RFAutomatic Protection Switching (APS) in SDH/SONET Networks
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In this article, we will discuss the protection of telecom networks in the event of failure, with examples from SDH/SONET networks. This is also referred to as APS or Automatic Protection Switching. There are two major ways of implementing protection by providing a standby unit/device: 1+1 and 1+N or N+1.
In a 1+1 configuration, one unit is the working unit, and the other unit is used as a hot standby. This other unit will be brought into the circuit in failure conditions. The advantage of the 1+1 configuration is that it provides 100% redundancy.
While in an N+1 configuration, ‘N’ number of units are in the working circuit and are normally connected in the path, and one unit will be connected as a hot standby. This standby unit will be brought into the circuit path when any one unit from the ‘N’ units fails.
In SDH/SONET, a frame consists of many fields, out of which K1 and K2 bytes are provisioned to take care of protection switching. In the K1 byte configuration, bits 1 to 4 are used for switch priorities, and bits 5 to 8 are used for the channel number (requesting the action). The K2 byte is mentioned in Table 2. Here, bits 1 to 4 represent the channel number used for protection, bit 5 represents the mode type, and bits 6-8 indicate conditions such as unidirectional switching, bidirectional switching, and MS-AIS (Multiplex Section Alarm Indication Signal).
Fig. 1, APS basic Architecture
As shown in Figure 1, in the event of failure over the working interface connected to router A, a connection will be made to the protection interface through router B. This is done by status information available in the K1 and K2 bytes of the SDH frame, which is monitored by the routers in the circuit.
Types of Protection Switching
There are two types of switching mechanisms: linear and ring. We will understand the protection with examples of ring networks below.
K1 Byte in SDH Frame
The following table mentions the fields of the K1 byte used in the SDH frame.
| Byte K1 | Bits 1-4 | Description |
|---|---|---|
| 1111 | Lockout of protection | |
| 1110 | Forced Switch | |
| 1101 | Signal Fail - High priority (not used in 1+1) | |
| 1100 | Signal Fail - Low priority | |
| 1011 | Signal degrade - high priority (not used in 1+1) | |
| 1010 | Signal degrade - low priority | |
| 1001 | NOT IN USE | |
| 1000 | Manual Switch | |
| 0111 | NOT IN USE | |
| 0110 | Wait to restore (revertive only) | |
| 0101 | NOT IN USE | |
| 0100 | Exerciser | |
| 0011 | NOT IN USE | |
| 0010 | Reverse request (Bi-directional only) | |
| 0001 | DO NOT REVERT (NONREVERTIVE ONLY) | |
| 0000 | NO REQUEST | |
| Bits 5-8 | These bits are only used in 1+N protection where they communicate the number of the working channel for which request is issued. 0-Null channel(indicates protection channel), 1-14 working channels/lines, 15-Extra traffic channel |
K2 Byte in SDH Frame
The following table mentions the fields of the K2 byte used in the SDH frame.
| Byte K2 | Bits 1-4 | Description |
|---|---|---|
| These bits shall indicate the number of the channel that is bridged onto protection unless channel 0 is received on bits 5to8 of byte K1, when they shall be set to 0000 | ||
| Bit 5 | 1 Provisioned for 1+N mode, 0 Provisioned for 1+1 mode | |
| Bits 6-8 | 111 AIS-L, 110 RDI-L, 101 Provisioned for bi-directional switching, 100 Provisioned for uni-directional switching, 011 Reserved for future use for other protection switching operations such as nested switching, 010, 001, 000 |
Fig. 2, Unidirectional ring protection
As shown in the figure, in the event of failures between the path from router C and router D, the data which is supposed to travel from D to C (as marked with a small pink circle) will route through the protection path. It will follow the route between routers D, A, B, and go to C as it was supposed to go earlier before the failure occurred. The same is depicted at various routers using a loop.
Fig. 3 bidirectional ring protection
As shown in Figure 3, in the event of a failure in a bidirectional four-fiber network, between router B and C, data flows through the protection path. The data flows through the route between routers B, A, D, and C.
Conclusion
As failures are realistic, protection of the network is very important. In this article, we have seen the protection carried out in SDH/SONET-based networks. Various means are adopted by telecom service providers to take care of failure events in wireless networks viz. GSM, CDMA, VSAT, LTE, and more.
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