the receiving side knows how many cells comprise a packet because the sending AAL5 uses the low-order bit of the "PAYLOAD TYPE" field of the ATM cell header to mark the final cell in a packet. This final cell header can be thought of as an "end-to-end bit". Thus, the receiving AAL5 collects incoming cells until it finds one with an end-of-packet bit set. ATM standards use the term "convergence" to describe mechanisms that recognize the end of a packet. Although AAL5 uses a single bit in the cell header for convergence, other ATM adaptation layer protocols are free to use other convergence mechanisms. Packet type and multiplexing The AAL5 trailer does not include a type field. Thus, an AAL5 frame does not identify its content. This means that either the two hosts at the ends of a virtual circuit must agree a priori that the circuit will be used for one specific protocol (e.g. the circuit will only be used to send IP datagrams), or the two hosts at the ends of a virtual circuit must agree a priori that some octets of the data area will be reserved for use as a type field to distinguish packets containing one protocol's data from packets containing another protocol's data. , Multiprotocol Encapsulation over ATM, describes two encapsulation mechanisms for network traffic, one of which implements the former scheme and one of which implements the latter scheme. The former scheme, in which the hosts agree on the high-level protocol for a given circuit, is referred to in RFC 2684 as "VC Multiplexing". It has the advantage of not requiring additional information in a packet, which minimises the overhead. For example, if the hosts agree to transfer IP, a sender can pass each datagram directly to AAL5 to transfer, nothing needs to be sent besides the datagram and the AAL5 trailer. The chief disadvantage of such a scheme lies in duplication of virtual circuits: a host must create a separate virtual circuit for each high-level protocol if more than one protocol is used. Because most carriers charge for each virtual circuit, customers try to avoid using multiple circuits because it adds unnecessary cost. The latter scheme, in which the hosts use a single virtual circuit for multiple protocols, is referred to in RFC 2684 as "LLC Encapsulation". The standards suggest that hosts should use a standard IEEE 802.2 Logical Link Control (LLC)