Automatic Protection Switching(APS) in SDH/SONET
In this article we will discuss protection of telecom networks in the event of failure with example of SDH/SONET networks. This is also referred as APS or Automatic Protection Switching.
There are two major ways of implementing the protection by providing standby unit/device viz. 1+1 and 1+N or N+1.
In 1+1 configuration 1 unit is the working one and the other unit is used as a hot standby.
This other unit will be brought in the circuit in the failure conditions. The advantage of 1+1 configuration is that
it provides 100% redundancy.
While in N+1 configuration, 'N' number of units are in 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 'N' units will fail.
In SDH/SONET, frame consists of many fields, out of which K1 and K2 bytes are provisioned to take care of protection switching. In K1 byte configuration bits 1 to 4 are used for switch priorities and bits 5 to 8 are used for channel number(requesting the action).
K2 byte is mentioned in the table-2, Here bits 1 to 4 channel number used for protection,bit-5 as mode type and bits 6-8 indicates conditions such as unidirectional switching,bidirectional switching and MS-AIS(Multiplex Section Alarm Indication Signal).
Fig.1, APS basic Architecture
As shown in the figure-1, in the event of failure over working interface connected with router-A, connection will be made to the protection interface through router-B. This is done by status information available in 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 mechanism linear and ring. We will understand the protection with example of ring networks below.
K1 byte in SDH frame
Following table mentions fields of K1 byte used in SDH frame.
| Byte K1 | ||
|---|---|---|
| Bits 1-4 | 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 | Exercizer | |
| 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
Following table mentions fields of K2 byte used in SDH frame.
| Byte K2 | ||
|---|---|---|
| Bits 1-4 | 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 filures between the path from router C and router D. The data which supposed to travel from D to C as marked with small pink circle will route through protection path. It will follow route between routers D,A,B and goes to C as it supposed to go earlier before failure occurred.The same is depicted at various routers using loop.
fig.3 bidirectional ring protection
As shown in the figure-3 in the event of failure in bidirectional four fibre network, between router B and C data flows through protection path. The data flows through route between routers B,A,D and C.
Conclusion:
As failures are realistic and hence protection to 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 the wireless networks viz.
GSM,CDMA,VSAT,LTE and more.
RELATED LINKS
SONET versus SDH
SONET SDH tutorial