The Synchronous Ethernet timing messages are sent through the chassis to support the network timing trails that are traceable to a high-quality timing source. The distributing clocking mode is handled through ESMC messages. The recovered line timing is driven out only by the line interface of the port Gigabit Ethernet MPC. Centralized clocking mode overcomes these limitations by distributing and driving timing out on all the chassis line interfaces.
This clock module functions as a centralized point within the chassis for clock monitoring, filtering, holdover, and selection. It has only one external clock interface. For more information, see Centralized Clocking Overview.
Starting from Junos OS Release The quality level of various clock sources in the network is determined by monitoring the Synchronization Status Messages SSMs from the clock sources. On Ethernet networks that use Synchronous Ethernet for clock synchronization, the SSM is not a part of the timing signal. By interpreting the SSM values, the router determines the clock quality associated with the clock source, and performs its clock selection accordingly. Note that the clock in the router goes into holdover mode in the absence of any clock sources with best quality level and in turn uses the timing information stored in its buffer to synchronize itself.
The following processes play a crucial role during external synchronization of the clock sources in the control board. Note that PTX Series routers need two best clock sources that act as primary and secondary clock sources, whereas MX Series routers need only one best clock source. The clock sync process clksyncd performs the clock selection and participates in ESMC message exchange.
For clock selection, in the absence of user-configured primary or secondary clock sources, the clksyncd runs a clock selection algorithm and selects the two best clocks available as the primary and secondary clock sources, respectively, for a PTX Series router or selects a best clock for an MX Series router.
ESMC packets are received on all the interfaces that are configured as clock sources. ESMC packets are also transmitted to the clock-source interfaces on other routers, as well as to the interfaces that are configured to receive ESMC packets on other routers. When it detects clock quality deterioration, it informs clksyncd to select another primary clock source. After clock selection chassisd is updated with the latest clock source information. Note that in the absence of user-configured primary and secondary clock sources on PTX Series routers, the clock sources are selected through the clock algorithm and chassisd is updated with the latest clock information.
Consequently, a new interprocess connection is established between chassisd and clksyncd. The periodic packet management process ppmd performs periodic transmission of ESMC packets to others routers in the network. These hardware and software devices facilitate data transmission in a computer network. Most times, this data transmission is conducted in either of the two modes — asynchronous or synchronous. Now, you may think of how these modes differ and which mode is better? Read this post to know the answers.
In synchronous data transmission, the data is transferred in the form of frames or chunks between a receiver and a sender. Synchronous data transmission is a data transfer method in which a continuous stream of data signals is accompanied by timing signals generated by an electronic clock to ensure that the transmitter and the receiver are in step synchronized with one another.
The data is sent in blocks called frames or packets spaced by fixed time intervals. Synchronous transmission modes are used when large amounts of data must be transferred very quickly from one location to the other. The speed of the synchronous connection is attained by transferring data in large blocks instead of individual characters.
Synchronous transmission synchronizes transmission speeds at both the receiving and sending end of the transmission using clock signals built into each component. A continual stream of data is then sent between the two nodes.
The data blocks are grouped and spaced in regular intervals and are preceded by special characters called syn or synchronous idle characters. See the following illustration. After the syn characters are received by the remote device, they are decoded and used to synchronize the connection. After the connection is correctly synchronized, data transmission may begin. An analogy of synchronous transmission would be the transmission of a large text document. Before the document is transferred across the synchronous line, it is first broken into blocks of sentences or paragraphs.
The blocks are then sent over the communication link to the remote site. The timing needed for synchronous connections is obtained from the devices located on the communication link. All devices on the synchronous link must be set to the same clocking. Ways to get around this problem include re-synchronization of the clocks and use of check digits to ensure the bytes is correctly interpreted and received.
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