X.25 and LAPB Protocols Overview
197
Once a virtual circuit is established between a pair of DTEs, it is assigned with a
unique virtual circuit number. When one DTE is to send a packet to the other, it
numbers this packet (with virtual circuit number) and sends it to DCE. According
to the number on the packet, DCE determines the method to switch this packet
within the switching network, so that this packet can reach the destination. Since
the X.25 layer 3 multiplexes the link established between DTE and DCE by the
X.25 layer 2 (LAPB), what finally viewed by the user will be multiple usable virtual
circuits.
The relation between packets and frames in various X.25 layers is shown in the
following diagram.
Figure 60
X.25 packet and LAPB frame
X.25 link layer specifies the frame switching process between DTE and DCE. In
terms of hierarchy, the link layer seems to bridge the packet layer interface of DTE
and that of DCE. Through this bridge, the packets can be transmitted continuously
between the packet layer of DTE and that of DCE. The link layer has such main
functions as follows:
■
Transmit the data effectively between DTE and DCE
■
Ensure the synchronization of information between the receiver and
transmitter
■
Detect and correct the error in the transmission
■
Identify and report the procedure error to the higher layer protocol
■
Inform the packet layer of the link layer state
As specified in international standards, X.25 link layer protocol LAPB adopts the
frame structure of high-level data link control (HDLC) and the frame structure is a
subset of LAPB. The bi-directional link will be established when either site sends an
SABM (Set Asynchronous Balanced mode) command and the other replies with
UA.
Defined as X.25 layer-2 protocol, LAPB is actually a separate link layer protocol,
which can transmit the data with LAPB bearing non-X.25 upper layer protocol.
3Com Router
series can configure the link protocol of serial interface to LAPB and
perform simple local data transmission. Meanwhile, X.25 of 3Com Router series
has switching function, that is to say, the router can be used as a small X.25
packet switch. The following diagram shows the relations among LAPB, X.25 and
X.25 switching.
Packet
header
User data
Data
Frame
header
FCS
Frame
delimiter
Bit stream
X.25 layer 3 Packet
X.25 layer 2 Frame
X.25 layer 1
Frame
delimiter
Summary of Contents for 3036
Page 1: ...http www 3com com 3Com Router Configuration Guide Published March 2004 Part No 10014299 ...
Page 4: ...VPN 615 RELIABILITY 665 QOS 681 DIAL UP 721 ...
Page 6: ...2 ABOUT THIS GUIDE ...
Page 7: ...I GETTING STARTED Chapter 1 3Com Router Introduction Chapter 2 3Com Router User Interface ...
Page 8: ...4 ...
Page 16: ...12 CHAPTER 1 3COM ROUTER INTRODUCTION ...
Page 34: ...30 ...
Page 60: ...56 CHAPTER 3 SYSTEM MANAGEMENT ...
Page 98: ...94 CHAPTER 6 DISPLAY AND DEBUGGING TOOLS ...
Page 110: ...106 ...
Page 114: ...110 CHAPTER 8 INTERFACE CONFIGURATION OVERVIEW ...
Page 158: ...154 CHAPTER 10 CONFIGURING WAN INTERFACE ...
Page 168: ...164 ...
Page 188: ...184 CHAPTER 13 CONFIGURING PPPOE CLIENT ...
Page 192: ...188 CHAPTER 14 CONFIGURING SLIP Router ip route static 0 0 0 0 0 0 0 0 10 110 0 1 ...
Page 248: ...244 CHAPTER 16 CONFIGURING LAPB AND X 25 ...
Page 320: ...316 ...
Page 330: ...326 CHAPTER 20 CONFIGURING IP ADDRESS ...
Page 362: ...358 CHAPTER 21 CONFIGURING IP APPLICATION ...
Page 374: ...370 CHAPTER 23 CONFIGURING IP COUNT ...
Page 406: ...402 CHAPTER 25 CONFIGURING DLSW ...
Page 408: ...404 ...
Page 452: ...448 CHAPTER 29 CONFIGURING OSPF ...
Page 482: ...478 CHAPTER 30 CONFIGURING BGP ...
Page 494: ...490 CHAPTER 31 CONFIGURING IP ROUTING POLICY ...
Page 502: ...498 ...
Page 508: ...504 CHAPTER 33 IP MULTICAST ...
Page 514: ...510 CHAPTER 34 CONFIGURING IGMP ...
Page 526: ...522 CHAPTER 36 CONFIGURING PIM SM ...
Page 528: ...524 ...
Page 532: ...528 CHAPTER 37 CONFIGURING TERMINAL ACCESS SECURITY ...
Page 550: ...546 CHAPTER 38 CONFIGURING AAA AND RADIUS PROTOCOL ...
Page 590: ...586 CHAPTER 40 CONFIGURING IPSEC ...
Page 599: ...IX VPN Chapter 42 Configuring VPN Chapter 43 Configuring L2TP Chapter 44 Configuring GRE ...
Page 600: ...596 ...
Page 638: ...634 CHAPTER 43 CONFIGURING L2TP ...
Page 649: ...X RELIABILITY Chapter 45 Configuring a Standby Center Chapter 46 Configuring VRRP ...
Page 650: ...646 ...
Page 666: ...662 ...
Page 670: ...666 CHAPTER 47 QOS OVERVIEW ...
Page 700: ...696 CHAPTER 49 CONGESTION MANAGEMENT ...
Page 706: ...702 CHAPTER 50 CONGESTION AVOIDANCE ...
Page 707: ...XII DIAL UP Chapter 51 Configuring DCC Chapter 52 Configuring Modem ...
Page 708: ...704 ...
Page 762: ...758 CHAPTER 52 CONFIGURING MODEM ...