Which reference point is used between the UTRAN and the Serving Gateway for user plane tunneling when a direct tunnel is established?

Question

Options

• AS4
• BS12
• CS8a
• DS5

• B
  89% (17)
• C
  5% (1)
• D
  5% (1)

Explanation

S12 is the defined 3GPP reference point between the UTRAN (RNC) and the Serving Gateway specifically for direct user plane tunneling, allowing data to bypass the SGSN entirely and flow straight from the radio access network to the core - this is the essence of the "direct tunnel" optimization in 3G/4G interworking.

S4 is wrong because it connects the SGSN to the SGW (control and user plane without a direct tunnel), meaning it still routes through the SGSN rather than bypassing it. S5 is the reference point between the SGW and the PDN Gateway within the same PLMN, which is a completely different leg of the EPC path. S8a is a variant of S8, the roaming interface between a visited SGW and the home PDN Gateway - unrelated to UTRAN connectivity.

Memory tip: Think "S12 = direct shortcut" - the number 12 skips over the SGSN (like jumping ahead), going straight from the RNC to the SGW. Any "S-number" in the single digits (S4, S5, S8) lives entirely within the core, not at the radio edge.

No community discussion yet for this question.