100 Chapter 3: OSI Reference Model & Layered Communication
Figure 3-14
802.2 SAP and SNAP Type Fields
Figure 3-15
Identifying Protocols over HDLC and Frame Relay
Referring to the top part of Figure 3-15, if Barney is using FTP to transfer files to the Sun system
and is also connected to the NetWare server (Fred) using IPX, then Barney will generate both
TCP/IP and NetWare IPX traffic. As this traffic passes over the HDLC controlled link, R2 will
need to know whether an IP or IPX packet follows the HDLC header. Mainly, this is so that the
802.3
03
CTL
OUI
802.5
SNAP
IP Data
802.3
AA
DSAP
AA
SSAP
0800
Type
802.3
14
14
1
1
1
1
1
4
1
3
2
4
CTL
IPX Data
802.3
E0
DSAP
E0
SSAP
R1
Barney
Fred
(NetWare
Server)
Sun
FTP
Server
R2
Frame Relay
R1
Barney
Fred
(NetWare
Server)
Sun
FTP
Server
R2
Point-to-Point
ch03.fm Page 100 Monday, March 20, 2000 4:58 PM