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).

APS basic architecture

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
1000 Manual Switch
0110 Wait to restore(revertive only)
0100 Exercizer
0010 Reverse request(Bi-directional only)
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

unidirectional ring protection
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.

bidirectional ring protection

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.

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.


SONET versus SDH
SONET SDH tutorial

RF and Wireless Terminologies