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Nv2 protocol is a proprietary wireless protocol developed by MikroTik for use with Atheros 802.11 wireless chips. Nv2 is based on TDMA (Time Division Multiple Access) media access technology instead of CSMA (Carrier Sense Multiple Access) media access technology used in regular 802.11 devices.

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Media access in Nv2 network is controlled by Nv2 Access Point. Nv2 AP divides time in into fixed size "periods" which are dynamically divided in into downlink (data sent from AP to clients) and uplink (data sent from clients to AP) portions, based on the queue state on AP and clients. Uplink time is further divided between connected clients based on their requirements for bandwidth. At the begginning beginning of each period, AP broadcasts a schedule that tells clients when they should transmit and the amount of time they can use.

In order to allow new clients to connect, Nv2 AP periodically assigns uplink time for "unspecified" client - this time interval is then used by a fresh client to initiate registration to AP. Then AP estimates propagation delay between AP and client and starts periodically scheduling uplink time for this client in order to complete registration and receive data from client.

Nv2 implements dynamic rate selection on a per-client basis and ARQ for data transmissions. This enables reliable communications across Nv2 links.

For QoS Nv2 implements variable number of priority queues with builtin built-in default QoS scheduler that can be accompanied with fine-grained QoS policy based on firewall rules or priority information propagated across network using VLAN priority or MPLS EXP bits.

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Nv2 protocol implementation status

As of version 5.0rc1 Nv2 has the following features:

• TDMA media access
• WDS support
• QoS support with variable number or priority queues

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• data encryption

As of version 5.0rc3:

• RADIUS authentication features

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• added missing statistics fields

As of version v6.40:

• Fixed Downlink mode support
• Uplink/Downlink ratio support
• Nv2 AP synchronization experimental support

Features that Nv2 DOES NOT HAVE YET:

• administrator controlled media access policy
• some other features...

Compatibility and coexistence with other wireless protocols

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• Media access is scheduled by AP - this eliminates hidden node problem and allows to implement centralized media access policy - AP controls how much time is used by every client and can assign time to clients according to some policy instead of every device contending for media access.
• Reduced propagation delay overhead - There are no per-frame ACKs in Nv2 - this significantly improves throughput, especially on long-distance links where data frame and following ACK frame propagation delay significantly reduces the effectiveness of media usage.
• Reduced per frame overhead - Nv2 implements frame aggregation and fragmentation to maximize assigned media usage and reduce per-frame overhead (interframe spaces, preambles).

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• Reduced polling overhead - instead of polling each client, Nv2 AP broadcasts an uplink schedule that assigns time to multiple clients, this can be considered "group polling" - no time is wasted for polling each client individually, leaving more time for actual data transmission. This improves throughput, especially in PtMP configurations.
• Reduced propagation delay overhead - Nv2 must not poll each client individually, this allows to create uplink schedule based on estimated distance (propagation delay) to clients such that media usage is most effective. This improves throughput, especially in PtMP configurations.
• More control over latency - reduced overhead, adjustable period size and QoS features allows for more control over latency in the network.

Configuring Nv2

As of version 5.0rc1 new wireless interface setting wireless-protocol has been introduced. This setting controls which wireless protocol selects and uses. Note that the meaning of this setting depends on the interface role (either it is AP or client) that depends on interface mode setting. Find possible values of wireless-protocol and their meaning in table below.

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• For Nv2 Synchronization a Master Nv2 AP should be chosen and "nv2-mode=sync-master" should be specified together with "nv2-sync-secret".
• For Nv2 Slave APs the same wireless frequency as Master AP should be used and "nv2-mode=sync-slave" should be specified with the same "nv2-sync-secret" as the in Master AP configuration.
• When Master AP is enabled Slave APs will try start searching for Master AP by matching it against specified "nv2-sync-secret".
• After Master AP is found the Slave AP will calculate the distance to the Master AP as it is possible that Master AP is located not on the same location.
• Then Slave AP starts operating as AP and it adapts the period size and downlink ratio from the synced Master AP.
• In addition after the Slave AP is operational other Slave APs can use this Slave AP to sync with.
• Slave AP periodically listens for the Master AP and checks if the "nv2-sync-secret" still matches and adapts the parameters again. If Master AP interface is disabled/enabled all the Slaves will be also disabled and will start the synchronization process from the beginning.
• If Master AP stops working Slave APs also will stop working as they do not have sync information.

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