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dash7_mode_2:network

Network Layer

The DASH7 Mode 2 Network Layer is responsible for the following functions:

  • Background Frame processing
  • Datastream forwarding
  • Unicast/Broadcast Addressing
  • Routing, if applicable
  • Network Layer Security

Background Frame Processing

Background frames are fixed-length, seven byte (56 bit) frames with no addressing and only subnet filtering. They are used for broadcast flooding of notification data. There are currently two background protocols defined by DASH7 Mode 2, and only one is currently implemented (M2AdvP).

Background Protocol Frame and Data Fields

When transferring frames to or from the Data Link Layer, the Background Protocols perform some frame reconfiguration in order to keep the data representation as compact as possible for lower layers and as consistent as possible for upper layers.

Data Link Layer to Background Network Protocol:

  1. Remove CRC16 field.
  2. Insert 1 byte Length field to the front of the frame, containing value 6.
  3. Insert 1 byte Link Control after the Length field, containing value 0.

Background Network Protocol to Data Link Layer:

  1. Remove Length field and TX EIRP field from front of frame.
Length Link Control TX EIRP Subnet Protocol Data
Not in transmitted frame 1 byte 1 byte 3 bytes
6 0 0-255 includes BPID

The BPID is a 4 bit identifier that is overlayed with the lower 4 bits of the Subnet field.

List of Background Protocols

BPID Name Description
15 Advertising Protocol Used for Ad-hoc group synchronization
3 Reservation Protocol A mechanism for advanced CSMA-CA methods

Background Advertising Protocol

The Mode 2 Advertising Protocol (M2AdvP) is a broadcast, simplex protocol used exclusively for rapid, ad-hoc group synchronization. This is a process designed to feed information about a request, that will occur in the future, to Hosts engaged in periodic Channel Scan Series.

The M2AdvP BPID is 15 (0xF), which is explicitly combined with the Subnet Mask of the background frame. Therefore, the Subnet Mask of transmitted M2AdvP frames shall be always 15 (0xF).

Advertising Protocol Payload (28 bits)
BPID Channel ID ETA
4 bits 8 bits 16 bits
Explicit, and combined with Subnet Mask Channel ID on which request will occur ticks until request, +/- 2, in big endian

Background Reservation Protocol

The Reservation Protocol (M2ResP) is not utilized by any current component of the Mode 2 specification. It is a broadcast, simplex protocol that may be used to reserve channel time, as part of a CSMA-CA process. One way to use the reservation protocol is for hosts to evaluate incoming duration values to prioritize channel access as part of an advanced CSMA-CA process. Another way is to jam the channel with a flood of packets until other Hosts back-off.

The M2ResP BPID is 3 (0x3), which is explicitly combined with the Subnet Mask of the background frame. Therefore, the Subnet Mask of transmitted M2ResP frames shall be always 3 (0x3).

Background Packet Train

The Advertising Protocol is used most productively when it floods packets onto the network during the moments before the advertised request occurs. Other Background Protocols may work in the same way.

[Diagram todo]

Foreground Frame Processing

Datastream Forwarding

Datastreams are conducted via M2DP protocol frames (Mode 2 Datastream Protocol). A Datastream frame is a Foreground Frame (a normal, variable length frame) designed for maximum payload utilization. Therefore, it has no addressing or routing fields, and handshaking must be conducted at higher levels (in practice this is done via certain M2QP features). Received datastream payloads are forwarded blindly to higher layers, which can decide what to do with them, or what to do with the dialog.

Unicast/Broadcast Addressing

Network Protocols like M2NP are enabled to deal with broadcast and unicast addressing. More sophisticated addressing methods (anycast, multicast) are interpreted by the Network Layer as broadcast methods, and the transport layer (typ M2QP) is responsible for managing the queries that control these sophisticated addressing methods.

Addressing Data Fields

[Table]

Routing

Network Protocols like M2NP are enabled to deal with multi-hop routing. Unicast routing is available, and Broadcast routing is performed through the Anycast addressing method. See the M2NP article for information on how M2NP manages routing. No other Mode 2 network protocols support routing at this time.

Network Layer Security

DASH7 Mode 2 Network Layer Security (NLS) is supported by M2NP, or other future protocols that utilize the addressing fields (enabled via the data link layer). The design of Mode 2 NLS as implemented by M2NP is quite similar to the data field design of IPsec (see M2NP article for more information).

dash7_mode_2/network.txt · Last modified: 2014/01/27 04:38 by jpnorair