Automatic Protection Switching (APS) in SDH/SONET Networks

sdh sonet
network protection
automatic switching
telecom network
k1 k2 byte

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).

APS basic architecture

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 K1Bits 1-4Description
1111Lockout of protection
1110Forced Switch
1101Signal Fail - High priority (not used in 1+1)
1100Signal Fail - Low priority
1011Signal degrade - high priority (not used in 1+1)
1010Signal degrade - low priority
1001NOT IN USE
1000Manual Switch
0111NOT IN USE
0110Wait to restore (revertive only)
0101NOT IN USE
0100Exerciser
0011NOT IN USE
0010Reverse request (Bi-directional only)
0001DO NOT REVERT (NONREVERTIVE ONLY)
0000NO REQUEST
Bits 5-8These 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 K2Bits 1-4Description
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 51 Provisioned for 1+N mode, 0 Provisioned for 1+1 mode
Bits 6-8111 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

unidirectional ring protection

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.

bidirectional ring protection

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.

Understanding Security Aspects of IoT Devices

Understanding Security Aspects of IoT Devices

Explore IoT security measures including encryption, authentication, and network protection to safeguard devices against cyber threats and ensure secure IoT operations.

iot security
data encryption
cyber threats
Wi-SUN Border Router: Function and Working Explained

Wi-SUN Border Router: Function and Working Explained

Explore the functions and working principles of a Wi-SUN border router within a Field Area Network (FAN). Learn about its role in providing WAN connectivity, node authentication, and network management.

wi-sun
border router
iot
WXC: Wavelength Crossconnect ROADM Explained

WXC: Wavelength Crossconnect ROADM Explained

Explore the WXC (Wavelength Crossconnect) in fiber optics, detailing its function as a ROADM in mesh networks, offering N x N connectivity and reliable configuration.

fiber optic
wavelength
optical