The full form of VCL is Video Coding Layer

The VCL is specified to efficiently represent the content of the video data. The NAL is specified to format that data and provide header information in a manner appropriate for conveyance on a variety of communication channels or storage media. All data are contained in NAL units, each of which contains an integer number of bytes. A NAL unit specifies a generic format for use in both packet-oriented and bitstream systems. The format of NAL units for both packet-oriented transport and byte stream is identical except that each NAL unit can be preceded by a start code prefix and extra padding bytes in the byte stream format.

To elaborate a little bit, VCL is to do more with video compression standard (not really to do with Network layer). Hence it doesn’t fall under any network protocol stack. NAL comes to aid network routers at times of congestion. To achieve better decoding at receiver end, critical packets shouldn’t be dropped. So, NAL gives a provision for sneak peak to understand importance of a given packet in decode process and helps to make an appropriate decision (either to drop a packet or not). It’s just one simple example of NAL.

VCL understanding is required if more attention needed at decoding syntax. NAL provides one layer of protection around VCL. VCL contains compressed video data in terms of slice. One video frame can be split into one or more slices. Slice contains one or more Macro blocks (16×16 pixel block) in compressed form, following the standard syntax.