z-wave MAC layer | z-wave MAC frame types

This page on z-wave MAC layer describes different z-wave MAC frame types used in a zwave network.

Following are the functions performed at z-wave MAC layer :

• unique network ID number (HomeID)
• up to 232 nodes in one network
• collision avoidance algorithm
• backoff algorithm
• automatic retransmission for reliable data transfer
• support for low-power operation via dedicated wakeup patterns.

z-wave MAC frame

The MAC layer has a collision avoidance mechanism that prevents nodes from starting to transmit while other nodes are transmitting. The collision avoidance is achieved by letting nodes be in receive mode when they are not transmitting, and then delay a transmit if the MAC layer is currently in the data phase in the receiver. The collision avoidance is active on all types of nodes when they have the radio activated.

The MAC layer is independent of the RF media, frequency and modulation method but the MAC layer requires either access to the frame data when received or to the whole signal in binary form either as an decoded bit stream or to the Manchester coded bit stream.

The transmission of the frame is delayed a random number of milliseconds. The data stream is Manchester coded and consists of a preamble, start of frame (SOF), frame data and an end of frame (EOF) symbol. The frame data is the part of the frame that is obtained from transport layer at the transmitter and given to the tranport layer at the receiver. Let us understand the z-wave MAC frame. The z-wave MAC layer uses frame formats based on channel configuration i.e. 1/2 and 3. The figure-1 depicts generic MAC frame format and multicast frame format for channel configuration 1 and 2. For channel configuration 3, 'sequence number' (1 byte) field is added after the 'length' field.

Channel configuration-1 supports 1 channel(Ch.B), channel configuration-2 supports 2 channels(Ch.A and Ch.B) and channel configuration-3 supports 3 channels(Ch.A, Ch.B, Ch.C).

Each MAC frame (MPDU) consists of MHR, MAC payload and MFR.
• MHR consists of addresses, frame control and length information
• MAC payload contains data as per frame type. Ack frames do not have payload field.
• MFR contains a FCS(Frame Check Sequence).

This field is 4 bytes in length. Z-wave HomeID specifies unique network identifier. All nodes in a z-wave network have the same HomeID. It is assigned by a primary node during inclusion.

Source Node ID:
This field is an 8 bit unique identifier of a node. Along with HomeID, NodeID identifies the node of the originated frame.

Frame Control:
This field is 16 bits in length. This frame control field contains information defining the frame type, addressing fields and other control flags.

Header type subfield:
This field defines different frame type i.e. single cast, multicast, ACK, routed frame etc.

It is 1 byte size and indicates length of the whole MPDU in bytes. A receiving node does not accept more bytes than the maximum length allowed for the actual data rate.

Sequence Number:
This field is a number provided by higher layers when transmitting. The valid range is 0x00 to 0xFF.

Destination Node ID:
The destination NodeID is used to address individual nodes.
0x00 - (Uninitialized NodeID)
0x01 - 0xE8 (NodeID) 0xE9 - 0xFE (Reserved) 0xFF -(Broadcast NodeID)

Data Payload:
This field is variable size in length. It contains information specific to individual frames. An acknowledgment frame do not have this field.

An 8-bit frame checksum is used for checking frame correctness for R1 and R2 data rates at the receiver. This error detection technique will help in finding the erroneous frame and hence will initiate retransmission in the z-wave network.

Multicast frames:
Multicast frames carry a destination bit map

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Other Standard MAC layer protocol

802.11ac PHY Frame Structure
Zigbee physical layer Frame Structure
Zigbee MAC layer Frame Structure