control information as a header, cr

control information as a header, creating an LLC protocol data unit (PDU).This control
information is used in the operation of the LLC protocol. The entire LLC PDU
is then passed down to the MAC layer, which appends control information at the
front and back of the packet, forming a MAC frame. Again, the control information
in the frame is needed for the operation of the MAC protocol. For context, the figure
also shows the use of TCP/IP and an application layer above the LAN protocols.
Logical Link Control
The LLC layer for LANs is similar in many respects to other link layers in common
use. Like all link layers, LLC is concerned with the transmission of a link-level PDU
between two stations, without the necessity of an intermediate switching node. LLC
has two characteristics not shared by most other link control protocols:
1. It must support the multiaccess, shared-medium nature of the link (this differs
from a multidrop line in that there is no primary node).
2. It is relieved of some details of link access by the MAC layer.
Addressing in LLC involves specifying the source and destination LLC users.
Typically, a user is a higher-layer protocol or a network management function in the
station.These LLC user addresses are referred to as service access points (SAPs), in
keeping with OSI terminology for the user of a protocol layer.
We look first at the services that LLC provides to a higher-level user, and then
at the LLC protocol.
LLC Services LLC specifies the mechanisms for addressing stations across the
medium and for controlling the exchange of data between two users. The operation
and format of this standard is based on HDLC. Three services are provided as alternatives
for attached devices using LLC:
• Unacknowledged connectionless service: This service is a datagram-style service.
It is a very simple service that does not involve any of the flow- and errorcontrol
mechanisms. Thus, the delivery of data is not guaranteed. However, in
most devices, there will be some higher layer of software that deals with reliability
issues.
• Connection-mode service: This service is similar to that offered by HDLC. A
logical connection is set up between two users exchanging data, and flow control
and error control are provided.
• Acknowledged connectionless service: This is a cross between the previous
two services. It provides that datagrams are to be acknowledged, but no prior
logical connection is set up.
Typically, a vendor will provide these services as options that the customer can
select when purchasing the equipment. Alternatively, the customer can purchase
equipment that provides two or all three services and select a specific service based
on application.
The unacknowledged connectionless service requires minimum logic and is
useful in two contexts. First, it will often be the case that higher layers of software
will provide the necessary reliability and flow-control mechanism, and it is efficient
15.3 / LAN PROTOCOL ARCHITECTURE 461
to avoid duplicating them. For example, TCP could provide the mechanisms
needed to ensure that data is delivered reliably. Second, there are instances in
which the overhead of connection establishment and maintenance is unjustified or
even counterproductive (for example, data collection activities that involve the
periodic sampling of data sources, such as sensors and automatic self-test reports
from security equipment or network components). In a monitoring application, the
loss of an occasional data unit would not cause distress, as the next report should
arrive shortly. Thus, in most cases, the unacknowledged connectionless service is
the preferred option.
The connection-mode service could be used in very simple devices, such as terminal
controllers, that have little software operating above this level. In these cases,
it would provide the flow control and reliability mechanisms normally implemented
at higher layers of the communications software.
The acknowledged connectionless service is useful in several contexts.With the
connection-mode service, the logical link control software must maintain some sort
of table for each active connection, to keep track of the status of that connection. If
the user needs guaranteed delivery but there are a large number of destinations for
data, then the connection-mode service may be impractical because of the large number
of tables required. An example is a process control or automated factory environment
where a central site may need to communicate with a large number of
processors and programmable controllers. Another use of this is the handling of
important and time-critical alarm or emergency control signals in a factory. Because
of their importance, an acknowledgment is needed so that the sender can be assured
that the signal got through. Because of the urgency of the signal, the user might not
want to take the time first to establish a logical connection and then send the data.
LLC Protocol The basic LLC protocol is modeled after HDLC and has similar
functions and formats. The differences between the two protocols can be summarized
as follows:
• LLC makes use of the asynchronous balanced mode of operation of HDLC, to
support connection-mode LLC service; this is referred to as type 2 operation.
The other HDLC modes are not employed.
• LLC supports an unacknowledged connectionless service using the unnumbered
information PDU; this is known as type 1 operation.
• LLC supports an acknowledged connectionless service by using two new
unnumbered PDUs; this is known as type 3 operation.
• LLC permits multiplexing by the use of LLC service access points (LSAPs).
All three LLC protocols employ the same PDU format (Figure 15.7), which
consists of four fields. The DSAP (Destination Service Access Point) and SSAP
(Source Service Access Point) fields each contain a 7-bit address, which specify
the destination and source users of LLC. One bit of the DSAP indicates whether the
DSAP is an individual or group address. One bit of the SSAP indicates whether the
PDU is a command or response PDU. The format of the LLC control field is identical
to that of HDLC (Figure 7.7), using extended (7-bit) sequence numbers.
For type 1 operation, which supports the unacknowledged connectionless service,
the unnumbered information (UI) PDU is used to transfer user data. There is