https://www.juniper.net/documentation/en_US/junos-mobility11.2/topics/concept/gateways-mobility-bearer-overview.htmlhttps://www.juniper.net/documentation/en_US/junos-mobility11.2/topics/concept/gateways-mobility-bearer-overview.html

In a mobile network using the Long Term Evolution (LTE) architecture, bearers are the tunnels used to connect the user equipment to Packet Data Networks (PDNs) such as the Internet. In practice, bearers are concatenated tunnels that connect the user equipment to the PDN through the Packet Data Network Gateway (P-GW).

In older architectures, bearers were known as packet data protocol (PDP) contexts. One PDP context connects to one PDN location by default (this was the default PDP context). Other PDP contexts (up to 11) could be established to or from the same user device. The maximum of 11 still holds in 4G/LTE networks. Figure 1 shows the relationship between bearers and P-GWs.

Figure 1: Bearers, Gateways, and Packet Networks

In an LTE mobile network, one default bearer is established to a default P-GW whenever the user equipment device is activated (this means the user equipment is on and has performed authentication). There must be at least one default bearer to one default P-GW, but up to 11 other bearers to the same or other P-GWs can be active to a single user equipment device.

Bearers encapsulate user data with the GPRS tunneling protocol, user plane (GTP-U). The GTP-U information is in turn sent with UDP and inside IP packets.

Every user equipment device has an “always on” default bearer for each P-GW to which it connects. For example, if user equipment connects to the Internet through one P-GW and a corporate intranet through another P-GW, two default bearers will be active. In addition, the user equipment can establish other dedicated bearers to other PDNs, based on quality-of-service (QoS) requirements. For instance, viewing a streaming video over the Internet could be done over a dedicated bearer. Dedicated bearers can use a bandwidth guarantee (a guaranteed bit rate, or GBR) or the user equipment can establish a non-GBR bearer.

The bearer itself is a concatenated tunnel consisting of three portions (in a non-roaming situation), established in the following order:

• The S5 bearer—This tunnel connects the Serving Gateway (S-GW) to the P-GW. (The tunnel can extend from P-GW to PDN service network, but this is not considered here.)
• The S1 bearer—This tunnel connects the evolved NodeB (eNodeB or eNB) radio cell with the S-GW. Handover establishes a new S1 bearer for end-to-end connectivity.
• The radio bearer—This tunnel connects the user equipment to the eNodeB (eNB). This bearer follows the mobile user under the direction of the Mobile Management Entity (MME) as the radio network performs handovers when the user moves from one cell to another.