wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

51-8 Industrial Communication Systems
Frag type Frag header Following headers/payload
First fragment
1 1 0 0 0
datagram_size/11
Subsequent fragment
1 1 1 0 0
datagram_tag/16
...
...
datagram_size/11
datagram_tag/16
datagram_offset/8
FIGURE 51.5
Fragmentation header structure.
dispatch field, which specifies the frame type and what specific frame type format follows this dispatch
field. Four groups are defined and identified by the first two bits of the dispatch field: Not a LoWPAN
frame (NALP), mesh frame (MESH), first fragmentation frame (FRAG1), and subsequent fragmentation
frame (FRAGN).
If the overall packet size exceeds the limit of the MAC layer, the fragmentation frame as depicted in Figure
51.5 has to be applied. Because the FRAG frame header is located at layer 2.5 between link layer and IP, the
FRAG header encloses the IP frame. The IP header is included in the first fragment only and interpreted
on the destination node after reassembling the IPv6 packet and not on intermediate nodes. A maximum
2.048 bytes can be transmitted within one IP packet. Packets with a bigger size have to be fragmented on IP
layer or above. The MTU of IPv6 requires only 1280 bytes. Different existing IPv6 stacks for computer and
server platforms autonomously fragment larger frames independent of the minimum available MTU on
the route to the destination to avoid the need of resending packets. This can reduce communication latency
and memory requirements because it is assured that the frame size complies with all link layer technologies
on the route to the destination. It might be applicable not to exceed the MTU of 1280 bytes per frame in
6LoWPANs also and realize a fragmentation on IP layer or above. The complete fragmentation frame header
size differs between 32 bit for the first fragment and 40 bit for subsequent fragments. In accordance to the
IPv6 reassembly timeout, a receiver of a fragmented packet must discard received fragments if the packet
could not be reassembled within 60.s after the first fragment was received. The 802.15.4 specification defines
a beacon-enabled mode, where RFDs only wake up in defined time slots to receive a beacon from their coordinator.
This beacon contains information if data are pending to be received by the endpoint. The beacons
are transmitted in periodical time slots between 15.ms and 252.s. If beacon-enabled mode is deployed on the
802.15.4 layers, the beacon distance has to fit—particularly in multi-hop scenarios depending on the network
topology—to ensure a maximum delay of 60.s between the endpoints.
51.5.3 Mesh Frame Type
For mesh network topologies (named peer-to-peer in IEEE 802.15.4) and multi-hop scenarios, an additional
mesh frame header is applied that encloses all other frame types including FRAG type, which in
turn encloses the IP frame (cf. Figure 51.6). The additionally required mesh header includes three parts. The
first part defines the format of the addresses for originator and final interface, which can be either the 16 bit
short addresses or 64 bit EUI-64 addresses format. The second part is the Hops Left field that defines after
how many hops the packet can be discarded by the forwarding node to avoid routing loops and can have a
maximum value of 256. The third field includes the originator and final address in the format specified by
the first field. Depending on the address format, the complete MESH frame header has a size of 5–17 bytes.
802.15.4 MAC/PHY
Includes information
elided in upper layers
MESH
Interpreted on each
intermediate node
FRAG
For reassembling on
destination node
IP
Interpreted on
destination node
...
802.15.4 MAC/PHY
FIGURE 51.6
Frame formats and order.
© 2011 by Taylor and Francis Group, LLC