Intel ER8100STUS - Express 8100 Router, Справочное руководство

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Reference.bok : Chapter05 36 Wed Apr 22 10:58:08 1998
36
X.25 Packets and Virtual Calls
X.25 Services
4. The remote DCE forwards the call Accepted Packet across the PSDN to the
local DCE which then passes it to the calling DTE. Again, only the Logical
Channel Identifier from the local DCE to the calling DTE is included in the
packet.
Window size
The window is the number of packets that may be transmitted without receiving
acknowledgment.
As X.25 is based upon LAPB which is resilient to packet loss, the window size
is used for controlling the flow of packets.
Packet and window sizes For PVCs, the packet and window sizes used when communicating across the
X.25 network are fixed (static) and are defined by the X.25 service provider (typ-
ically packet size of 128 bytes and a window size of 2 packets).
For SVCs, some X.25 networks allow negotiation of the packet and window sizes
between the DTE and the DCE. The packet and window sizes used are arrived
upon using an algorithm defined by the X.25 standard. Parameter negotiations
can give more efficient data transfers but be aware that some X.25 network pro-
viders charge extra for the negotiation services.
For SVCs across X.25 networks or subscriptions which do not allow negotiation,
the packet and window sizes are fixed (static) and are defined by the X.25 service
provider (typically 128 bytes, 2 packets).
Note
The packet and window sizes are only used between a DTE and the
DCE. The packet and window size used at the remote DTE and DCE
can be different.
1236
X.25 X.25
Logical Channel
Packet Path
Packet Switched
Data Network
(PSDN)
n
.
.
4
3
2
1
Calling
DTE
DCE DCE
Call
Connected
Call
Accepted
1
2
3
4
.
.
n
Called
DTE