PLUGGABLE PACKET MASTER CLOCK
RELATED APPLICATIONS
[0001] The present application claims the benefit under 35 U.S.C. §120 of U.S. Application
13/625,876 filed on September 25, 2012, the disclosure of which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] Embodiments of the invention relate to timing distribution in packet switched networks.
BACKGROUND
[0003] Modern communication networks typically link many different types of mobile and/or
stationary communication terminals. Among the communication networks are the well known
cellular phone backhaul networks and the less familiar smart grid networks, which operate to
control and distribute energy. The networks may link communication terminals, such as by way
of example, cellular phones, computers, and communication enabled industrial plant equipment,
and provides the terminals with an ever increasing menu of voice, video, and data
communication services.
[0004] The networks operate to transport information from one to another of their respective
terminals using signals containing information relevant to the services that the networks provide.
In propagating from a source terminal to a destination terminal of a given communication
network, the signals generally propagate through a plurality of network nodes. At each node the
signals are received and then, optionally after processing in the node, are forwarded toward their
destination. Typically, the networks are packet switched networks (PSNs) and information
propagated by the networks are packaged in packets configured in accordance with a suitable
packet technology such as multiprotocol label switching (MPLS), internet protocol (IP), and/or
Ethernet.
[0005] Various services, such as cellular telephony and data, when transported over PSN
networks, require for their proper operation that devices in the PSN network be synchronized to
highly accurate timing information. Timing information comprises a highly accurate reference
frequency and/or time of day (ToD). ToD defines a current year, month, day, hour, minute,
second, and fractions of a second, referenced to some standard, such as International Atomic
Time (TAI) or Universal Coordinated Time (UTC). A network device in a PSN network may
receive timing information from various sources and in accordance with various synchronization
procedures. For example, a network device in a PSN may receive timing information directly
from Global Navigation Satellite System (GNSS) transmissions, such as the US Global
Positioning System (GPS), the Russian GLONAS, or the Chinese Beidou satellite transmissions,
or by participating in a PSN "timing over packet" procedure, executed in accordance with a
suitable packet timing distribution protocol, whereby a "packet slave clock" is synchronized to a
"packet master clock".
[0006] Timing over packet is generally provided by a system of clocks comprising a single
reference clock referred to as a "packet grand master clock" that communicates and synchronizes
time with at least one packet slave clock. The packet grand master clock receives a frequency
reference and a time of day (ToD) reference from a highly accurate reference clock, such as a
Primary Reference Time Clock (PRTC). A PRTC may provide the frequency reference as an
isochronous train of pulses, referred to as "clock pulses", characterized by an accurate and stable
pulse repetition frequency (e.g., 10MHz). The PRTC may provide signals for determining ToD
as a sequence of narrow pulses having a repetition rate at one pulse per second (1-PPS), with
each pulse accompanied by a time code that associates the pulse with a year, month, day, hour,
minute, and second, referenced to a standard such as TAI or UTC. The PRTC may comprise a
highly stable Cesium or Rubidium atomic clock in order to maintain highly accurate frequency.
Additionally or alternatively, it may comprise a GNSS radio receiver in order to receive accurate
ToD information from GNSS satellite transmissions. The reference frequency provided by a
PRTC is generally required to be accurate to better than 1 part in 10^ and the ToD accurate to
±100ns (nanosecond) relative to UTC. The packet master clock repeatedly, and usually at regular
time intervals, synchronizes each packet slave clock responsive to the reference frequency and
ToD that it receives from the PRTC, in a process referred to as "timing distribution".
[0007] Timing distribution involves a packet master clock and packet slave clock exchanging a
sequence of timing packets configured in accordance with communication protocols of the
network. Commonly used protocols are Network Time Protocol (NTP), versions of which are
defined in RFC-1305 and RFC-5905, and Precision Time Protocol (PTP), versions of which are
defined in IEEE- 1588-2002 and IEEE-1588-2008 (often called 1588-v2). The timing packets
comprise timing information, such as "timestamps", which the packet slave clock records, and
which define times at which the timing packets egress and/or ingress the master clock and/or the
slave clock. Upon completion of a transaction, the packet slave clock has a record comprising a
set of timestamps that it uses to synchronize itself to the packet master clock.
[0008] Typically, a packet master clock of a PSN distributes timing to a plurality of packet slave
clocks, each of which is located at a different node of a plurality of nodes of the network. To
provide a satisfactory degree of reliability, the packet master clock may comprise redundant
components, such as a back-up power supply and redundant packet transmission and reception
circuitry. Due to the expense of the PRTC and packet master clock, and in order to ensure
consistency, the network generally comprises a single packet master clock connected to the
physical layer of the network at a central location in the network.
[0009] Packets propagating between the same two nodes in a PSN may experience different
transit times, referred to as propagation delays, in propagating between the two nodes. Variations
in propagation delay (propagation delay variation - PDV) may result from variations in queuing
delays in network elements along paths that packets travel between the nodes and/or from
packets traveling along different paths between the nodes. A difference between a propagation
delay from a first node to a second node, and a propagation delay from the second node to the
first node is referred to as delay asymmetry. Consistent delay asymmetry may result from the
path from the second node to the first node not coinciding with the path from the first node to the
second. In addition to causes of delay asymmetry and PDV resulting from the logical structure
and operation of the PSN network noted above, delay asymmetry and PDV may also be
generated by changes in physical hardware comprised in the physical layer supporting the PSN.
Such changes may for example comprise changes in physical connectors, fiber and/or copper
cabling, and/or interface circuitry. For example, delay asymmetries and PDVs of hundreds of
nanoseconds (depending on cable length) may be caused by environmental changes affecting
cabling and cable connectors. Overall, delay asymmetries and PDVs can be on the order of 10s
or even 100s of milliseconds.
[00010] Delay asymmetries and PDVs inherent in a PSN network degrade the quality of timing
recovered by a packet slave clock using timing over packet protocols. Furthermore, as the
number of nodes in a PSN network increases and a number of alternative communication paths
between a master clock and slave clocks increases, the difficulty, expense, and bandwidth
overhead incurred in distributing timing to the network clocks increases.
SUMMARY
[00011] An embodiment of the invention relates to providing a packet master clock housed in a
housing that may be plugged into a conventional small form factor (SFP) compliant cage located
in, by way of example, a PSN switch, router, or end device, that enables connecting the packet
master clock to the PSN. Optionally, the packet master clock distributes timing according to the
IEEE 1588 protocol. Optionally, the packet master clock comprises a GNSS receiver from which
it receives timing information. Hereinafter, a packet master clock in accordance with an
embodiment of the invention may be referred to as a pluggable packet master clock (PPMC). A
PPMC enhances functionalities of a host device into which it is plugged with functionalities of a
packet master clock.
[00012] An SFP compliant cage refers to a socket configured to receive a small communication
module such as a Small Form-factor Pluggable (SFP) module, an Enhanced Small Form-factor
Pluggable (SFP+) module, a 10G Form-factor Pluggable (XFP) module, a 100G Form-factor
Pluggable (CFP) module, and a Gigabit Interface Converter (GBIC) module, specified by an
industry groups in agreements known as "multisource agreements (MSA)". Multisource
agreements specify electrical, optical, and physical features of the modules and sockets, referred
to as "cages", into which the modules may be plugged. "SFP" may be used generically to
reference small communication modules and cages that are compliant with any of the MSA
agreements.
[00013] In an embodiment of the invention, a PPMC comprises a GNSS receiver that generates a
ToD and a reference frequency responsive to signals received from GNSS satellites. The PPMC
comprises a PRTC that receives said GNSS signals and generates the ToD and the reference
frequency, and a packet master clock to which it provides the received reference frequency and
ToD. In the following we will usually refer to GNSS satellites, transmissions, and receiver, as
GNSS satellites, transmissions, and receiver, without limiting ourselves to a specific GNSS.
[00014] In an embodiment of the invention, each of a plurality of network elements of a
communication network is provided with its own PPMC, thus creating a network having a
dispersed configuration of packet master clocks, each packet master clock maintaining
synchronization with other packet master clocks in the network without recourse to a timing
distribution protocol. These PPMCs may then distribute timing to nearby network elements,
resulting in a network with a plurality of packet master clocks, instead of a single centrally
located packet grand master clock.
[00015] By providing a plurality of network element each with its own PPMC, in accordance with
an embodiment of the invention, end point nodes that may require accurate timing, are generally
closer than they would be in a conventionally configured network to a source of accurate timing
information. As a result, PDV and delay asymmetries that may degrade synchronization of the
end point nodes are reduced, and the endpoints may benefit from improved quality of timing and
functioning of services that are dependent on accurate synchronization with other
communication devices in the network. In addition, distributed PPMCs may reduce bandwidth
dedicated to distributing timing.
[00016] The network may also benefit from reduced sensitivity to timing degradation resulting
from loss of line of sight reception of signals from GNSS satellites. In an urban environment,
loss of line of sight to GNSS satellites and consequential loss of GNSS timing signals may for
example, result from masking by high rise buildings. Loss of GNSS timing signals may also
result from deliberate or unintentional jamming.
[00017] By providing a plurality of the network elements with independent PPMCs, the network
may comprise a relatively large number of clocks capable of functioning as PRTCs and packet
master clocks that are spatially dispersed. The relative large number and spatial dispersion of the
PPMCs reduces a probability that all the PPMCs in the network will simultaneously suffer a
GNSS outage and leave the network without an accurate ToD and frequency reference. The
disposition of a large number of spatially dispersed PPMCs may provide the network with
substantially improved robustness in maintaining timing quality.
[00018] In the discussion, unless otherwise stated, adjectives such as "substantially" and "about"
modifying a condition or relationship characteristic of a feature or features of an embodiment of
the invention, are understood to mean that the condition or characteristic is defined to within
tolerances that are acceptable for operation of the embodiment for an application for which it is
intended
[00019] This Summary is provided to introduce a selection of concepts in a simplified form that
are further described below in the Detailed Description. This Summary is not intended to identify
key features or essential features of the claimed subject matter, nor is it intended to be used to
limit the scope of the claimed subject matter.
BRIEF DESCRIPTION OF FIGURES
[00020] Non-limiting examples of embodiments of the invention are described below with
reference to figures attached hereto that are listed following this paragraph. Identical structures,
elements or parts that appear in more than one figure are generally labeled with a same numeral
in all the figures in which they appear and a numeral labeling an icon representing a given feature
in a figure may be used to reference the given feature. Dimensions of components and features
shown in the figures are chosen for convenience and clarity of presentation and are not
necessarily shown to scale.
[00021] Fig. 1A schematically illustrates a PPMC in accordance with an embodiment of the
invention;
[00022] Fig. IB schematically illustrates components that the PPMC shown in Fig. 1A in
accordance with an embodiment of the invention;
[00023] Fig. 2 schematically shows a cellular network comprising elements that are synchronized
by PPMCs in accordance with an embodiment of the invention; and
[00024] Fig. 3 schematically shows the cellular network shown in Fig. 2, in which elements of the
network are synchronized by a conventional packet master clock and packet slave clocks.
DETAILED DESCRIPTION
[00025] Fig. 1A schematically shows a PPMC 20 in accordance with an embodiment of the
invention. PPMC 20 is housed in a small form factor pluggable housing 22 suitable for insertion
into a standard SFP cage. Optionally, housing 22 comprises input connectors 24 and 26, which
may be BNC connectors, for connecting a GNSS antenna 30 and an external reference frequency
source 40 respectively to internal circuitry of the PPMC.
[00026] Fig. IB shows a schematic block diagram of circuit components comprised in PPMC 20,
in accordance with an embodiment of the invention. The PPMC optionally has a PRTC, 49
comprising a GNSS receiver 50 that includes an RF front end 5 1 for receiving transmissions
from GNSS satellites, and a GNSS processor 52. GNSS processor 52 processes transmissions
received by front end 5 1 to recover a ToD, noted as t* in Fig. IB, and a prospective reference
frequency, f*(l), from the GNSS satellite signals.
[00027] In an embodiment of the invention, GNSS receiver 50 transmits t* to a packet master
clock module 60 and optionally transmits the prospective reference frequency f*(l) to a
reference frequency selector 70 that also receives prospective frequency signals f*(2), f*(3) and
f*(4). Prospective reference frequency f*(2) is generated by external reference frequency source
40 (Fig. 1A). Prospective reference frequency f*(4) is generated by a local oscillator 72, such as
a quartz oscillator or a miniature atomic clock, internally housed in housing 22 and optionally
comprised in PRTC 49. Prospective reference frequency f*(3) is recovered by a "clock
extractor" 74, optionally comprised in PRTC 49, from the physical data rate of signals that
PPMC 20 receives from a backplane of a PSN network element (not shown) into which it is
plugged. PPMC 20 is connected to the backplane by transmit and receive ports 8 1 and 82
respectively when the PPMC is plugged into an SFP cage of the network element. Reference
frequency selector 70 determines a reference frequency f* signal responsive to prospective
reference frequencies f*(l), f*(2), f*(3), and f*(4). Reference frequency selector 70 transmits
reference frequency f* to packet master clock module 60 for use in distribution of timing through
transmit port 81 to the PSN and thence to packet slave clocks connected to the PSN.
[00028] It is noted that whereas in the above description a ToD is generated responsive to a GNSS
transmission, a PPMC, such as PPMC 20, in accordance with an embodiment of the invention
may generate and/or receive a plurality of prospective ToDs and comprise a ToD selector that
selects a reference ToD from amongst the plurality of prospective reference ToDs. For example,
the ToD selector may receive a prospective ToD generated by processor 52 and an additional
prospective ToD via a suitable receive port from an independent source, such as an external
GNSS receiver or an external PRTC.
[00029] It is further noted that whereas PRTC 49 determines a reference frequency from a
plurality of four reference frequencies, a PPMC in accordance with an embodiment of the
invention may determine a reference frequency from a number of prospective frequencies
different from four. Optionally, the PPMC comprises a PRTC that does not determine a
reference frequency from a plurality of prospective reference frequencies but provides a
reference frequency that is always either f*(l), f*(2), f*(3), or f*(4).
[00030] Fig. 2 schematically shows PPMCs 20 deployed to provide timing over a PSN 100, in
accordance with an embodiment of the invention. PSN network 100 is assumed, by way of
example, to be a cellular phone backhaul network comprising a plurality of four access networks
121, 122, 123, and 124, which connect subscribers to an aggregation network 101. Aggregation
network 101 connects the access networks to a cellular phone core network (not shown). Each
access network may comprise a plurality of base stations 140.
[00031] Aggregation network 101 comprises a plurality of switches and/or routers 102,
hereinafter referred to generically as aggregation routers 102, connected to each other by
communication links 103, which may, by way of example, be provided by, optical fibers, copper
cables, and/or microwave links, and suitable connectors. Aggregation routers 102 transmit and
receive voice, video, and data communication packets via communication channels between a
plurality of edge devices 106, which may router or switches, that interface access networks 121,
124 with aggregation network 101. Edge devices 106 and aggregation routers 102 generally
comprise a plurality of SFP cages (not shown) for connecting edge devices 106 and routers 102
to network 100.
[00032] Each access network 121, 124 may comprise a plurality of base stations 140 each of
which transmits and receives wireless signals at accurately defined radio frequency (RF) carrier
frequencies to and from user equipment (UE) (not shown) in a limited geographical area referred
to as a "cell" of the network. The wireless signals carry voice, video, and/or data to and from the
UEs, which may be cellular phones, laptops configured with cellular access, tablets, ebook
readers, etc. Base stations 140 in a given access network 121, 124, are connected to an
associated edge device 106 that interfaces the given access network to aggregation network 101
by a configuration of communication links 103.
[00033] Each base-station 140 comprises or has access to its own "local" clock 141 that provides
clock signals for frequency and ToD referencing the base-station operations. To transport
communication packets and provide network services at an acceptable QoS, all base-station
clocks 141 comprised in access networks 121, 124 of cellular phone network 100 generally
have to be synchronized to substantially a same reference frequency, and in many cases to a ToD
that coincides substantially with UTC.
[00034] In an embodiment of the invention, a PPMC 20 is plugged into an SFP cage (not shown)
of each edge device 106. Each PPMC 20 directly provides the edge device 106 into which it is
plugged with a substantially same reference frequency f*, and with ToD t*, that coincides
substantially with UTC time, responsive to GNSS satellite transmissions. As a result, edge
devices 106 in aggregation network 101 may be maintained substantially synchronized to each
other in accordance with an embodiment of the invention without, in general, transmission of
timing packets over links connecting the edge routers to aggregation network 101. Aggregation
network 101 may therefore not only be able to function with a reduced allocation of bandwidth
for distributing timing, its network elements may need not be upgraded to introduce on-path
support for timing packets. Edge devices 106 may therefore benefit from improved timing
quality and stability, and hence improved QoS relative to cellular phone networks that are
conventionally configured to distribute timing.
[00035] In addition, an edge device 106 hosting a PPMC 20 in accordance with an embodiment of
the invention, may function as a PRTC that provides timing information responsive to GNSS
timing information, and as a packet master clock referenced to the GNSS timing information for
distributing timing to base-station clocks 141 in an access network 121, or 124. A given edge
device 106 may, for example, transmit timing packets to distribute timing to base station slave
clocks 141 in the access network 121, 124 to which the given edge device 106 is connected.
Propagation paths of timing packets between the given edge device 106 and slave clocks 141 in
the access network to which the PPMC is connected are indicated by dashed arrow lines 180.
Generally, propagation paths 180 are physically shorter and pass through a smaller number of
communication links 103 and network nodes than propagation paths of timing packets in a
conventionally configured cellular phone network. Base stations 140 in each access network
121, 124 may therefore evidence improved timing quality and stability, and hence improved
QoS relative to cellular phone networks that are conventionally configured to distribute timing.
[00036] For comparison with cellular phone network 100 configured to distribute timing using
PPMCs 20 in accordance an embodiment of the invention, Fig. 3 schematically shows a cellular
phone network 200 conventionally configured to distribute timing. Cellular phone network 200
is identical to cellular phone network 100 except that it does not comprise PPMCs, such as
PPMCs 20, and distributes timing from a conventional PRTC (not shown) and packet grand
master clock 202.
[00037] Packet grand master clock 202 is connected to aggregation network 101 so that it may
exchange timing packets with any of slave clocks 141 in network 200 via communication
channels provided by the backhaul network. Propagation paths of the timing packets are
indicated by dashed arrow lines 280 and are readily seen to be substantially longer and passing
through a greater number of communication links 103 and network nodes than propagation paths
180 of cellular phone network 100 shown in Fig. 2. As a result, quality and stability over time of
synchronization of cellular phone network 200 may be inferior to that provided by cellular phone
network 100.
[00038] Whereas quality and stability of synchronization of cellular phone network 200 may be
improved by providing network 200 with more than one conventional packet master clock 202,
conventional packet master clocks are relatively expensive. Conventional packet master clocks
are also substantially more complicated to install than a PPMC 20, and the cabling and cable
connectors used to connect a conventional packet master clock to a closest network element are a
source of asymmetry and packet delay variation that contribute to degrading synchronization.
[00039] In the description and claims of the present application, each of the verbs, "comprise"
"include" and "have", and conjugates thereof, are used to indicate that the object or objects of the
verb are not necessarily a complete listing of components, elements or parts of the subject or
subjects of the verb.
[00040] Descriptions of embodiments of the invention in the present application are provided by
way of example and are not intended to limit the scope of the invention. The described
embodiments comprise different features, not all of which are required in all embodiments of the
invention. Some embodiments utilize only some of the features or possible combinations of the
features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments,
will occur to persons of the art. The scope of the invention is limited only by the claims.

CLAIMS
1. Apparatus for providing timing information, the apparatus comprising:
a primary reference time clock (PRTC) that provides a reference time of day (ToD) and a
reference frequency;
a packet master clock that receives the ToD and reference frequency and is configured to
distribute timing to a slave clock in accordance with a timing over packet procedure responsive
to the ToD and the reference frequency; and
a housing that houses the PRTC and packet master clock which may be plugged into a
conventional small form factor (SFP) compliant cage to connect the packet master clock to a
packet switched network (PSN).
2. Apparatus according to claim 1 wherein the PRTC comprises:
a global satellite navigation system (GNSS) receiver that receives GNSS transmissions
transmitted by GNSS satellites; and
a GNSS processor that processes received GNSS transmissions to generate the reference
frequency.
3. Apparatus according to claim 1 or claim 2 wherein the PRTC comprises an internal
oscillator that generates the reference frequency.
4. Apparatus according to any of claims 1-3 wherein the PRTC comprises an input for
receiving the reference frequency from an external frequency source.
5. Apparatus according to any of claims 1-4 wherein the PRTC comprises a clock extractor
that generates the reference frequency responsive to a physical data rate of a backplane of a
network element of the SFP comprising the SFP cage into which it the apparatus is plugged.
6. Apparatus according to any of claims 1-5 wherein the PRTC comprises:
a global satellite navigation system (GNSS) receiver that receives GNSS transmissions
transmitted by GNSS satellites; and
a GNSS processor that processes received GNSS transmissions to generate the ToD.
7. Apparatus according to any of claims 1-6 wherein the PRTC comprises an input for
receiving the reference ToD from an external source.
8. Apparatus according to any of claims 1-7 wherein the PRTC comprises a frequency
selector that receives a plurality of prospective reference frequencies and selects the reference
frequency from the plurality of prospective reference frequencies.
9. Apparatus according to claim 8 wherein the PRTC comprises:
a global satellite navigation system (GNSS) receiver that receives GNSS transmissions
transmitted by GNSS satellites; and
a GNSS processor that processes received GNSS transmissions to generate a prospective
reference frequency of the plurality of reference frequencies.
10. Apparatus according to claim 8 or claim 9 wherein the PRTC comprises an internal
oscillator that generates a prospective reference frequency of the plurality of reference
frequencies.
11. Apparatus according to any of claims 8-10 wherein the PRTC comprises an input for
receiving a prospective reference frequency of the plurality of reference frequencies from an
external frequency source.
12. Apparatus according to any of claims 8-11 wherein the PRTC comprises a clock
extractor that generates a prospective reference frequency of the plurality of reference
frequencies responsive to a physical data rate of a backplane of a network element comprising
the SFP cage into which it the apparatus is plugged.
13. Apparatus according to any of the preceding claims wherein the PRTC comprises a ToD
selector that selects a reference ToD from amongst a plurality of prospective reference ToDs.
14. Apparatus according to claim 13 wherein the PRTC comprises:
a global satellite navigation system (GNSS) receiver that receives GNSS transmissions
transmitted by GNSS satellites; and
a GNSS processor that processes received GNSS transmissions to generate a prospective
reference ToD of the plurality of reference ToDs.
15. Apparatus according to claim 13 or claim 14 wherein the PRTC comprises an input for
receiving a prospective reference ToD of the plurality of reference ToDs from an external
source.
16. Apparatus according to any of the preceding claims wherein the timing over packet
procedure conforms to IEEE-1588-2008.
17. Apparatus according to any of the preceding claims wherein the timing over packet
procedure conforms with Network Time Protocol (NTP).
18. A PSN comprising a plurality of apparatuses according to any of the preceding claims.
19. A PSN according to claim 18 and comprising a slave clock that receives timing
information distributed by a packet master clock comprised in an apparatus of the plurality of
apparatuses.