![]() ![]() Note: You need to have registered users in order to get the full access to the document. The full specifications are available here: 1588-2008 - IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems. PTP is defined under IEEE standard 1588-2008. This means that PTP clock stability issues impact all modems on a chassis, even those on Integrated CCAP (I-CCAP) line cards, when you use a mix of cards in a chassis. Note: Once the cBR-8 is configured to use PTP for timing, all line cards rely on this clock, even line cards with RF PICs. The RPD and the cBR-8 can only function as slave clocks in current software, although the cBR-8 roadmap adds support for it as a grand master and boundary clock. It is recommended that the RPD does not pass through the cBR-8 to access the master clock. The cBR-8 can access the master clock via Wide Area Network (WAN) interfaces on the supervisor Physical Interface Card (PIC) or via Digital PIC (DPIC) interfaces on the cable line card (the DPIC option was added in 16.8.1 release). The RPD should reach the master clock via the CIN. Non-symmetrical connections like Ethernet Passive Optical Network (EPON) are listed in the R-DTI specification for use as a CIN but rely on a different timing method, currently not supported by Cisco. Warning: One of the main causes of DOCSIS issues in R-PHY is created by non-symmetrical PTP links which lead to clock instability. The formulas for this calculation assume a symmetrical connection between the two clocks. The master and slave devices exchange messages that include time stamps before the slave runs an algorithm to determine these values. PTP enables a slave clock to determine its time offset from a master clock (difference in time between the clocks), as well as the propagation delay in the transport network between the two clocks. Precision Time Protocol (PTP) Introduction ![]() In the current implementation from Cisco, both the cBR-8 and the RPD act as a slave device to a PTP master clock. For Ethernet based networks, PTP is used to achieve this timing. The Remote DOCSIS Timing Interface Specification (R-DTI) is the specification from CableLabs that details how this timing takes place. However, the downstream and upstream signals now physically originate and terminate on the RPD, so the RPD needs to have the same concept of time as the CCAP core. The CCAP core handles upstream scheduling and generation of the MAP messages. In order to link the RPDs to the CCAP, a Converged Interconnect Network (CIN) is used, which is IP-based, and can be dedicated to cable access or shared by other applications. The timing element of DOCSIS needed for upstream scheduling, is still present in R-PHY. To avoid this situation, the CCAP uses a MAP advance timer, where it does not schedule traffic for a modem until a point in time later than the MAP advance timer. The CCAP must assure that it does not assign a time slot to a modem too quickly following a request, to avoid that the modem has no time to receive the MAP message and process it, and misses the chance to use that minislot. It is important for the CCAP and each modem to have the same concept of time, in order to avoid overlaps. Low latency DOCSIS (LLD) provides options to reduce this value below 2 ms. The MAP messages from the CCAP allocate approximately 2 milliseconds (ms) worth of time. A modem only transmits traffic on a minislot assigned to it (or on a contention slot if making a bandwidth request or other station maintenance activity). The modems look at these messages to determine which minislots are allocated to which modems and which are for contention-based activities. These MAP messages are sent in the downstream and received by all modems. ![]() In order to grant modems the time slots (minislots) to transmit on a upstream channel, the CCAP maps out upstreams minislot assignments by means of upstream bandwidth allocation map (MAP) messages. If your network is live, ensure that you understand the potential impact of any command. All of the devices used in this document started with a cleared (default) configuration. The information in this document was created from the devices in a specific lab environment. Tip: Refer the Cisco 1x2 RPD cisco article for more information. The information in this document is based on these software and hardware versions: cBR-8 Converged Cable Access Platform (CCAP).Prerequisites RequirementsĬisco recommends that you have knowledge of these topics: This document describes the Precision Time Protocol (PTP) which is use in cable network with cBR-8 and Remote PHY (R-PHY) networks. The goal is to give a global understanding of the protocol and how to configure it in cBR-8 and RPHY deployments. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |