Configuring X.25 8-15
Priority
prioritizes this logical port for intra-nodal tr affic; the higher the number, the highe r the
priority. (This priority has no effect on traffic exiting a node.)
Priorities are configured on all logical interfaces that use a physical frame relay port.
The device processor in the node uses these priorities to help determine the order in
which it will process protocols.
When configuring priorities, be careful to consider the types of
traffic being routed on other connections in the node.
Bandwidth Allocation Group
assig ns the logical port to on e of si x te e n g roups whose para met ers regu l at e ban dwidth
usage by outgo ing tr a ffic on the ph ys ical l i nk. S e e "Configuring Ba ndwidth Allo-
cation Groups" on page 7-6.
Encapsulation Method
is the method by which traffic over the logical port will be enveloped within frame
rela y fr ames for tra nsmissi on. T he valid meth ods are T1 .617 A nnex G an d T1.6 17
RFC1490 .
With Annex G, a LAPB frame is encapsulated immediatel y follo wing the frame relay
header (flag, 2-byte T1.618 header, LAPB address, LAPB control, LAPB I-field
[x.25/x.75]). With RFC1490, the order is: T1.618 header, Q.922 control byte, Q.933
NLPID, 2-byte level-2 protocol ID, 2-byte level-3 protocol ID, LAPB frame.
Although it may appear that RFC14 90 encaps ulatio n uses more overh ead, t he number
of protocols being encapsulated are important in determining the method:
●RFC1490 – The protocols will be identified on the DLCI based on their NLPIDs
and (if applicable) level-2 and level-3 headers (e.g., X.25: flag, T1.618 2-byte
header, 0x03 Q.922 control, 0x08 Q.933 NLPID, T1.617 level-2 LAPB PID
0x51 81, T 1.61 7 l e vel-3 PID 0x 67 80, LA PB fram e).
●Annex G – Eac h pr ot ocol must be en ca ps u l a te d i n X. 2 5 fo r tr ans mission over a
singl e DLCI, beca us e Ann e x G allows the DL CI to be used only by X. 25 . E ac h
protoc ol in wo uld req uire it s own X. 25 vir t ua l cir cuit a nd level -3 wind ows.
Modulo
is the number of values used in X.25 level-2 sequenced packets. The actual packet
numbering is 0–7 for modulo 8 and 0–127 for modulo 128.
Maximum LAPB Window Size
is the maximum number of sequentially numbered I-frames that can be waiting for
acknowledgment by the next statio n in the path . If this number is exceeded , no frames
will be transmitted until an acknowledgment is received. A larger value allows faster
throughput, but increases the number of frames that may have to be retransmitted if
there is a problem.
N2 Retransmit Count
is the maximum number of times the node will attempt to send an I-frame after a
Retransmit Period expiration. A larger value for this parameter increases the proba-
bility of an eventual correct transfer between DTE and DCE, but a smaller value
permits faster detection of a permanent error condition.
(conti nued)