Vowifi – Voice over Wifi

The voice of wifi is a necessity that arose from the growing trend of WFH during COVID-19. People were confined to four walls at the time, and cellular connections were sometimes spotty, particularly in high-rise apartments along with increasing congestion over cellular path.

In all fairness Wi-Fi finally becomes inevitable change back then and even now it still continues strong sustaining stiff competition from cellular technologies like 5g during its commercial availability across pan india. There is another problem solved with the raise of vowifi is offloading traffic from cellular. currently 5G may have lot of bandwidth but back then in 4g where maximum available bandwidth not more than 20MHz. so user once back to home or office will be offloaded unknowingly to wifi which helps to free cellular channels.

The architecture shown below. the highlighted components are involved the path for enabling voice over wifi. The common element between both transports is PGW where this provides differentiates the path by providing different IP addresses. This also involves in corodinating/triggering the handover when device tries to move from one transport to other.

IPSEC connection:

IPSEC (Internet Protocol security) connection establishment between UE and ePDG. any IP packet enters into IP network needs to go through certain security protocols to protect them from any middle man attacks. so these packets will be authenticated and encrypted properly before entering into data stream. after ePDG packets go through IP networks through GTP tunnel towards PGW.

Finding ePDG address:

UE tries to construct a tunnel between itself and ePDG(evolved packet domain gateway) but sending a FQDN query to DNS server to resolve by constructing query as per 3GPP it takes the form mcc.mnc.3gppnetwork.org. This helps by find working epdg addresses. it can be either an IPv4 or IPv6 based on the query from device capability, if it mentions type A or AAAA where former for v4 and later for v6 address. if device has capability for both IPv4v6 then device send both its capabilities to the network DNS.

FQDN query epdg.epc.mncXXX.mccXXX.pub.3gppnetwork.org.: IPv4 or IPv6 address. protocol used between UE and ePDG connection is IKE internet key exchange.

Once the tunnel is established successfully device will sent IMS register request message which is similar like it does in cellular network and gets 401 challenge and device replies with nonce and gets 200ok to say successful registration. from there device icon changes like displayed below. at this stage both MO and MT calls will flow through wifi.

VoWIFI registration can be identified through SIP signalling. In SIP invite and its subsequent transactions indicate the transport, here in this case it shows i-WLAN unlike EUTRAN in 4G volte and NR in 5G vonr call.

SIP invite
P-Access-Network-Info: IEEE-802.11;i-wlan-node-id=ffffffffffff

Mobility between WIFI to Cellular and vice versa:

wifi to wifi handover happens through process called Reassociation.During these handovers UE gets assigned with two IP addresses, one from local WIFI AP and other one from cellular network particularly PGW component. local will tend to change depending on different wifis device switch in between whereas network IP will not change.

Below are the possible handovers:

1. Wifi to Wifi
2. wifi to cellular and vice versa.

WIFI to cellular and cellular Handover where network IP will be static and only local IP will be changed.

Refercence:

3gpp 23.402 and 3gpp 23.203