Protocol Analysis - Unit 2 Sec 1

Unit 2

Lesson 1 - LAN Protocols - IEEE 802 Family

LAN Protocols

The Data Link Layer Protocols Diagram illustrates the two sublayers, 802.2 LLC and the various 802.n MAC technologies.

Data Link Layer Protocols

The following are sublayers of the Data Link Layer protocol:

	LLC-- IEEE 802.2
	MAC--Some of these are: - 802.3--Ethernet - 802.4--Token Bus - 802.5--Token Ring - 802.6--Metropolitan Area Network (MAN) - 802.11--Wireless

Ethernet V2, or the original Ethernet, preceded the IEEE 802 specification.

MAC functionality was discussed earlier in the addressing lesson and is primarily concerned with handling the mechanics of address recognition and framing while controlling user access to the medium.

The LLC area of an 802.3 Ethernet frame is shown highlighted on the 802.2 LLC Header Diagram, which illustrates the LLC header as the first part of the Information field. The header contains reserved codes that are used to deliver the data to the proper Network Layer service. It does this by the use of service access points (SAPs), which indicate the protocol in the Network Layer for which the frame is destined. An IEEE 802.2-encoded frame uses SAPs, while the older Ethernet Version 2 frame uses a Type field to accomplish this function.

LLC Header

802.2 LLC Header

The LLC Frame Format Diagram shows the LLC header broken out into its respective fields.

LLC Frame Format

The LLC Header field descriptions are as follows:

	DSAP (Destination SAP, 1 byte [octet])--The service destination above the Data Link Layer for which the frame is intended. For example, if the frame were for Novell Internetwork Packet Exchange (IPX), the DSAP would have the contents, E0h. Some reserved SAPs are: - 04h--IBM System Network Architecture (SNA) Path Control - 06h--IP (Internet Protocol) - AAh--SNAP (Subnetwork Access Protocol) - E0h--Novell - F0h--IBM NetBIOS - FEh--OSI Network Layer It is interesting to note that in their earlier releases, Novell used an 802.3 frame that does not have an 802.2 LLC header. This is sometimes referred to as "raw 802.3."
	SSAP (Source SAP, 1 byte)--The source service (protocol) of the frame. DSAP values and SSAP values always specify the identical protocol.
	Control (1 or 2 bytes)--Indicates a supervisory frame or an information frame and contains send counts and receive counts used for data acknowledgment, error recovery, and flow control, depending on frame type. Most frames that flow on the LAN will contain either information with appropriate send--N(S) and receive--N(R) frame counts, or will contain just the receive count with the command "RR" (supervisory frame), indicating the Receiver is Ready to accept more frames. If the receiving node cannot accept more data, an "RNR" frame will be forwarded to the sender to indicate the Receiver is Not Ready to process more data. This commonly occurs when a slow device, such as a printer, is busy printing a page. When the receiver is ready it will send an "RR" frame. This is how flow control is achieved between the end nodes. Several protocols, such as NetBOIS Extended User Interface (NetBEUI) and HDLC use this mechanism. If something other than NetBEUI, such as IPX or TCP/IP, is being used as the transport protocol, the Control field will contain a 03h, and no send or receive counts are present.
	Information--Follows the Control field in the frame. In the case of Ethernet, the Information field can contain up to 1,500 bytes of data.

The following frame fragment illustrates the LLC portion of the frame in bolded text. Notice on the first line of the hexadecimal data (the line with ADDR 0000), the field immediately following the 81h (Length field) is the DSAP. Its value is F0h, indicating the frame is carrying Network Basic Input Output System (NetBIOS) and is to be delivered to the NetBIOS process in the layer above. The LLC portion of the trace is actually NetBEUI, which is an 802.2 LLC protocol. The command is an "I frame," which means a frame containing user information is being sent.

DLC:  ----- DLC Header -----
DLC:
DLC:  Frame 1 arrived at 09:18:37.3543; frame size is 143 (008F hex) bytes.
DLC:  Destination = Station 3Com 676974
DLC:  Source      = Station 3Com 741178
DLC:  802.3 length = 129
DLC:
LLC:  ----- LLC Header -----
LLC:
LLC:  DSAP = F0, SSAP = F0, Command, I frame, N(R) = 29, N(S) = 109
LLC:
NETB: ----- NETBIOS Data Only Last -----

ADDR  HEX                                                  ASCII
0000  02 60 8C 67 69 74 02 60   8C 74 11 78 00 81 F0 F0  .`.git.`.t.x....
0010  DA 3A 0E 00 FF EF 16 00   00 00 00 00 6B 16 19 01  .:..........k...

The NetBEUI receive count represents the next frame number the station that transmitted this Ethernet frame expects to get. That is, in the above trace the source station had previously gotten frame number 28, and now expects frame number 29. It shows this by the value in its N(R) count--N(R) = 29. It is important to note that the send and receive counts are the source station's send and receive counts, because the source station is the one transmitting.

The N(S) count is 109, which means that this is frame number 109, but it does not necessarily indicate that it is the 109th frame that was sent (or 110th counting frame number 0). The reason for this is that the frame counts go from 0 to 127 and then wrap around to 0 again. There is no way to look at the trace and be able to tell how many times the count has wrapped. The window size is set for a large window (called modulo 128), which simply means the count goes from 0 to 127. A smaller window size (modulo 8) would mean the count goes from 0 to 7 and then wraps around to 0 again. With a small window size, only 7 frames can be transmitted at one time, and then the other end must acknowledge the frames before any more frames can be sent. With the larger window, 127 frames can be sent before an acknowledgment is required. The larger window size is more efficient, but on a "noisy" line a smaller window size may be required to keep the number of retransmitted frames to a minimum. Remember, more than one frame at a time can be sent, up to the maximum number represented by the modulo number minus one.

[ Previous Section ] [ Unit Contents ] [ Next Lesson ]