> Direct Tunnel
> This chapter briefly describes the 3G UMTS direct tunnel feature, indicates how it is implemented on various systems (for example, the Serving GPRS Support Node (SGSN), the Gateway GPRS Support Node (GGSN), and the Home NodeB Gateway (HNB-GW), and provides feature configuration procedures. Direct tunnel is an enhanced feature, so some products may require a feature implementation license and all relevant products will require completion of basic service configuration. Refer to your product’s administration guide for feature licensing and basic service configuration information.
> Currently, the SGSN is the only product that enables configuration of this feature. All other products that support direct tunnel do so by default. It is the SGSN that determines if setup of a direct tunnel is to be allowed or disallowed.
> After the feature overview description, this chapter provides configuration procedures for the following:

> • | Enabling and Disabling GTP-U Direct Tunnels

> • | Disabling or Enabling DT Access to Specific GGSN(s)

> • | Disabling or Enabling Direct Tunnels to Specific RNC(s)

> Important: This chapter provides a limited instruction set for configuring direct tunnel. Basic service configuration for the SGSN is provided in the SGSN Service Configuration Procedures chapter of the SGSN Administration Guide. Command details are provided in the Command Line Interface Reference.
> Direct Tunnel Feature Overview
> The direct tunnel architecture allows the SGSN to establish a direct user plane tunnel between the radio access network (RAN) and the GGSN. Once a direct tunnel is established the SGSN continues to handle the control plane signaling. This improves the user experience (e.g., expedites web page delivery, reduces round trip delay for conversational services). Additionally, direct tunnel functionality implements the standard SGSN optimization to improve the usage of user plane resources (and hardware) by removing the requirement from the SGSN to handle the user plane processing.
> Typically, the SGSN sets up a direct tunnel at PDP context activation. The SGSN uses an Update PDP Context Request towards the GGSN to establish a GTP user plane (GTP-U) tunnel directly between the RNC and the GGSN. This means a significant increase in control plane load on both the SGSN and GGSN components of the packet core. Hence, deployment requires highly scalable GGSNs since the volume and frequency of Update PDP Context messages to the GGSN will increase substantially. The SGSN’s platform capabilities ensure control plane capacity will not be a limiting factor with direct tunnel deployment.
> GTP-U Direct Tunnel
> The following figure shows the logic used within the SGSN to determine if a direct tunnel can be used.
> Direct Tunnel Activation Logic
> Direct Tunnel Configuration
> There are three aspects to direct tunnel configuration on the SGSN:

> • | enabling or disabling support for direct tunnel functionality on the SGSN

> • | enabling or disabling direct tunnel (DT) access to specific GGSN(s)

> • | enabling or disabling direct tunnel (DT) access to specific radio network controller(s) (RNCs)

> The procedures in the remaining portion of this chapter cover all three of the aspects noted above.
> Enabling and Disabling GTP-U Direct Tunnels
> By default, direct tunnel support is disallowed on the SGSN and allowed on the GGSN. The SGSN’s direct tunnel functionality is enabled within the SGSN operator policy configuration.
> SGSN operator policies specify the rules governing the services, facilities, and privileges available to subscribers depending on factors such as roaming agreements between operators, the subscription restrictions for visiting or roaming subscribers, provisioning of defaults, over-riding of standard behavior.
> One aspect of a policy is to allow or disallow the setup of direct GTP-U tunnels. If no operator policies are configured, the system looks at the settings in the system operator policy named default. If direct tunnel is allowed in the default operator policy, then any incoming call that does not have an applicable operator policy configured will have direct tunnel allowed.
> Enabling a Direct Tunnel
> To enable a direct tunnel in an SGSN operator policy, use the following procedure:

> Step 1 | Enter the global configuration mode.

> Step 2 | Create a new SGSN operator policy or Identify an existing SGSN operator policy.

> Step 3 | Enable direct tunnel functionality.

> Example Configuration
> The following is an example of the commands used to enable direct tunneling on the SGSN:
> config
> sgsn-operator-policy { name <policy_name> | default }
> direct-tunnel attempt-when-permitted
> Disabling a Direct Tunnel
> To disable a direct tunnel in an SGSN operator policy, use the following procedure:

> Step 1 | Enter the global configuration mode.

> Step 2 | Identify the existing SGSN operator policy with the DT enabled.

> Step 3 | Disable the direct tunnel functionality.

> Example Configuration
> The following is an example of the commands used to disable direct tunneling on the SGSN:
> config
> sgsn-operator-policy { name <policy_name> | default }
> remove direct-tunnel
> Disabling or Enabling DT Access to Specific GGSN(s)
> In each SGSN operator policy, SGSN APN policies are configured to connect to a GGSN and to control the direct tunnel (DT) access to that GGSN.
> Multiple SGSN APN policies can be configured per SGSN operator policy.
> By default, DT functionality is allowed in SGSN APN policies.
> Disabling a Direct Tunnel to a GGSN
> To disable a direct tunnel access to a GGSN configured in an SGSN APN policy, use the following procedure:

> Step 1 | Enter the global configuration mode.

> Step 2 | Identify the existing SGSN operator policy with the SGSN APN policy.

> Step 3 | Identify the SGSN APN policy with the GGSN supporting DT.

> Step 4 | Disable the direct tunnel functionality.

> Example Configuration
> The following is an example of the commands used to disable direct tunneling with a GGSN:
> config
> sgsn-operator-policy { name <policy_name> | default }
> apn network-identifier <net_id> | operator-identifier <op_id> }
> direct-tunnel not-permitted-by-ggsn
> Re-enabling a Direct Tunnel to a GGSN
> To re-enable a direct tunnel access to a GGSN configured in an SGSN APN policy, use the following procedure:

> • | Enter the global configuration mode.

> • | Identify the existing SGSN operator policy with the SGSN APN policy.

> • | Identify the SGSN APN policy with the GGSN.

> • | Enable the direct tunnel functionality.

> Example Configuration
> The following is an example of the commands used to re-enable direct tunneling with a GGSN:
> config
> sgsn-operator-policy { name <policy_name> | default }
> apn network-identifier <net_id> | operator-identifier <op_id> }
> default direct-tunnel
> Disabling or Enabling Direct Tunnels to Specific RNC(s)
> SGSN access to radio access controllers (RNCs) is configured in the IuPS service.
> Each IuPS service can include multiple RNC configurations that determine communications and features depending on the RNC.
> By default, direct tunnel functionality is enabled for all RNCs.
> Disabling a Direct Tunnel to an RNC
> To disable direct tunnel access to an RNC, use the following procedure:

> Step 1 | Enter the global configuration mode.

> Step 2 | Identify the context containing the IuPS service configuration for that RNC.

> Step 3 | Identify the IuPS service configuration.

> Step 4 | Identify the RNC configuration

> Step 5 | Disable the direct tunnel access for the RNC.

> Example Configuration
> The following is an example of the commands used to disable direct tunneling with an RNC:
> config
> context <ctx_name>
> iups-service <service_name>
> rnc id <rnc_id>
> direct-tunnel not-permitted-by-rnc
> Re-enabling a Direct Tunnel to an RNC
> To re-enable a direct tunnel access to an RNC configured in an IuPS service configuration, use the following procedure:

> Step 1 | Enter the global configuration mode.

> Step 2 | Identify the context containing the IuPS service configuration for that RNC.

> Step 3 | Identify the IuPS service configuration.

> Step 4 | Identify the RNC configuration

> Step 5 | Re-enable the direct tunnel access for the RNC.

> Example Configuration
> The following is an example of the commands used to re-enable direct tunneling with an RNC:
> config
> context <ctx_name>
> iups-service <service_name>
> rnc id <rnc_id>
> default direct-tunnel

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