VLAN Identifier

Description

VID, or VLAN Identifier, is a 12-bit identifier defined in IEEE 802.1Q standards and adopted in 3GPP specifications for 5G networks. It is used to tag Ethernet frames with a VLAN membership, allowing multiple logical networks to coexist on the same physical infrastructure. In the context of 3GPP, VID is employed in the user plane and control plane interfaces, such as N3, N6, and N9, to segregate traffic belonging to different network slices, QoS flows, or user sessions. This enables efficient network resource utilization, enhanced security, and simplified management by isolating traffic streams based on service requirements or administrative domains.

Architecturally, VID operates within the Ethernet frame header, specifically in the 802.1Q tag, which includes a 12-bit VID field and a 3-bit Priority Code Point (PCP) for QoS. When a packet is transmitted over an Ethernet interface in a 5G network, the VID is assigned based on policies defined by the network slice selection function or the session management function. For example, in the 5G Core (5GC), the User Plane Function (UPF) may apply VLAN tags to packets destined for different Data Network Names (DNNs) or network slice instances. The VID allows switches and routers in the transport network to forward frames only to ports that are members of the corresponding VLAN, effectively creating isolated broadcast domains and preventing unauthorized cross-traffic.

The mechanism of VID assignment and processing involves several network functions. During session establishment, the Session Management Function (SMF) may configure the UPF with rules for VLAN tagging, including the VID values to use for specific PDU sessions or QoS flows. These rules are enforced by the UPF when handling user plane traffic. Additionally, the Network Exposure Function (NEF) or Policy Control Function (PCF) may influence VID assignment based on subscription profiles or service requirements. On the receiving end, network elements like the gNB or other UPFs use the VID to identify the appropriate processing context, such as mapping to a specific network slice or applying QoS policies. This ensures that traffic is correctly routed and managed according to the slice's characteristics, such as latency, bandwidth, or security levels.

In 5G networks, VID plays a crucial role in supporting network slicing, which is a key feature for enabling diverse services like enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communications (URLLC), and massive IoT. By using VLANs, operators can create logical networks that are isolated from each other, providing dedicated resources and tailored performance for each slice. This isolation is essential for meeting the stringent requirements of different services, such as ensuring low latency for autonomous vehicles or high bandwidth for video streaming. Moreover, VID facilitates traffic engineering and simplifies network operations by allowing centralized management of slice-specific policies through standardized interfaces like N4 between the SMF and UPF.

Purpose & Motivation

The adoption of VLAN Identifier in 3GPP specifications, starting from Release 15, was motivated by the need for efficient traffic segregation and network slicing in 5G systems. Prior to 5G, mobile networks primarily used GTP tunnels for user plane separation, which added overhead and complexity in the transport layer. With the transition to cloud-native architectures and Ethernet-based fronthaul/midhaul/backhaul, there was a demand for lighter-weight mechanisms to isolate traffic for different services or tenants. VID provides a standardized way to leverage existing Ethernet VLAN capabilities, which are widely supported in networking equipment, to achieve this isolation without introducing proprietary protocols.

Historically, VLANs have been used in enterprise and data center networks for decades to segment broadcast domains and improve security. In the context of 5G, the introduction of VID addresses the limitations of previous mobile core networks where traffic differentiation was often handled at the IP layer or through dedicated tunnels, which could be inefficient for large-scale deployments with diverse service requirements. By incorporating VID into 3GPP standards, operators can reuse their existing Ethernet infrastructure and skills, reducing deployment costs and accelerating the rollout of 5G services. This also aligns with the trend towards convergence of mobile and fixed networks, where Ethernet is a common transport technology.

Furthermore, VID solves the problem of scalable network slicing by providing a simple yet effective method for identifying and isolating slice-specific traffic. In 4G and earlier networks, creating separate logical networks often required complex configurations of multiple GTP tunnels or virtual routing instances. With VID, a single physical interface can carry traffic for multiple slices, each identified by a unique VID, simplifying the network design and management. This is particularly important for 5G use cases like network-as-a-service, where operators need to offer customized slices to different vertical industries. The use of VID also enhances security by preventing traffic leakage between slices, which is critical for applications with strict confidentiality requirements, such as healthcare or financial services.