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10: Ethernet - IEEE 802.3 LLC frame & IEEE 802.3 SNAP frame part 16




IEEE 802.3 LLC frame & IEEE 802.3 SNAP frame

IEEE 802.3 LLC  (Logical Link Control) frame:

By understanding and absorbing the ingredients of Ethernet II (DIX) frame, you do not need much effort to grasp what is contained in IEEE 802.3 (LLC) frame.  It is just a repetition of the fields except for three fields which pertain to IEEE 802.3 (LLC) frame. This Ethernet frame is currently not used except for  large corporate NetWare installations, which is an  obsolete computer network operating system. The fields that are peculiar to this frame format are  DSAP, SSAP, and Control fields. The following figure shows the structure of the frame.
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Note: IEEE 802.3 LLC frame and IEEE 802.3 SNAP frame are rarely if not at all used in today's networks; instead, DIX Ethernet is the dominant one which is widely relied on in data transmission over networks, that is, the latter frame supersede the former ones.
As shown in the figure, there are three new fields in 802.3 frame. There is no need to explain the the other fields.

SSAP (Source Service Access Point) : is an 8-bit long field. It has the same function as Ethertype. It is used to distinguish between data transmitted on the same station; or in other words, to determine whether the data being transmitted is IPv4, or IPv6, or ARP request, or 802.1Q tagged frame etc. ; for each type of data is assigned a value through which data is identified.  Here are some of these values  0x0800 means IPv4,  0x8100 means 802.1Q tagged frame, 0x0806 means ARP, and 0x86DD means IPv6. Thus, The SSAP indicates from which service the LLC data unit was sent. To put it another way, SSAP represents the logical address of the network layer entity through which the message is carried, and which created the message, or rather the  network layer protocol that is sending. For example, a message might be carried by ipv4 ; so it can be said that IPv4 protocol type is the SSAP, which, for easy identification, puts its value 0x0800 in SSAP field.

DSAP (Destination Service Access Point) : is also an 8-bit long field. This field functions as SSAP, but it contains the the value of the protocol to which data is addressed. SSAP and DSAP values must be the same because the protocol through which a message is generated in the source machine should be the same one the is going to receive the message in the destination machine. For example, a message sent by IPv6 network layer protocol must be received by IPv6 network layer protocol of the destination machine ; so both SSAP and DSAP fields must comprise IPv6 value which is 0x86DD. By and large, DSAP represents the logical addresses of the network layer entity intended to receive the message, or rather the network layer protocol that is receiving.
Control :  is an 8-bit long field. This field indicates whether the communication is connection-less or connection-oriented (TCP or UDP protocols), which is usually performed by the upper layers (transport layer).

IEEE 802.3 SNAP (Subnetwork Access Protocol) frame:

This frame has emerged because of the fact that the LLC header of IEEE 802.3 LLC frame, which contains SSAP, DSAP, and Control fields, is too small to accommodate a large number of protocols. Thus, due to the short size of SSAP DSAP fields in the header that are unable to address all the layer three protocols or services, a need for another frame type, or rather the extention of IEEE 802.3 LLC frame was highly required. As a result, IEEE 802.3 SNAP frame was the one to have been created to solve the problem. 
As the name implies, this frame type has added a new header called SNAP, whose size is 5 bytes (40 bits)  that  comprises  OUI and ID fields. SNAP header comes into play when both source and destination (SSAP and DSAP) are set to the value 0xAA, and the control field is set to 0x03. In other words, if the SSAP and DSAP in LLC header are set to 0xAA and the control field is set to 0x03, then the SNAP service is requested, and as a result the SNAP encapsulation will occur.

OUI (Organizationally Unique Identifier) and ID fields : is a 3-byte field that SNAP frame encompasses. If this field is assigned 000000 (hexadecimal), the ID is the Ethernet type (EtherType)  field value for the protocol running on top of SNAP. But if the OUI field is an identifier of a specific organization, the ID field is a value allocated to a protocol assigned by that organization. For example, OUI is an advantage to vendors because they can create their own types to use, such as Cisco company, which has protocols that are peculiar to it and thus encapsulated with an ID value specific to the organization.
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A network might use different Ethernet frames. So how one can tell what frame is used in a given transmission !. The answer is found in the Length/type field. Notwithstanding The length field is used to indicate how many bytes of data are following this field before the FCS field, It can also be utilized to draw a distinction between DIX frame and 802.3 frame. If the field's value is greater than or equal to 1536, the frame must be an Ethernet v2 frame, with that field being a type field. If it's less than or equal to 1500, it must be an IEEE 802.3 frame, with that field being a length field.

10: Ethernet - IEEE 802.3 LLC frame & IEEE 802.3 SNAP frame part 16 Reviewed by BOUFTIRA on 2:00:00 PM Rating: 5

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