lock to worse case in this category, that
is, ±10ppm.
This category is the minimum expected from public networks and high-level
systems.
Category
B:
Long-term accuracy (20-year) is less than or equal to 32ppm.
Control range sufficient to lock to worst case in this category, that
is, ±64ppm.
This category covers the capability of lower level PBXs and high level
multiplexers.
Category
C:
Long-term accuracy (20-year) is less than or equal to 100ppm
This category covers low-level multiplexers and "channel banks".
They may have no synchronising capability or very simple loop timing.
These devices should only be used at end points where they are not passing
timing signals to another node in the network.
The following rules apply to the design of a network synchronization scheme:
Only the devices with the highest category clocks should be designated as network masters. A violation of this rule will eventually raise alarms or take a whole link out of service.
Timing loops should be avoided. Although timing loops are quite easy to avoid in the primary network design, they are often very easy to find in the fall-back plans when, for example, the normal network master device fails.
A private digital network with a DASS II digital line to the local exchange must derive its timing from the public network.
Avoid long series timing chains and provide as many alternative links, cables and paths as possible. Both of these items will lower the network's susceptibility to link failure.
A timing loop exists when two devices are each trying to derive timing information from the other device. Timing loops can occur if the network master fails, another node takes over as master and creates timing chains which include a number of intermediate steps.
