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Femtocell Synchronization Analysis
Rock Xie
HUAWEI TECHNOLOGIES CO., LTD.
HUAWEI TECHNOLOGIES CO., LTD.
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3GPP Structure for Femtocell
The 3GPP TR23.830 report defines the architecture aspects of Home NodeB (3G
UMTS) and Home eNodeB (4G LTE). The HNB represents the femtocell base
station and HNB GW represents the femtocell gateway. The iuh is the
interface specifying the communication between the gateway and base
station.
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Femtocell Forum Structure for Femtocell
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The Femtocell Forum also provides a reference model very similar
to 3GPP
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The challenges in Femtocell synchronization
The Femtocell network architecture shows that a
public network is used to establish the connectivity
between the femtocell and core network elements,
This presents a set of unique challenges:
Femtocell synchronization security
Femtocell synchronization connectivity
Femtocell synchronization network scalability
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Security Issue
The security is mandatory since the Gateway and Clock server communicate to
the Femtocells via a public backhaul broadband connection (also known as the
3GPP iuh interface or Femtocell Forum Fa interface).
The important point worth noting is the large number of Femtocells that might have
to be served, as well as the additional bandwidth overhead IPSec represents to
the offered load (bandwidth consumption) of synchronization messages.
The 3GPP has considered some of the security aspects with respect to
synchronization performance. The [3GPP.33.820] report lists the following:
1. it may be possible to leave some of the messages unprotected, but there
might be security risks. Care should be taken in considering which messages
are protection or not.
2. there may be bandwidth, delay and delay variation problems if all the
messages (eg., [IEEE1588]) are to be protected.
3. if delays become excessively long, reliance on internal clock might be
important and requires further investigation.
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Connectivity Issue
The Home GW provides Internet sharing for home devices. One of the
challenges is to define the end-to-end connectivity model between the
femtocell and clock server when a function such as network address
translation (NAT) is present within the Home GW.
network address translation (NAT) protects the internal home network, but
prevents the direct connection between devices sitting in the home
network and devices that are outside (reachable through the public
Internet). With respect to synchronization, this could limit the ability of
a femtocell to communicate directly to a Clock server. Therefore,
some mechanism to handle NAT translation and to guarantee proper
flow of synchronization messages should be provided.
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Scalability issue
The femtocell network could easily extend to millions of devices and the
offered load of synchronization messages (bandwidth consumption)
could become quite large when unicast addressing is used.
The scalability issue could impact the clock server throughput but more
importantly could increase the difficultly in capacity planning and
placement of clock servers in the network
The use of load balancing techniques might be beneficial to help in sync
capacity planning but might impact performance. The use of multicast
technologies currently being developed might present opportunities to
reduce the bandwidth consumption of synchronization messages and
to considerably increase the number offemtocells a server can support.
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Conclusion
The use of IPSec links in public networks could degrade the transport of
synchronization message and its performance. In order to reduce the impact
of security issue in public networks, it is possible to leave some of the
synchronization messages unprotected. Caution should take place on the use
of IPSec and sync performance.
Synchronization messages traversing Network Address Translation (NAT)
functions need to be considered, in order to guarantee proper connectivity
between a Clock Server and Femtocell.
Scalability aspects (large number of femtocells), bandwidth consumption and the
placement of clock servers are important as the network architecture need to
be developed.
If TICTOC intends to develop some form of synchronization profile covering mobile
backhaul for instance, then the aspects of femtocell should be taken into
account.
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Thank You
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