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ISO/IEC 11573:1995 (Confirmed 2004) — Synchronization of Access Methods for Local and Metropolitan Area Networks
A Technical Guide to the Scope, Requirements, and Compliance of the Distributed Synchronization Standard for CSMA/CD and Token Ring LANs
Scope and Overview
ISO/IEC 11573:1995 (confirmed 2004) is a joint international standard developed by ISO and IEC under the ISO/IEC JTC 1 subcommittee for telecommunications and information exchange between systems. It specifies mechanisms for distributed synchronization of access methods in local area networks (LANs) and metropolitan area networks (MANs). The standard addresses the need for coordinating the timing of data transmission among multiple stations operating in shared-medium environments, particularly those using CSMA/CD (Carrier Sense Multiple Access with Collision Detection) and token-ring topologies.
The standard was reaffirmed in 2004, confirming its continued relevance in preserving interoperability and deterministic behavior in legacy and evolving LAN/MAN infrastructures. Its scope includes the definition of synchronization entities, protocol data units (PDUs), and state machines required to maintain consistent timing across distributed network nodes. While modern Ethernet-dominated networks rely largely on central clocking (e.g., IEEE 1588), ISO/IEC 11573 provides alternative mechanisms that are still foundational for understanding distributed access synchronization and for retrofitting or maintaining older networks where deterministic timing is critical.
Tip: Although ISO/IEC 11573:1995 is widely superseded in greenfield deployments, its synchronization concepts underpin the logical link control (LLC) layer and remain in use in specialized industrial and vehicular networks. Engineers working on legacy system migrations should consult this standard for backward-compatible timing requirements.
Technical Requirements for Synchronization
ISO/IEC 11573 defines two primary synchronization modes: centralized and distributed. The standard mandates specific parameters, PDUs, and timer values to ensure clock alignment among stations sharing a common medium. The following subsections outline the core technical elements.
Synchronization Entities and PDUs
The standard introduces a Synchronization Sublayer (SS) residing between the Medium Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer. The SS communicates via two types of PDUs:
- Synchronization Request (SyncReq): issued by a station to initiate a timing reference.
- Synchronization Response (SyncResp): returned to confirm reception and adjust local clocks.
Each PDU includes a timestamp field, a sequence number, and a mode indicator. The standard specifies a maximum drift rate of $10^{-4}$ (100 ppm) for station clocks without external correction, and a tolerance window of ±1 bit time at the MAC layer for a successful synchronization exchange.
Timer and Counter Values
Table 1 summarizes the critical timers and counters defined in ISO/IEC 11573 for distributed synchronization in a typical 10 Mbps CSMA/CD environment. These values are network-dependent and must be scaled for higher data rates or different MAC types.
| Timer / Counter | Symbol | Default Value (CSMA/CD, 10 Mbps) | Description |
|---|---|---|---|
| Sync Request Interval | T_sr | 100 ms | Time between consecutive SyncReq transmissions |
| Sync Response Timeout | T_srt | 500 µs | Maximum wait for a SyncResp after sending SyncReq |
| Holdover Counter | N_hold | 10 | Number of consecutive missed SyncResp before entering holdover |
| Drift Compensation Factor | K_drift | 0.0001 | Fractional adjustment applied per timer tick (max 100 ppm) |
Warning: Using incorrect timer values for networks with higher data rates (e.g., 100 Mbps Fast Ethernet) can cause synchronization instability. The standard provides scaling formulas in Annex C, but implementors must verify compliance with the specific MAC physical layer in use.
Implementation Highlights
Implementing ISO/IEC 11573 requires careful integration with the MAC sublayer and consideration of the physical layer’s timing characteristics. The following practices are recommended based on industry experience and the standard’s conformity guidelines:
- State Machine Design: The standard defines four states — Idle, Wait, Synced, and Holdover — with explicit transition conditions. A well-defined finite state machine (FSM) ensures deterministic behavior during loss of synchronization.
- Timestamp Resolution: Timestamps must have a resolution of at least one bit time (e.g., 100 ns for 10 Mbps). Lower resolution increases jitter and may cause protocol violations.
- Interoperability Testing: Because many legacy devices implement proprietary variations, conformance testing against the reference PDUs and timer values is essential. The standard includes a test suite in Annex B (informative) for self-testing.
- Performance Impact: On a lightly loaded network, synchronization overhead is minimal (less than 1% of bandwidth). Under heavy load, SyncReq/Resp exchanges can add up to 5% of collisions. Administrators should configure the Sync Request Interval (T_sr) accordingly.
Success Story: A mid-sized industrial automation network replaced a proprietary synchronization method with ISO/IEC 11573-compliant devices, reducing clock skew from 10 ms to 50 µs and achieving deterministic control loop cycles. The migration was validated using the standard’s conformance tests.
Compliance and Validation Notes
ISO/IEC 11573:1995 was confirmed in 2004 without technical changes, meaning devices manufactured before or after 2004 can claim compliance if they meet the 1995 specifications. However, compliance involves formal validation of:
- PDU encoding and decoding (exact bit patterns per Section 5.4)
- Timer accuracy within the specified drift limits
- State machine sequences (Idle → Wait → Synced → Holdover)
- Scalability: if data rates exceed 20 Mbps, the standard requires a “high-speed amendment” (non-normative) or use of additional external references like IEEE 1588.
Verification is typically performed by an accredited test laboratory using the test procedures described in ISO/IEC 11573:1995/Amd.1 (2002) — an amendment that clarifies test conditions for token-ring networks. It is important to note that the amendment is not incorporated into the confirmed 2004 edition; both documents must be used together for complete compliance.
Important: Since 2004, many national standards bodies have withdrawn ISO/IEC 11573 for new installations. Before deploying any synchronization solution based solely on this standard, verify with your local standards organization (e.g., ANSI, BSI, DIN) whether its status is “confirmed” or “cancelled”. In most jurisdictions, it remains valid only as a historical reference.
Q: Is ISO/IEC 11573 still applicable to modern Gigabit Ethernet networks?
A: No, for data rates above 100 Mbps the standard’s timers and drift tolerances are not directly scalable. However, the synchronization state machine and PDU definitions are often used as a reference for proprietary timing protocols in industrial Ethernet. Use IEEE 1588v2 (PTP) for modern high-speed networks requiring sub-microsecond synchronization.
A: No, for data rates above 100 Mbps the standard’s timers and drift tolerances are not directly scalable. However, the synchronization state machine and PDU definitions are often used as a reference for proprietary timing protocols in industrial Ethernet. Use IEEE 1588v2 (PTP) for modern high-speed networks requiring sub-microsecond synchronization.
Q: Does the 2004 confirmation change any technical requirements?
A: No. ISO confirmation only indicates that a systematic review found no need for revision. All technical clauses, including timers, PDUs, and state machines, remain as published in 1995. The confirmation simply extends the standard’s validity for another five-year period.
A: No. ISO confirmation only indicates that a systematic review found no need for revision. All technical clauses, including timers, PDUs, and state machines, remain as published in 1995. The confirmation simply extends the standard’s validity for another five-year period.
Q: How can I test if my device is compliant with ISO/IEC 11573?
A: Obtain the test suite from ISO/IEC 11573:1995/Amd.1:2002. It includes reference PDUs, test timings, and acceptance criteria. Many commercial protocol analyzers offer built-in templates for validating synchronization exchanges. For formal certification, contact an ISO/IEC-accredited test laboratory.
A: Obtain the test suite from ISO/IEC 11573:1995/Amd.1:2002. It includes reference PDUs, test timings, and acceptance criteria. Many commercial protocol analyzers offer built-in templates for validating synchronization exchanges. For formal certification, contact an ISO/IEC-accredited test laboratory.
Q: What is the recommended holdover strategy for distributed synchronization?
A: The standard recommends entering holdover after 10 consecutive missed SyncResp (N_hold = 10). During holdover, the station uses a free‑running local clock while continuing to listen for SyncReq or SyncResp. The protocol automatically resyncs once a valid exchange occurs. Exponential backoff of SyncReq transmissions during holdover is permitted but not mandatory.
A: The standard recommends entering holdover after 10 consecutive missed SyncResp (N_hold = 10). During holdover, the station uses a free‑running local clock while continuing to listen for SyncReq or SyncResp. The protocol automatically resyncs once a valid exchange occurs. Exponential backoff of SyncReq transmissions during holdover is permitted but not mandatory.
Last updated 2026. This article is for informational purposes and does not replace official ISO/IEC documentation.