2.4 Synchronization of communication network

Sync - a means of support for all digital equipment in the communications network at a medium speed. To transfer the digital information is converted into discrete pulses. When sending impulses through these lines and communication units Digital Network all its components must be synchronized.

To synchronize networks most administrators telecommunications networks uses the hierarchical source - receiver (driving - driving). The main source of reference signal synchronization network is one or more primary reference generator (PEG). The reference signal generator that is shared on the network, consisting of generators - receivers or driving generators set (CCA).

Host with the most stable generator assigned node - source. Node - the source transmits synchronization reference to one or more receiving nodes. The performance of the host course units are the same or worse than the source node. Receiver unit captures the reference clock frequency source and then transmits it to another node receiver

Host nodes typically designed for receiving one or more the number of reference signals. A reference signal is active. All other alternative reference signals are redundant. If the active reference signal is lost, the receiver can switch node reference signals and connects to an alternative reference signal.Thus, each receiving node has access to sync from one or more sources. Most networks are designed in such a way that all generators receivers filed two or more different reference signals. In private networks, it may not be possible because of the limited possibility of connections between nodes. Generators are located under the hierarchy based on levels of performance.

Initial reference generator (PEG) is a generator for managing network providing frequency setting accuracy better than 1 ∙ 10 ^-11. One of the classes is PEG generator Generator words words 1. 1, by definition, is a free generator. It does not use a reference clock signal to obtain or start syncing. Generators words one usually consist of several cesium frequency standards.

However, PEG can be made ​​not only on the basis of primary atomic frequency standards. Other examples are the generators of PEG coordinated universal time (GPS). These systems use local or rubidium crystal oscillators that run timing information derived from the GPS. They are forced to run, but in their parameters are classified as primary reference generators. These generators are able to maintain error from a few units to a few 10 ^-13 10 ^-12 units.

The main purpose of the generator receiver (CCA) is to restore synchronization with the reference signal and sync support as close to synchronization source node. This requires that the generator receiver performed two main functions. First, he must play sync generator reference signal source, even despite the fact that the reference signal may be erroneous. Second, it must maintain adequate temporary state in the absence of standard synchronization.

The usual mode of operation is CCA extraction clock signal with a reference signal PEG. In this mode, the receiver oscillator must have the ability to detect possible errors brief reference signal. These errors may be unstable or short synchronization reference signal interruption. These errors are usually caused by a device that transports the reference signal from the generator source to the generator receiver.

The main requirement for the formation of network synchronization is the presence of the main and backup synchronization signal path.

To create a network of network synchronization is necessary to have a highly stable source of PM (DTCH) and provide network architecture, which should ensure its quality performance. In a qualitative operation mean ensuring high stability DTCH allowable withdrawal phase of face value, and high network reliability. The network consists of independent DTCH (can work offline and sync mode) transmission line clock frequency and buffer device processing delay and clock signals.

Consider two means of network synchronization network:

• Network with mutual synchronization DTCH;

• Network with forced synchronization.

The network of mutual synchronization of all DTCH united among themselves on the principle of each with each. High reliability - an advantage of this option. A disadvantage is the long transition process to the state established. Also, such a network and management are difficult and expensive dynamic processes. So far area using mutual synchronization networks is limited.

Currently, creating a network of network synchronization using the second option. And it is characterized by the following variants of the network:

• with direct synchronization;

• departmental network synchronization generators;

• hierarchical forced synchronization;

• oligarchic network synchronization tree structure.  The most common is a third option.

PRG - primary reference generator; SRG - secondary reference generator.

Figure 5 - Hierarchical forced synchronization

This system provides strict control of known devices synchronization. It synchronous and stable. Therefore, the current system, built on this principle.

When telecommunications network covers a large area, thereby picks large number of network nodes, the DNN to build oligarchic principle. In such a network is several PRG synchronized with each other. For this purpose CFS that are on satellite.

CFS should have high stability. The higher stability, so they are the best, and therefore more expensive.

Will result CFS quality levels (Table 3.1).

Table 3.1 - Levels quality CFS

Symbols of quality	Name of Quality
PRC	The primary reference genenrator
TNC	Timer transit hub
LNC	Timer localhost
SETS	Linear timer
Do not use	For the purposes of synchronization is not used
Uknown	The level of quality is not known

To improve the reliability of the network is generally used not one but several primary reference generators. In Ukraine there are three PRG:

• basic;

• reserve;

• spare.

Thus, the construction of network synchronization is so that each MW fit several clock signals from different sources. Thus, when an accident appears select the best alternative DTCH.

Figure 3.1 shows the scheme of distribution of source clock synchronization in normal operation.

LNC SNN-8 from PRC LNC

S ETS SETS SETS

Don’t Use Don’t Use Don’t Use

DNN-7 DNN-1

L NC TNC

D NN-6 DNN-2

LNC

DNN-5 DNN-4 DNN-3

Don’t Use Don’t Use Don’t Use Don’t Use

SETS SETS SETS SETS

LNC LNC LNC

Figure 3.1. Distribution of sources and the status of network nodes in normal operation mode.

Table 3.2 - Distribution of sources and the status of network nodes in normal operation mode.

Network node (NN)	First priority (the main road)		The second priority (reserve)
	Source synchronization	Status	Source synchronization	Status
SNN- 8	External PRC	TNC	-	-
DNN- 1	SETS SNN of 8	LNS	SETS of DNN-2	Don 't Use
DNN-2	SETS of DNN-1	LNS	SETS of DNN-3	Don 't Use
DNN- 3	SETS of DNN-2	LNS	SETS of DNN-4	Don 't Use
DNN- 4	SETS of DNN-3	LNS	SETS of DNN-5	Don 't Use
DNN-5	SETS of DNN-4	LNS	SETS of DNN-6	Don 't Use
DNN- 6	SETS of DNN-5	LNS	SETS of DNN-7	Don 't Use
DNN-7	SETS of DNN-6	LNS	SETS SNN of 8	Don 't Use
SNN 8	-	-	External PRC	TNC

LNC from PRC LNC

SETS SETS SNN-8 SETS

Don’t Use Don’t Use Don’t Use Don’t Use

LNC DNN-7 DNN-1

TNC LNS

DNN-6 DNN-2

S ETS

DNN-5 DNN-4 DNN 3

D on’t Use Don’t Use Don’t Use Don’t Use

SETS SETS SETS

LNC LNC LNC

Figure 3.2. Distribution of sources and the status of network nodes in emergency operation mode.

Table 3.3. Distribution of sources and the status of network nodes in emergency operation mode.

Network node (NN)	First priority (the main road)		The second priority (reserve)
	Source synchronization	Status	Source synchronization
SNN- 8	External PRC	TNC	-
DNN- 1	SETS of SNN- 8	LNS	SETS of DNN-2
DNN-2	SETS of DNN-1	LNS	SETS of DNN-3
DNN- 3	SETS of DNN-2	LNS	SETS of DNN-4
DNN- 4	SETS of DNN-3	LNS	SETS of DNN-5
DNN-5	SETS of DNN-4	LNS	Is absent
DNN- 6	Is absent	LNS	SETS of DNN-7
DNN-7	Don 't Use of DNN-1	LNS	SETS of SNN -8
SNN-8	-	-	External PRC