UE access the network

Network Attach
http://beyond-3g-wireless.blogspot.com/

When a UE is turned on it needs to attach to the network so that it can able receive or initiate communication. Until the attach operation is not completed, UE will not be able to access the network.


In order to Attach UE is allocated with Uplink/Downlink Signal thru which it can able to read the System Information sent by the network.It is essential for terminals to understand which kind of network is present in terms of available technology, operators,and which channels and parameters shall be used to connect to the network in order to attempt a registration procedure.

After UE is able to read the System Information the next step would be Cell Selection. This procedure is same as it is done in other technologies.

UE sends the NAS PDU as a part of RRC message to eNB. This NAS PDU is then extracted by eNB and sent to MME as a part of S1AP message(Initial UE message)

Let us take a look at IE's in Attach Request NAS PDU mentioned below

(IMSI or old GUTI, last visited TAI (if available), UE Core Network Capability, UE Specific DRX parameters, PDN Type, Protocol Configuration Options, Ciphered Options Transfer Flag, Attach Type, KSIASME, NAS sequence number, NAS-MAC, additional GUTI, P-TMSI signature)

IMSI is International Mobile Subscriber Identity #.
Because the IMSI uniquely addresses each subscriber, it is seen as critical information from a security point of view and its transmission clearly has to be avoided as much as possible. By spying on and monitoring the IMSI, attackers could, for example, track a subscriber’s location,movement and activity, determine user home country and operator.

IMSI shall be included if the UE does not have a valid GUTI or a valid P TMSI available.NAS procedures make use of the GUTI for temporary identity as much as possible instead of the IMSI.

If available, the last visited TAI shall be included in order to help the MME produce a good list of TAIs for any subsequent Attach Accept message. Selected Network indicates the PLMN that is selected for network sharing purposes.

The UE network capabilities indicate also the supported NAS and AS security algorithms

PDN type indicates the requested IP version (IPv4, IPv4/IPv6, IPv6)

For EUTRAN attach, NAS message is PDN Connectivity request. This message is used by UE to inform network that it needs a bearer to transmit data

NAS sequence number and NAS-MAC are included if the UE has valid EPS security parameters. NAS sequence number indicates the sequential number of the NAS message

MME reads this NAS message and understands that UE needs a default bearer and an IP address. MME creates a GTP message Create Session Request and forwards it to SGW. At this point MME assigns and EPS bearer ID to the bearer.Then S-GW responds to GTP Create Session response message with SGW FTEID for user plane, EBI, and Bearer level QoS values. SGW communicates to PCRF to pull the QOS values.
MME receives the session response. It takes the SGW FTEID, EBI and QoS values and places it in Activate Default Bearer Context Request DL_NAS message(Attach Accept) and sends it to eNB in S1AP Initial Context Setup Request message. At this point EPS bearer is established and a Radio Bearer has to be established so that UE can start transmitting the data.
eNB receives the S1AP message, pulls out the NAS message places it in RRC RECONFIGURATION REQ and sends it UE.

UE responds with RRC_RECONFIGURATION_COMPLETE message. It also starts procession the NAS message. Initial Context Setup Response message is attached to the RRC message. At this point UE knows the Bearer ID, an IP address and corresponding QoS values.

eNB informs that UE accepted the default bearer to MME in a S1-AP UL NAS Transport message -Attach Complete-(EPS Bearer Identity, NAS sequence number, NAS-MAC message). Also eNB indicates its FTEID for user plane communication to MME.

MME now has to indicate the eNB user plane info to SGW. It does the same in GTP message Modify bearer request. SGW learns the eNB user plane info from it. After this the user plane data shall flow on the default bearer