The amount of user plane data will increase significantly during the
next few years because of High Speed Packet Access (HSPA) and IP Multimedia
Subsystem technologies. Direct tunneling of user plane data between the RNC and
the S-GW can be employed to scale UMTS system architecture to support higher
traffic rates.
Direct Tunnel (DT) offers a solution that optimizes core architecture
without impact to UEs and can be deployed independently of the LTE/SAE
architecture.
Important:
Direct tunnel is a licensed Cisco feature. A separate feature
license is required for configuration. Contact your Cisco account
representative for detailed information on specific licensing requirements. For
information on installing and verifying licenses, refer to the
Managing License Keys section of the
Software Management Operations chapter in the
System Administration Guide.
Important:
Establishment of a direct tunnel is controlled by the SGSN; for 4G
networks this requires an S4 license-enabled SGSN setup and configured as an
S4-SGSN.
Once a direct tunnel
is established, the S4-SGSN/S-GW continues to handle the
control
plane (RANAP/GTP-C) signaling and retains the responsibility of making
the decision to establish direct tunnel at PDP context activation.
Figure 1. GTP-U Direct
Tunneling
A direct tunnel
improves the user experience (for example, expedites web page delivery, reduces
round trip delay for conversational services) by eliminating switching latency
from the user plane. An additional advantage, direct tunnel functionality
implements optimization to improve the usage of user plane resources (and
hardware) by removing the requirement from the S4-SGSN/S-GW to handle the user
plane processing.
A direct tunnel is
achieved upon PDP context activation when the S4-SGSN establishes a user plane
tunnel (GTP-U tunnel) directly between the RNC and the S-GW over an S12
interface, using a Create Bearer Response or Modify Bearer Request towards the
S-GW.
Figure 2. Direct
Tunneling - LTE Network, S12 Interface
A major consequence
of deploying a direct tunnel is that it produces a significant increase in
control plane load on both the SGSN/S-GW and GGSN/P-GW components of the packet
core. Hence, deployment requires highly scalable GGSNs/P-GWs since the volume
and frequency of Update PDP Context messages to the GGSN/P-GW will increase
substantially. The SGSN/S-GW platform capabilities ensure control plane
capacity will not be a limiting factor with direct tunnel deployment.