Delivering
Synchronization across the Packet Network
With
the powerful driver from the Mobile Wireless Operator to reduce OPEX and
increase ARPU, the decisions on CAPEX investment on equipment infrastructure
are hugely important and the challenge to reduce development, installation and
maintenance costs has never been so great.
When
considering a typical Third Generation (3G) mobile network, a significant
portion of the operating cost, in some installations up to 50%, can be
attributed to the radio access network and the recurring costs of the backhaul
between the Base Station and the BSC/MSC, leased on a monthly basis from the
local carrier as well as remote monitoring and management of equipment.
For
the next generation mobile network, the “Holy Grail” is quickly evolving
towards a Packet-Everywhere architecture. Just like the SDH/SONET networks, the
need to synchronize the network is mandated and needs careful consideration. In
wireless networks, the synchronization needs are driven more by the air interface
than by the services being carried, and so tight synchronization is definitely
still needed. Indeed, wireless network have some of the strictest requirements
for synchronization, and they are not at all easily met.
Synchronization within a base station
UTRAN mandates a common timing reference amongst adjacent Node B’s and is
defined to ensure that Node B’s are traceable to the RNC and to minimise cross
interference between adjacent Node B’s. The synchronization method depends on
whether a Frequency Division Duplex (FDD) or Time Division Duplex (TDD) mode is
being deployed within the UTRAN.
The
delivery of synchronization today typically uses either a leased line (E1 or
DS1), a re-timed clock from a SDH/SONET multiplexer or GPS. Synchronising a base
station to a common clock is usually attempted, in GSM and WCDMA systems at
least, using the frequency of the landline signal. Although a landline clock is
usually traceable to an atomic clock, and therefore has excellent long-term
accuracy, the amount of noise allowed on a land line makes most of them
unsuitable for this purpose. Either special synchronization links have to be assigned,
at greater expense, or noise filters have to be used but these can introduce
their own problems. CDMA2000 systems have mandated that the air interface
carries GPS time. Because the cable delay of a landline is not known, the
CDMA2000 base stations cannot be given GPS from a common point in the network but
must acquire it individually, and so base stations are usually equipped with a
GPS receiver, at great installation and maintenance expense.
A
more ideal synchronisation scheme would control the air interface by providing
accurate frequency and phase whilst also providing an accurate copy of GPS
time, all without requiring specially-conditioned lines or individual GPS
receivers.
As
the links from the BSC/MSC to the base stations start to migrate to packets, it
becomes possible to adopt a fundamentally different synchronisation scheme
which closely approaches the ideal scheme outlined above. It provides
equivalent performance to the best of today’s methods but avoids many of their
pitfalls whilst being robust and extremely cost-effective. In 2002, the IEEE
ratified IEEE 1588, which is a new standard for distributing a high precision time
base around a network. In 2003, this was adopted by the IEC and given the
number IEC 61588. Although it was originally targeted towards relatively
compact LANs, it was seen as being potentially useful in wider networks,
particularly in the connections to wireless networks. In 2004, the IEEE opened
a PAR to enhance IEEE 1588 to suit this larger audience. The benefit of this new
standard is that it will allow a time base to be generated in remote equipment
without relying on quiet transmission links. It can operate with reasonable
network delays and packet jitter without deviating away from the synchronization
requirements. The common clock can either be owned by the land network operator
or the wireless network operator. Because it builds a time base, which can be
GPS-locked, it provides frequency, phase and time accuracy, and it suits both
FDD and TDD systems. To get it to work, especially in the presence of typical
levels of packet delay and jitter, requires some careful design of the slave
clock. Semtech have employed their extensive
experience of conventional designs for SDH and PDH equipment to this problem
and have prototyped an IEEE 1588 master slave pair with the right levels of performance. Semtech are also key contributors to the IEEE 1588 PAR,
with the aim of making sure the new version of the standard has the right
enhancements to suit this, and similar, applications.
The
basic operation of IEEE 1588 is similar to that of NTP (IETF RFC 2030) in that
master and slave clocks exchange timestamp values at a relatively slow rate,
and the slave uses these timestamp values to align its timebase
to that of the master. The difference between IEEE 1588 and NTP is that IEEE
1588 uses hardware-assisted timestamping which avoids
the considerably-variable latency suffered by software methods and this leads
to much-improved accuracy.
In
the Semtech patent pending technology, strong
filtering of perceived packet offsets finds the average path delay and
maintains the stability of the slave’s time base even when packet jitter is
high. Strong filtering, though, particularly with the low rate of transmission
of IEEE 1588 timing packets, has to work over long time constants and the local
oscillator in the slave clock must have suitable stability to support this.
Since there is no getting away from the holdover requirement, a decent OCXO has
still to be provided in each base station, and this can be at the heart of the
IEEE 1588 slave clock. Because the OCXO will have good short- and medium-term
stability, the slow rate of IEEE 1588 packets does not limit the performance
which can be obtained for this application. The basic performance seen on the
prototypes is more than adequate for wireless base station requirements.
The
original version of IEEE 1588 did not have the right features to provide
sufficient resiliency for this application and this is being addressed in the
PAR. Things like redundant network clocks and paths will be provided, so that
the wireless base station can continue uninterrupted by a network failure.
This
new synchronisation scheme offers a way to simplify and cost-reduce the
synchronisation of a wireless network. Being able to work independently of the
land network operator’s clock removes the need for expensive conditioned land
lines, whilst the ability to transport GPS time from an RNC to its base
stations means dedicated GPS receivers are no longer needed at each base
station, thereby saving considerable installation and maintenance costs. GPS
reception has always been subject to good antenna positioning and is vulnerable
to heavy weather. Worse, availability of GPS signals has always been at the
behest of the US Navy, which owns and operates the GPS system and they have the
ability to make all CDMA2000 systems, and other GPS-based systems, inoperable.
This potential weakness has been feared for some time in these applications.
Being able to take GPS (directly, or via UTC) from a protected central point
means the CDMA2000 networks can continue even during GPS outages.
Semtech is not a provider of backhaul services and quoting
a “guarantee” of savings would be unwise. Semtech has
however discussed the potential savings with a large and well respected service
provider, and under conditions of anonymity, we can offer some “numbers” using
the Pound Sterling.
Traditional access pricing
model;
Assumed 100Mbps of an STM1 is
used at RNC and 3xE1 at each Node B (giving 17 Node Bs in RNS).
Assumed distance between RNC
and provider network POP to be 5 miles, and average distance between provider
network POP and Node Bs to be 5 miles.
Then, yearly rental costs:
RNC-end: £100,295 + £18,000 = £118,295
Node B ends: 100M/E1 x (£4098 + £1000 + £1192) =
£314,500
Total annual cost: =
£432,795
Simple
Ethernet backhaul pricing model;
Assumed 100Mbps Ethernet at
RNC: =
£14,000
Assumed 6Mbps Ethernet at each
of 17 Node Bs: 17
x £2600 = £44,200
Total annual costs: =
£58,200
Potential ANNUAL SAVING : £432,795 -
£58,200 = £374,595.
While this is an example and cannot be promised, certainly not by Semtech, it provides a glimpse into the new paradigm of
cost structure for mobile wireless network backhaul services.
To
learn more about a new timing delivery paradigm for next generation IP/packet
networks, please contact Phil Tolcher (ptolcher@semtech.com).