[cisco-nas] 96K availainle bandwidth vs 128K BW on Vi1 bundle consisting of 2 B-channels during 64K Unrestrictet call via ISDN-TA PPP-multilink to 5350 NAS

[&y0]

Hi everybody. I have problem while customers places calls to 5350 NAS via
ISDN using ISDN-TA and PPP-multilink. During the call 2 B-channels are bound
to Virtual-access interface which parameters then looks like this:
Router#sh int virtual-access 1
Virtual-Access1 is up, line protocol is up
  Hardware is Virtual Access interface
  MTU 1500 bytes, BW 128 Kbit, DLY 100000 usec,
     reliability 255/255, txload 1/255, rxload 7/255
  Encapsulation PPP, loopback not set
  DTR is pulsed for 5 seconds on reset
  Time to interface disconnect:  idle 3w3d
  LCP Open, multilink Open
  Open: IPCP
  Last input 00:00:00, output never, output hang never
  Last clearing of "show interface" counters 02:28:22
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: weighted fair
  Output queue: 0/1000/64/0 (size/max total/threshold/drops)
     Conversations  0/50/256 (active/max active/max total)
     Reserved Conversations 0/0 (allocated/max allocated)
     Available Bandwidth 96 kilobits/sec
  30 second input rate 4000 bits/sec, 2 packets/sec
  30 second output rate 1000 bits/sec, 1 packets/sec
     32892 packets input, 15179159 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     31164 packets output, 18584822 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions
Router#sy
  Interface      User        Mode                     Idle     Peer Address
  Vi1          fim        Virtual PPP (Bundle) 00:00:06 10.2.2.1
  Se3/0:1      fim        Sync PPP                    -   Bundle: Vi1
  Se3/0:2      fim        Sync PPP                    -   Bundle: Vi1
Accordingly customer can obtain the data transfer speed no more ~88K.
It looks like 16K is spent for unknown reason per each B-channel as
arithmetic of 2*(64K-16K)=96K shows I have ever trusted that signalling
bandwidth should be taken from D-channel bandwidth (in my case
Serial3/0:15).
Am I correct? Have anybody solution for this problem? Below is configuration
of ISDN and D-channel isdn switch-type primary-net5 controller E1 3/0
framing NO-CRC4  pri-group timeslots 1-31 interface Serial3/0:15  ip
unnumbered FastEthernet0/0  ip nat inside  encapsulation ppp  load-interval
30  dialer idle-timeout 2147483  dialer load-threshold 1 outbound
dialer-group 1  autodetect encapsulation ppp v120  isdn switch-type
primary-net5  isdn incoming-voice modem  isdn sending-complete  no cdp
enable  ppp authentication pap contract_tac  ppp authorization contract_tac
ppp multilink  ppp timeout multilink link add 1

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AO618-RIPE
108248909