Types of VTs
There are several option for payload are actually mapped into the VT. Locked mode VTs bypass the pointers with a fixed byte oriented mapping of limited flexibility. Floating mode mapping use the pointers to allow the payload to float within the VT payload. There are three different floating mode mappings asynchronous bit synchronous and bytes synchronous.
To accommodate mixes of different VT types within an STS ISPE the VTs are grouped together. An SPE can carry a mix of any of the seven groups. They have no overhead or pointer they are just a means of organizing the different VT with in an STS -1 SPE. Because each of the VT group is allocated 12 columns of the SPE a VT group would contain one the of the followings combinations:
a.Four VT ( with 3 columns per VT 1.05)
b.Three VT ( with 4 columns per VT2)
c.Two VT 3 ( with 6 columns per VT3)
d.One VT 6( with 12 columns per VT6)
e.VT 1.5 this virtual tributary canaries one DSI signal which in itself carriers 24 DSO signals at 64 kbps each. VT 1.5 is contained in three columns that is it takes up 27 bytes. Four VT can be transported in signal VTG.
f.VT2 this virtual tributary carries an E1 signal of 2.048 mbps. Which consists of thirty two 8 bit time slots of which thirty raw used for voice and the remaining two are used for synchronization and control. VT2 is contained into four columns that is it takes up 36 bytes. Three VT2 can be carried in a single VTG.
g.VT3. This virtual tributary transport the un channelized version of the DSI signal. Where the time slot boundaries are ignored by the sending and receiving equipment. All 192 bits are used to transport data followed by the 193 framing bit. It is also possible to use the entire frame ( including the framing bit) in an un channelized manner. The un channelized version is known as the concatenated channel. It is indicated by the capital c ( as opposed to the lowercase c used in SONET . VT3. Is contained in six columns that is it takes up 54 bytes. This means that a VTG can carry two VT3s.
h.VT6: this virtual tributary transport a DS2 signal which carries 96 voice channels. VT6 is contained in twelve columns that is it takes up 108 bytes. AVTG can carry exactly one VT6.