The network access in LTE consist of a set of steps:
The UE acquires frequency synchronization by detecting the DC carrier (null subcarrier), which is transmitted in the center subcarrier of the bandwidth used. Then, the UE acquires time slot and frame synchronization by means of primary and secondary synchronization signals. These signals occupy the middle 62 subcarriers (with five subcarriers reserved on either side for later use) at pre-defined times within each frame. The UE uses the signals to establish slot, sub-frame, and frame timing, and to determine the Physical Cell ID. Once the UE has synchronized with the downlink, it can then read the Broadcast Channel and receive key system configuration parameters. In particular, the Master Information Block (MIB) contains the actual channel bandwidth, allowing the UE to begin using the entire channel for sending and receiving data.
Once the UE has performed system acquisition, the UE then initiates an RRC connection to create the signaling radio bearers SRB 1 and SRB 2. The signaling connection, once created, allows the UE to exchange information with both, the E-UTRAN and the EPC.
After the UE has acquired the network and has established a Signaling Radio Bearer (SRB) with the eNB, it performs an initial attach procedure. Network Attachment is a registration procedure where the UE registers with the network to receive services. During the initial attach procedure, the eNB selects an MME and then establishes a UE-specific S1 signalling bearer with the MME. The attach procedure triggers a default bearer establishment procedure to establish a default EPS bearer for that UE. The IP address can be allocated during default bearer setup, or it can be allocated after default bearer setup.
The next step is authentication. Authentication in LTE is mutual (i.e., the UE and network authenticate each other). Then, the Serving Gateway (S-GW) and Packet Gateway (P-GW) are selected by the MME.
The EPS bearer has three portions associated with it: first is the radio bearer between the UE and the eNB, the second is the S1-U tunnel between the eNB and the S-GW, and the third is the S5/ S8 tunnel between the S-GW and the P-GW. The MME keeps track of all the bearers created for a UE through a bearer identity. The MME initiates the bearer creation procedure based on the subscription information from the HSS. During the attach procedure, GTP-C control tunnels are set up between the S-GW and P-GW and also between and also between the MME and S-GW to transfer signaling messages.
Once the default radio bearer is created, data traffic can flow in both UL and DL.