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Ethernet II framing is an attempt to simplify framing and header overhead.

Header Type Field Size
802.3 destination address 6 bytes
802.3 source address 6 bytes
802.3 length 2 bytes
802.2 0xaa 1 byte
802.2 0xaa 1 byte
802.2 UI 1 byte
SNAP protocol ID 1 byte
SNAP type 1 byte
data x

Table 4-4. Ethernet SNAP framing

Ethernet SNAP framing allows networks to use Ethernet II frames on IEEE compliant networks without any modification to the network. Notice the use of three protocol headers in the frame: IEEE 802.2, IEEE 802.3, and SNAP.

Header Type Field Size
802.5 AC 1 byte
802.5 FC 1 byte
802.5 destination address 6 bytes
802.5 source address 6 bytes
802.5 routing data 0-18 bytes
802.2 destination SAP 1 byte
802.2 source SAP 1 byte
802.2 control 1 byte
data x

Table 4-5. Token Ring framing

Token Ring framing includes the SAP field which Novell sets to 0xe0 to indicate that the upper layer protocol is IPX. Token Ring framing is specified by IEEE 802.5 and IEEE 802.2

Header Type Field Size
802.5 AC 1 byte
802.5 FC 1 byte
802.5 destination address 6 bytes
802.5 source address 6 bytes
802.5 routing data 0-18 bytes
802.2 0xaa 1 byte
802.2 0xaa 1 byte
802.2 UI 1 byte
SNAP protocol ID 1 byte
SNAP type 1 byte
data x

Table 4-6. Token Ring SNAP framing

Token Ring SNAP framing allows networks to use Ethernet II frames on IEEE compliant networks without any modification to the network. Notice the use of three protocol headers in the frame: IEEE 802.2, IEEE 802.5, and SNAP.

But What Does It All Mean?

Framing is really an ATM issue and not an ADSL issue. The ATM protocol fits in with the first three layers of the OSI model. The OSI model features and constraints of the first three layers are well represented in the features offered by ATM as a transmission and switching technology. ATM framing is similar to the legacy framing structures from which it is derived.

ADSL, though, is a multiplexing/demultiplexing technology that does fit in as a higher level implementation of the OSI model. ATM fits in with the first three layers of the OSI model, and ADSL fits in with the higher layer protocols.


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