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.