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Femtocell Synchronization Analysis

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Rock Xie

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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

• 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 IssueThe 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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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 of femtocells a server can

support.

Scalability issue

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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.

Conclusion

Thank You

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