Image Processing Reference
In-Depth Information
18.5.1 Transport Layer (ISO/DIS 15765-2.2)
The ISO transport layer [] provides USDT services to higher software layers (e.g., ISO diagnos-
tics [see Section ..]) by facilitating the transmission of messages, whose length is greater than
the MTU of the underlying communication system. On the sender's side, the transport layer will
split such long messages into multiple segments, each small enough for the underlying communi-
cation system. On the receiver's side, the transport layer reassembles these segments again. As far
as the frame format is concerned, the ISO transport layer distinguishes between single frames (SF),
first frames (FF), consecutive frames (CF), and flow control frames (FC). Hereby, SFs are used for
reduction of protocol overhead in case the amount of data to be transmitted does not exceed the
MTU. In that case, only an SF is used to exchange this data. Otherwise an FF, which contains the
total number of data bytes to be sent as well as the first few data bytes, is transmitted followed by
one or more CFs, which then contain the remaining data bytes. For flow control reasons (i.e., to pre-
vent the sender from outpacing the receiver), the receiver is allowed to send FCs at defined points in
time. Figure . illustrates the frame exchange in an unsegmented (Figure .a) and a segmented
transmission (Figure .b).
Hereby, the frame format depicted in Figure . is used by the ISO transport layer. The source
and “destination address” fields are used to identify the sender and the receiver(s) of the TP frame.
The “address type” field is used to define the semantics of the destination address. Here, ISO TP pro-
vides support for single ECU addresses (called “physical addresses” by the ISO TP specification) and
multicast addresses (called “functional addresses” by the ISO TP specification). In case of multicast
transmissions, only unsegmented data transfer is allowed in ISO TP.
Sender
Receiver
FF
Flow Control Clear to Send (FC_CTS)
CF
CF
CF
CF
CF
Flow Control Clear to Send (FC_CTS)
Receiver
Sender
CF
SF
CF
(a) Unsegmented
(b) Segmented
FIGURE .
ISO transport layer—transmission sequence.
