IEEE802.2 - Logical Link Control Layer (LLC)

The Logical Link Control Layer resides in the upper portion of the Data Link Layer. The LLC layer performs these functions:

  1. Managing the data-link communication
  2. Link Addressing
  3. Defining Service Access Points (SAPs)
  4. Sequencing

The LLC provides a way for the upper layers to deal with any type of MAC layer (ex. Ethernet - IEEE 802.3 CSMA/CD or Token Ring IEEE 802.5 Token Passing).

The Data field of the MAC layer Frame transmits the LLC Protocol Data Unit.

LLC PDU Format

Service Access Ports (SAPs)

SAPs are Service Access Ports. A SAP is a port (logical link) to the Network layer protocol. If we were operating a multiprotocol LAN, each Network Layer protocol would have its own SAP. This is the method that the LLC uses to identify which protocol is talking to which. For example, Unix's TCP/IP, Novell's SPX/IPX and IBM's Netbios would all have different SAPs to identify which was which.

	Address		Assignment

	00		Null LSAP
	02		Individual LLC Sublayer Management Function
	03		Group LLC Sublayer Management Function
	04		IBM SNA Path Control (individual)
	05		IBM SNA Path Control (group)
	06		ARPANET Internet Protocol (IP)
	08		SNA
	0C		SNA
	0E		PROWAY (IEC955) Network Management & Initialization
	18		Texas Instruments
	42		IEEE 802.1 Bridge Spannning Tree Protocol
	4E		EIA RS-511 Manufacturing Message Service
	7E		ISO 8208 (X.25 over IEEE 802.2 Type 2 LLC)
	80		Xerox Network Systems (XNS)
	86		Nestar
	8E		PROWAY (IEC 955) Active Station List Maintenance
	98		ARPANET Address Resolution Protocol (ARP)
	BC		Banyan VINES
	AA		SubNetwork Access Protocl (SNAP)
	E0		Novell NetWare
	F4		IBM LAN Management (individual)
	F5		IBM LAN Management (group)
	F8		IBM Remote Program Load (RPL)
	FA		Ungermann-Bass
	FE		ISO Network Layer Protocol
	FF		Global LSAP
DSAP stands for Destination Service Access Port and is the receiving station's logical link to the Network Layer protocol. SSAP stands for Source Service Access Port and is the transmitting station's logical link to the Network Layer Protocol.

SAPs ensure that the same Network Layer protocol at the Source talks to the same Network Layer protocol at the Destination. TCP/IP talks to TCP/IP, Netbios talks to Netbios and IPX/SPX talks to IPX/SPX.

Types of LLC Operation

LLC defines 2 types of operation for data communication:

  • Type 1: Connectionless
  • Type 2: Connection Oriented

Type 1: Connectionless

Connectionless service for data communications is very similar to sending mail with the postal system (hand delivered mail). The data is sent and we hope it arrives at its destination. There is no feedback from the destination to indicate whether it arrived or not.

Type 1: Connectionless Service

Type 2: Connection Oriented

Connection Oriented service for data communications is very similar to having a phone conversation. First a connection is made and established by dialing the number, waiting for it to ring, someone picking up the line and saying hello. This establishes the connection. During the conversation, confirmation that the other person is still there (hasn't fallen asleep or died) and listening is given by hearing things like: yeah, oh really, uh huh, etc..

This is the acknowledgement of receipt of data. If the destination party did not hear something correctly, they ask to have it repeated which is called automatic repeat request (ARQ).

Connection Oriented service

NOTE: These models for connectionless and connection-oriented can be used for any protocol.

Type 2: Connection Oriented operation for the LLC layer provides 4 services:

  1. Connection establishment
  2. Confirmation and acknowledgement that data has been received.
  3. Error recovery by requesting received bad data to be resent.
  4. Sliding Windows (Modulus: 128)

Sliding Windows are a method of increasing the rate of data transfer. Type 2 Connection Oriented operation calls for every Protocol Data Unit (LLC frame) sent to be acknowledged. If we waited for every PDU to be acknowledged before we sent the next PDU, we would have a very slow data transfer rate.

For example: If we were contacting Microsoft in Sunnyvale California, it might take 2 seconds for our LLC PDU to reach Microsoft and another 2 seconds for the acknowledgement to return. This would mean that we are only sending 1 PDU every 4 seconds. If our PDU was IEEE 802.3 MAC's limit of 1500 octets (8x1500 = 12 Kbits), we would actually be transferring at 3 Kbps (12 kbits/4 seconds). This would be regardless of our actual transfer rate! Waiting for an acknowledgement is controlling the data transfer rate!

Sliding Windows

To overcome this problem, a Sliding Window system of data transmission is used. Each PDU is sequentially numbered (0 - 127). Rather than wait for an acknowledgement, the next PDU is numbered and sent out. The receive station LLC layer acknowledges with the received PDU's numbers back to the transmit station. The LLC will allow up to 128 PDUs to be sent and not acknowledged before it sounds an error alarm.

The received station LLC layer keeps track of the PDUs it is receiving and if one should be lost during transit, it requests the Source to restart transmitting at that PDU number. All PDUs since the lost PDU are discarded.

It is called a Sliding Window because the number of unacknowledged PDUs is determined by the time it takes to get to the destination and for the destination to acknowledge the receipt of the PDU. This time is dependant on the transfer rate and the physical distance the PDU must travel. It is set automatically and we do not have to worry about it.

Classes of LLC

There are 2 Classes of Logical Link Control defined:

 Class I : Type 1 operation only (connectionless)

 Class II: Both Type 1 (connectionless) and
   Type 2 (connection-oriented) operation allowed.

LLC PDU Control Field Formats

There are 3 LLC PDU Control field formats:

  1. Un-numbered (U-Format PDU)
  2. Information Transfer (I-Format PDU)
  3. Supervisory (S-Format PDU)

Un-numbered (U-Format PDU)

The last 2 bits are set to 1, to indicate U-Format Control Field.

M - Modifier bits, they are set depending on the mode of operation: Command, Response or Data

P/F - Poll/Final bit, this bit is used by the Source to solicit a response from the Destination. It is used by the Destination to respond to a solicit from the Source.

The Un-numbered LLC Control field is used mainly in Type 1 (connectionless) operation. The PDUs are not numbered, they are sent out and hopefully arrive at their destination. U-Format PDUs can be commands, responses and data. There are only 8 bits in a U-Format LLC PDU. In the U-Format (Unnumbered), there are 8 commands & responses:

UI - Unnumbered information    (here's some data - hope it arrives)
DISC - Disconnect      (we're done, shut her down)
SABME - Set Asynchronous Balanced Mode Extended  (start now)
XID - Exchange IDs     (Here's who I am, who are you?)
TEST - Test the link     (Here's a test, send me back a test)
UA - Unnumbered Acknowledgement   (Yes, I'm still here)
DM - Disconnect Mode     (I'm disconnecting)
FRMR - Frame Reject      (Bad frame - reject)

Information Transfer (I-Format PDU)

It is used for transferring information or data between Source and Destination in a Type 2 (connection oriented) operation. It is the only LLC PDU allowed to transfer information in Type 2 operation.

I-Format Control Field Format

The last bit is set to 0, to indicate that it is an I-Format Control Field.

P/F - Poll/Final bit, this bit is used by the Source to solicit a response from the Destination. It is used by the Destination to respond to a solicit from the Source.

N(R) - PDU number received. Used with the Sliding Window and for acknowledging PDUs.

N(S) - PDU number sent. Used with the Sliding Window and for acknowledging PDUs.

The N(R) bits are commonly called "Piggyback Acknowledgment" because the response is acknowledged along with the transfer of data. The acknowledgement is piggybacked onto a data transfer.

In the I-Format (Information), there are no commands & responses but typically indicated by:

	I	- Information (data transfer)

Supervisory (S-Format PDU)

Supervisory (S-Format) LLC Control fields are used for Data Link supervisory control functions (handshaking). The S-Format Control fields are used for acknowledging I-Format PDUs, requesting retransmission, requesting a temporary suspension of transmission (buffers full - wait).

S-Format LLC PDU Control Field

The last 2 bits are set to 0 1, to indicate that it is a S-Format Control Field

S - Supervisory function bits. Determines the purpose of the control field

The four 0s in a row are reserved bits and are always set to 0.

P/F - Poll/Final bit, this bit is used by the Source to solicit a response from the Destination. It is used by the Destination to respond to a solicit from the Source.

N(R) - PDU number received. Used with the Sliding Window and for acknowledging PDUs.

In the S-Format (Supervisory), there are 3 commands & responses:

	RR	- Receive Ready (awake & ready to receive)
	RNR 	- Receive Not Ready (got problems, hold off for awhile)
	REJ 	- Reject (received a bad PDU, send the PDU with this number again)

LLC PDU Control Field Formats

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Copyright July 2013 Eugene Blanchard