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Synchronization is the means of keeping all digital equipment in a communications network operating at the same average rate. For digital transmission, information is coded into discrete pulses. When these pulses are transmitted through a network of digital communication links and nodes, all entities must be synchronized. Synchronization must exist at three levels: bit, time slot, and frame.
Bit synchronization refers to the requirement that the transmit and receive ends of the connection operate at the same clock rate, so that bits are not misread. The receiver may derive its timing from the incoming line to achieve bit synchronization. Bit synchronization involves timing issues such as transmission line jitter and ones density. These issues are addressed by placing requirements on the clock and the transport system.
Time slot synchronization aligns the transmitter and receiver so that time slots can be identified for retrieval of data. This is done by using a fixed frame format to separate the bytes. The main synchronization issues at the time slot level are reframe time and framing loss detection.
Frame synchronization refers to the need of the transmitter and receiver to be phase aligned so that the beginning of a frame can be identified. The frame in a DS1 or E1 signal is a group of bits consisting of twenty four or thirty bytes, or time slots, respectively, and a single framing pulse. The frame time is 125 microseconds. The time slots are associated with particular circuit users.
A network clock located at the source node controls the rate at which the bits, frames, and time slots are transmitted from the node. A second network clock is located at the receiving node, controlling the rate that the information is being read. The objective of network timing is to keep the source and receive clocks in step, so that the receiving node can properly
interpret the digital signal. Differences in timing at nodes within a network will cause the receiving node to either drop or reread information sent to it. This is referred to as a slip.
For example, if the equipment that is sending information is operating with a clock rate that is faster than the receiving equipment’s rate, the receiver cannot keep up with the flow of information. When the receiver cannot follow the sender, the receiver will periodically drop some of the information sent to it. The loss of information is referred to as a slip of deletion.
Similarly, if the receiver is operating with a clock rate faster than the sender, the receiver will duplicate information, so that it can continue to operate at its speed and still communicate with the sender. This duplication of information is called a slip of repetition.
In DS1 and E1 communications, buffers are used to control slips (see below figure). The data is clocked into the receiving equipment’s buffer at a rate determined by the source end’s clock rate. Data is read from the buffer using the receiving equipment’s clock. Buffers of varying sizes are used. Typically, the buffer will hold more than one frame of data. In this case, the receiving equipment will drop or repeat an entire frame of data when it slips. This is called a controlled slip.
IMPACT OF SLIPS
Voice ⏩Occasional audible clicks
Faxes ⏩Distorted lines
Voiceband Data ⏩Corrupted data
Video ⏩Frame freeze
Encrypted Data ⏩Loss of communications
SONET/SDH ⏩Pressure on pointer budgets. Impairment at PDH boundary.
Poor synchronization affects quality of service. The impact ranges from annoying for voice services to disastrous for encrypted services.
The basic objective of network synchronization is to limit the occurrence of controlled slips. Slips can occur for two basic reasons. The first is the lack of frequency synchronization among the clocks in the connection, resulting in differences in clock rates. The second is phase movement either on the communications link (such as jitter and wander) or between the source and receiver clock. The latter, phase movement between the source and receiver
clock, will be shown to be the largest contributor to slips in communication networks.
Slips, however, are not the only impairment caused by lack of synchronization. In SDH and SONET networks, poor synchronization can lead to excessive jitter and misframes in the transport of digital signals.
SDH and SONET Synchronization Needs
With the introduction of SDH and SONET, new requirements and demands are being placed on network synchronization. SDH and SONET are high-speed, synchronous transport systems. SDH and SONET network elements require synchronization, since the optical signal they transmit is synchronous. If the SDH/SONET network elements lose synchronization, they will not cause slips, however. This is due to the fact that the payload in SDH and SONET is transmitted asynchronously. SDH and SONET use pointers to identify the beginning of a frame. A mismatch in the sending and receiving rate would cause
a change in the pointer.
A pointer adjustment, however, can cause jitter and wander in the transported signal. Jitter is a fast (³10 Hz) change in the phase of a signal. Wander is slow (<10 Hz) phase change. Excessive jitter from SDH/SONET can cause misframes (loss of frame synchronization). Excessive wander can cause terminating equipment to slip. Therefore, the goal of network synchronization in an SDH/SONET network is to limit the number of pointer adjustments made by the SDH/SONET network elements. This is achieved by limiting the short term
(<100 second) noise in the synchronization network by using better network clocks throughout the network.
Telecommunications Synchronization
Most telecommunication administrations use the hierarchical source-receiver method to synchronize its E1/DS1 network. The master clock for a network is one or more Primary Reference Sources. This clock reference is distributed through a network of receiver clocks.
A node with the most stable, robust clock is designated as a source node. The source node transmits a timing reference to one or more receiver nodes. Receiver nodes usually have equal or worse performance than the source node. The receiver node locks onto the timing reference of the source node and then passes the reference to other receiver nodes. Timing is thereby distributed down a hierarchy of nodes.
Receiver nodes are usually designed to accept two or more references. One reference is active. All other alternate references are standby. In the case where the active reference is lost, the receiver node can switch references and lock to an alternate reference. Thus, each receiver node has access to timing from two or more source nodes. Most networks are engineered so that all receiver clocks are given two or more diverse references. In private networks, this may not be possible due to limited connectivity between nodes.
Clocks are placed into the hierarchy based upon performance levels. ANSI designates performance levels as stratum levels: stratum 1, 2, 3, 4E, and four, in order of best performance to worst.
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| Role of Synchronization in Telecommunication Network |
Synchronization is the means of keeping all digital equipment in a communications network operating at the same average rate. For digital transmission, information is coded into discrete pulses. When these pulses are transmitted through a network of digital communication links and nodes, all entities must be synchronized. Synchronization must exist at three levels: bit, time slot, and frame.
Bit synchronization refers to the requirement that the transmit and receive ends of the connection operate at the same clock rate, so that bits are not misread. The receiver may derive its timing from the incoming line to achieve bit synchronization. Bit synchronization involves timing issues such as transmission line jitter and ones density. These issues are addressed by placing requirements on the clock and the transport system.
Time slot synchronization aligns the transmitter and receiver so that time slots can be identified for retrieval of data. This is done by using a fixed frame format to separate the bytes. The main synchronization issues at the time slot level are reframe time and framing loss detection.
Frame synchronization refers to the need of the transmitter and receiver to be phase aligned so that the beginning of a frame can be identified. The frame in a DS1 or E1 signal is a group of bits consisting of twenty four or thirty bytes, or time slots, respectively, and a single framing pulse. The frame time is 125 microseconds. The time slots are associated with particular circuit users.
A network clock located at the source node controls the rate at which the bits, frames, and time slots are transmitted from the node. A second network clock is located at the receiving node, controlling the rate that the information is being read. The objective of network timing is to keep the source and receive clocks in step, so that the receiving node can properly
interpret the digital signal. Differences in timing at nodes within a network will cause the receiving node to either drop or reread information sent to it. This is referred to as a slip.
For example, if the equipment that is sending information is operating with a clock rate that is faster than the receiving equipment’s rate, the receiver cannot keep up with the flow of information. When the receiver cannot follow the sender, the receiver will periodically drop some of the information sent to it. The loss of information is referred to as a slip of deletion.
Similarly, if the receiver is operating with a clock rate faster than the sender, the receiver will duplicate information, so that it can continue to operate at its speed and still communicate with the sender. This duplication of information is called a slip of repetition.
In DS1 and E1 communications, buffers are used to control slips (see below figure). The data is clocked into the receiving equipment’s buffer at a rate determined by the source end’s clock rate. Data is read from the buffer using the receiving equipment’s clock. Buffers of varying sizes are used. Typically, the buffer will hold more than one frame of data. In this case, the receiving equipment will drop or repeat an entire frame of data when it slips. This is called a controlled slip.
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| The Need for Synchronization in Telecommunication Network |
IMPACT OF SLIPS
Voice ⏩Occasional audible clicks
Faxes ⏩Distorted lines
Voiceband Data ⏩Corrupted data
Video ⏩Frame freeze
Encrypted Data ⏩Loss of communications
SONET/SDH ⏩Pressure on pointer budgets. Impairment at PDH boundary.
Poor synchronization affects quality of service. The impact ranges from annoying for voice services to disastrous for encrypted services.
The basic objective of network synchronization is to limit the occurrence of controlled slips. Slips can occur for two basic reasons. The first is the lack of frequency synchronization among the clocks in the connection, resulting in differences in clock rates. The second is phase movement either on the communications link (such as jitter and wander) or between the source and receiver clock. The latter, phase movement between the source and receiver
clock, will be shown to be the largest contributor to slips in communication networks.
Slips, however, are not the only impairment caused by lack of synchronization. In SDH and SONET networks, poor synchronization can lead to excessive jitter and misframes in the transport of digital signals.
SDH and SONET Synchronization Needs
With the introduction of SDH and SONET, new requirements and demands are being placed on network synchronization. SDH and SONET are high-speed, synchronous transport systems. SDH and SONET network elements require synchronization, since the optical signal they transmit is synchronous. If the SDH/SONET network elements lose synchronization, they will not cause slips, however. This is due to the fact that the payload in SDH and SONET is transmitted asynchronously. SDH and SONET use pointers to identify the beginning of a frame. A mismatch in the sending and receiving rate would cause
a change in the pointer.
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| SDH and SONET Synchronization Needs |
A pointer adjustment, however, can cause jitter and wander in the transported signal. Jitter is a fast (³10 Hz) change in the phase of a signal. Wander is slow (<10 Hz) phase change. Excessive jitter from SDH/SONET can cause misframes (loss of frame synchronization). Excessive wander can cause terminating equipment to slip. Therefore, the goal of network synchronization in an SDH/SONET network is to limit the number of pointer adjustments made by the SDH/SONET network elements. This is achieved by limiting the short term
(<100 second) noise in the synchronization network by using better network clocks throughout the network.
Telecommunications Synchronization
Most telecommunication administrations use the hierarchical source-receiver method to synchronize its E1/DS1 network. The master clock for a network is one or more Primary Reference Sources. This clock reference is distributed through a network of receiver clocks.
A node with the most stable, robust clock is designated as a source node. The source node transmits a timing reference to one or more receiver nodes. Receiver nodes usually have equal or worse performance than the source node. The receiver node locks onto the timing reference of the source node and then passes the reference to other receiver nodes. Timing is thereby distributed down a hierarchy of nodes.
Receiver nodes are usually designed to accept two or more references. One reference is active. All other alternate references are standby. In the case where the active reference is lost, the receiver node can switch references and lock to an alternate reference. Thus, each receiver node has access to timing from two or more source nodes. Most networks are engineered so that all receiver clocks are given two or more diverse references. In private networks, this may not be possible due to limited connectivity between nodes.
Clocks are placed into the hierarchy based upon performance levels. ANSI designates performance levels as stratum levels: stratum 1, 2, 3, 4E, and four, in order of best performance to worst.
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