In modern telephone networks the use of digital technology has become widespread. Utilizing digital technology in telephone networks has a number of advantages. One advantage is that the digital transmission of data is less susceptible to noise, which improves the quality of the transmission. While another advantage is that the digital format is ideal for being implemented on solid state technology such as integrated circuits. This is significant because most of the developments in technology has been in this area.
In order to exploit the advantages of digital technology, new techniques and equipment had to be developed. These new developments have included new modulation techniques, digital switches and various digital interfaces.
An example of a system utilized in digital telephone networks is shown in FIG. 1, which is known as a Digital Loop Carrier or an Integrated Digital Loop Carrier (IDLC) system 10. The IDLC system 10 is utilized to couple subscriber lines 22, 24, 26, 28 to a switching system 12, such as an EWSD.RTM. switching system, which routes calls from the subscriber lines 22, 24, 26, 28 to other parts of the phone network.
The IDLC system 10 includes a remote digital terminal (RDT) 30 which interfaces the subscriber lines 22, 24, 26, 28 to a number of 1.544 MPBS highways 14,18. The 1.544 MPBS highways 14,18 are also known as Digital Signal Level 1 lines (DS1) and are utilized to carry calls from the subscriber lines 22, 24, 26, 28 to the switching system 12. Each DS1 includes 24 individual 64 KBPS digital signal carrying facilities, which are also known as Digital Signal Level 0 lines (DS0). For discussion purposes, only one of the 24 DS0s is shown per each DS1.
The RDT 30 is utilized as an interface to assign and connect the DS0s to the subscriber lines. The assignment and connection of the DS0s is either accomplished on a per call basis or on a provisioned basis. The per call basis is utilized when a large concentration of subscriber lines are required. This means that the RDT 30 has to dynamically assign and connect the DS0s to the subscriber lines. The subscriber lines utilizing a per call basis interface are known as concentrated lines 24,26. While DS0s assigned and connected on a provisioned basis are known as dedicated DS0s and the connected subscriber lines are known as non-concentrated lines 22,28. The dedicated DS0s 16,20 are nailed up which means semi-permanently connected to the respective subscriber lines 22,28 at the RDT 30.
A problem with utilizing a provisioned type of interface is that the non-concentrated subscriber lines 22,28 often lose access to the network. Very often this is caused by a failed or blocked DS1, which causes the DS0s to become unavailable to the subscriber lines. This is a serious problem since the subscriber lines connected to the blocked DS1 are unable to be utilized to make calls. The DS1s are often blocked due to technical problems or maintenance purposes.
The above discussed problem is partially removed by incorporating DS1 protection switching capability within the RDT 30. An example of a IDLC system having DS1 protection switching is shown in FIG. 2. In such a system, a standby DS1 36 is reserved in the event one of the other DS1s 32,34 fail or is blocked. When a DS1 fails, the traffic from that DS1 32 is switched to the standby DS1 36 as shown in FIG. 3. Thus, the DS1 protection switching partially solves the problem of a non-concentrated line losing access. However, the problem remains if a subsequent DS1 34 fails before the previous failed DS1 32 is repaired as shown in FIG. 4. In this situation, the non-concentrated line 40 loses access. Thus, DS1 protection switching is inadequate when there are consecutive DS1 failures within a IDLC system.
It is therefore an object of the present invention to provide a device which is capable of providing continual network access for subscriber lines even in the event of consecutive DS1 failures within a Digital Loop Carrier System.