Page 287 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 273

Network 3:
Time until DRX is enabled: 200 ms;
●
DRX short cycle time: none;
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DRX long cycle time: 80 ms;
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On‐duration: 4 ms;
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Time alignment: 1.92 seconds;
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Time until idle: 30 seconds.
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Network 3 is provisioned quite differently. While it also enters DRX mode in a fraction
of a second, the DRX cycle time is much shorter than in networks 1 and 2. Equally, the
on‐duration is much shorter. And finally, the time alignment can be considered lost
after only 1.92 seconds, which means power saving is much higher than in the two
examples above. After no data transmission for 30 seconds, the network sets the
connection to RRC idle.
Network 4:
No DRX configured;
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Time until idle: 5 seconds.
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And finally, networks that have no DRX configured at all are also common. Instead,
the networks go from RRC connected to RRC idle state of the air interface after an
inactivity period of only 5 seconds. In other words, a new air interface connection and
a new context in the MME and S‐GW has to be created for every web page that is
loaded. From a signaling point of view and also from a core network point of view this
setting is far from ideal.

4.8 LTE Security Architecture

The LTE security architecture is similar to the mechanisms already used in UMTS and
discussed in Section 3.9. The architecture is based on a secret key which is stored on the
SIM card of the subscriber and in the HSS in the network. The same key is used for
GSM, UMTS and LTE. It is therefore possible to efficiently move the security context
between network nodes when the user roams between different RATs.
During the initial contact with the LTE network, that is, during the attach procedure
described earlier, security procedures are invoked between the UE, the MME and the
HSS. During this process, the UE authenticates to the network and the network authen-
ticates to the UE. This prevents man‐in‐the‐middle attacks. The authentication algo-
rithms required for the process are stored and executed in the SIM card and in the HSS.
This way, the secret key remains in a protected environment and cannot be read by
potential attackers eavesdropping on the message exchange on an interface between the
SIM and the mobile device or the HSS and the MME. SIM cards must be capable of
performing UMTS authentication. Consequently, old GSM‐only SIM cards cannot be
used for authentication in LTE and the attach procedure is rejected with such SIM
cards. If a GSM‐only SIM card is used in an LTE‐capable device that then tries to access
an LTE network, the MME at first queries the HSS for authentication and ciphering
keys. As the HSS is receiving the request from an LTE network node, it rejects the
request as the subscriber’s HSS entry contains only GSM authentication information.