AAL5 can be used to transport packets from routed or bridged protocols. (In practice, this means connectionless protocols.) Examples of such protocols are IP, IPX (Internet Packet eXchange), but also Ethernet. In RFC 1483 [JH93] two methods are discussed to encapsulate and transmit Connectionless packets, in particular IP packets, over AAL5 Common Part Convergence Sublayer Protocol Data Units (CPCS PDUs).
The first method is called LLC/SNAP
LLC encapsulation and uses a single VCC to
transport packets of different higher layer protocols over the same
VCC. A LLC/SNAP header is used to differentiate between the packets of
different protocols. The other method is called ``null'' encapsulation
or VC multiplexing. With this method, a single
VC is used per protocol (and per destination), so there is no need to
differentiate the packets using a header with a format identifier. This
method does use more connection resources if multiple protocols are
used on the same host.
A protocol entity must be defined to setup and manage the VCCs for connectionless traffic. The logical endpoint of the VCC used with LLC/SNAP encapsulation is at the ``lower end'' of this entity. When each upper protocol has its own VCC when VC multiplexing is used, the logical endpoint is at the interface with the upper protocol entity.
To show how the packets of higher protocol layers are encapsulated in
the lower layer payload or Service Data Unit ( SDU) fields. The data
passed to the lower layer is called the Protocol Data Unit (PDU),
because it contains all the protocol information in the header
necessary for the protocol entity to do its work. In
Figure 
is shown how the PDUs of the upper layers are
passed down as SDUs of the lower layers. In this figure, no signalling
is regarded, just the encapsulation of packets over an already
established VCC is shown.
Figure: Packet encapsulation in the user plane of the protocol stack