Pulse or digital communications – Synchronizers – Network synchronizing more than two stations
Reexamination Certificate
1999-07-22
2001-11-13
Chin, Stephen (Department: 2734)
Pulse or digital communications
Synchronizers
Network synchronizing more than two stations
C370S503000, C370S509000
Reexamination Certificate
active
06317475
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to synchronization of telecommunications networks, and particularly to a method for determining the synchronization hierarchy in networks in which nodes send synchronization status messages to one another. A synchronization status message indicates the signal quality level relative to the synchronization, and thus the node can decide on the basis of the quality levels received which signal it is to use as its synchronization source. Hence, it is ensured by means of synchronization status messages that the node utilizes the best possible signal as its synchronization source. The method of the invention is intended particularly for the synchronization of SDH (Synchronous Digital Hierarchy) devices, but it can also be used in conjunction with conventional PDH (Plesiochronous Digital Hierarchy) devices.
BACKGROUND OF THE INVENTION
In this description, the term node (or node equipment) is employed for the intersection point of links in a telecommunications network. A node may be any device or equipment, for example a branching device or a cross-connection device.
In present-day (plesiochronous) telecommunications systems, synchronization may be performed either by means of separate synchronization connections or by utilizing the normal data connections between the system nodes. Separate synchronization connections are used only in isolated cases and very seldom to synchronize an entire network. When data links are used for the synchronization, the line code must be such that the nodes are also capable of recognizing the clock frequency from the incoming data signal. Synchronization of the network nodes from these clock frequencies can be achieved by two basic methods: mutual synchronization and slave synchronization. In mutual synchronization, each node forms its own clock frequency from the mean value of the incoming signal frequencies and its current clock frequency. Hence, all nodes in the network drift towards a common mean frequency and in a steady state have reached said frequency. However, a network employing mutual synchronization cannot be synchronized with a desired source, and thus it will be difficult to interconnect different networks, as in that case the operating frequency of the entire network cannot be precisely determined in advance. In slave synchronization, on the other hand, all network nodes are synchronized with the clock frequency of the master node of the network. Each node selects one incoming signal frequency as the source for its clock frequency. The node seeks to select a signal having the clock frequency of the master node of the network.
In independent slave synchronization, each node makes its decisions about synchronization without receiving any external information to support the decision-making. When the nodes make their decisions on synchronization independently, each node must determine with which node it is synchronized. These determinations are often made in the form of a priority list, and thus the node selects from valid incoming signals the one having the highest priority, i.e. the one highest on the list, as its synchronization source. If this signal is lost or its quality deteriorates so that it is no longer acceptable as a synchronization source, the node selects from the list the signal having the next-highest priority. The priority list must be compiled in such a way that all nodes on the list are located between the node concerned and the master node of the network, and thus synchronization is distributed from the master node to the lower levels.
However, independent slave synchronization poses limitations to network synchronization: in looped networks, all links cannot be used for synchronization, and hence the dynamic adaptability of the network in different situations is limited. Communication must be present between the nodes in order for the information possessed by an individual node to be sufficient for decision-making in all situations without any need to strongly limit the number of links utilized for synchronization, in which case the clock frequency of the master node could not be distributed as easily to the network nodes. There are two methods for such communication, which will be described in the following.
A simple method for expanding independent slave synchronization to be communicative is loop protected synchronization (LP). LP synchronization seeks to prevent the timing from drifting into inoperative state in looped networks by using two state bits mcb and lcb as an aid in the above priority lists, the bits being transmitted between network nodes. The first state bit, the master control bit (mcb), indicates whether the synchronization is derived from the master network node. The master node defined for the network sends this bit as a logical zero in its outgoing signals, and the other nodes relay it further, if they are synchronized with a signal in which the mcb bit has the value zero. The other state bit, the loop control bit (Icb), indicates whether there is a loop in the synchronization. Each node in the network sends this bit as a logical one in the direction in which it is synchronized and as a logical zero in other directions.
Another way in which independent slave synchronization has been expand to be communicative is to use a synchronization status message (SSM) in accordance with the ITU-T standards G.704 and G.708. Standard G.704 defines the frame structure of a digital transmission system operating at a rate 2048 kbit/s. In accordance with the recommendation, bits
4
-
8
in every second frame are spare bits and may be used e.g. to transport the above synchronization status messages. Only one of bits
4
-
8
in a frame can be used for this purpose, and thus a four-bit synchronization status message is made up by a selected bit (
4
-
8
) in frames
1
,
3
,
5
, and
7
and in frames
9
,
11
,
13
, and
15
of the multiframe. The same synchronization status messages (SSM) are in standard G.708 for SDH networks. In an SDH network, the synchronization status messages are transported in bits b
5
. . . b
8
of byte S
1
in the section overhead (SOH) of the STM-N frame.
The table below presents the synchronization quality levels (QL) indicated by the bit patterns formed by these selected bits San
1
-San
4
(n=4, 5, 6, 7 or 8) S
1
(b
5
. . . b
8
). The last column shows the expressions in accordance with to the recommendations.
San1-San4
QL
or S1 (b5 . . . b8)
Synchronization Quality (QL) Description
0
0000
Quality unknown (Existing Sync. Network)
1
0001
Reserved
2
0010
G.811
3
0011
Reserved
4
0100
G.812 Transit
5
0101
Reserved
6
0110
Reserved
7
0111
Reserved
8
1000
G.812 Local
9
1001
Reserved
10
1010
Reserved
11
1011
Synchronization Equipment Timing Source
(SETS)
12
1100
Reserved
13
1101
Reserved
14
1110
Reserved
15
1111
Do not use for Synchronization
As will be seen from the table, ITU-T has decided on four synchronization levels, and additionally a meaning has been given to two further levels; one indicates that the synchronization level is unknown and the other that the signal should not be used for synchronization (QL=1111).
FIGS. 1 and 2
illustrate the operation of the SSM method in a ring-shaped network having five nodes in all, denoted by references N
1
. . . N
5
. Within each node, the quality level of the internal clock of the node (QL:1011) is indicated at the top of the column. Therebeneath the priority list of the node is shown, wherein the selected timing source is indicated in italics. As stated previously, each node selects as its timing source the signal having the highest quality level as indicated by the synchronization message included therein. If several signals have the same quality level, the one highest on the priority list is selected. The synchronization status message transmitted by each node is shown with reference “QL:xxxx” beside each port of the node. External timing sources S
1
and S
2
are connected to the master node N
1
and to node N
3
respectively. The quality levels of the synchronization status
Altera Law Group LLC
Chin Stephen
Ha Dac V.
Nokia Telecommunications Oy
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