for flow control and error control.

for flow control and error control. Either LLC entity can terminate a logical LLC
connection by issuing a disconnect (DISC) PDU.
With type 3 operation, each transmitted PDU is acknowledged. A new (not
found in HDLC) unnumbered PDU, the Acknowledged Connectionless (AC)
Information PDU, is defined. User data are sent in AC command PDUs and must be
acknowledged using an AC response PDU. To guard against lost PDUs, a 1-bit
sequence number is used. The sender alternates the use of 0 and 1 in its AC command
PDU, and the receiver responds with an AC PDU with the opposite number
of the corresponding command. Only one PDU in each direction may be outstanding
at any time.
Medium Access Control
All LANs and MANs consist of collections of devices that must share the network’s
transmission capacity. Some means of controlling access to the transmission
medium is needed to provide for an orderly and efficient use of that capacity. This is
the function of a medium access control (MAC) protocol.
The key parameters in any medium access control technique are where and
how. Where refers to whether control is exercised in a centralized or distributed
fashion. In a centralized scheme, a controller is designated that has the authority to
grant access to the network.A station wishing to transmit must wait until it receives
permission from the controller. In a decentralized network, the stations collectively
perform a medium access control function to determine dynamically the order in
which stations transmit. A centralized scheme has certain advantages, including
• It may afford greater control over access for providing such things as priorities,
overrides, and guaranteed capacity.
• It enables the use of relatively simple access logic at each station.
• It avoids problems of distributed coordination among peer entities.
The principal disadvantages of centralized schemes are
• It creates a single point of failure; that is, there is a point in the network that, if
it fails, causes the entire network to fail.
• It may act as a bottleneck, reducing performance.
The pros and cons of distributed schemes are mirror images of the points just
made.
The second parameter, how, is constrained by the topology and is a tradeoff
among competing factors, including cost, performance, and complexity. In general,
we can categorize access control techniques as being either synchronous or asynchronous.
With synchronous techniques, a specific capacity is dedicated to a connection.
This is the same approach used in circuit switching, frequency division
multiplexing (FDM), and synchronous time division multiplexing (TDM). Such
techniques are generally not optimal in LANs and MANs because the needs of the
stations are unpredictable. It is preferable to be able to allocate capacity in an asynchronous
(dynamic) fashion, more or less in response to immediate demand. The
asynchronous approach can be further subdivided into three categories: round
robin, reservation, and contention.
464 CHAPTER 15 / LOCAL AREA NETWORK OVERVIEW
Round Robin With round robin, each station in turn is given the opportunity to
transmit. During that opportunity, the station may decline to transmit or may transmit
subject to a specified upper bound, usually expressed as a maximum amount of data
transmitted or time for this opportunity. In any case, the station, when it is finished,
relinquishes its turn, and the right to transmit passes to the next station in logical
sequence. Control of sequence may be centralized or distributed. Polling is an example
of a centralized technique.
When many stations have data to transmit over an extended period of time,
round-robin techniques can be very efficient. If only a few stations have data to
transmit over an extended period of time, then there is a considerable overhead in
passing the turn from station to station, because most of the stations will not transmit
but simply pass their turns. Under such circumstances other techniques may be
preferable, largely depending on whether the data traffic has a stream or bursty
characteristic. Stream traffic is characterized by lengthy and fairly continuous transmissions;
examples are voice communication, telemetry, and bulk file transfer.
Bursty traffic is characterized by short, sporadic transmissions; interactive terminalhost
traffic fits this description.
Reservation For stream traffic, reservation techniques are well suited. In general,
for these techniques, time on the medium is divided into slots, much as with synchronous
TDM. A station wishing to transmit reserves future slots for an extended
or even an indefinite period. Again, reservations may be made in a centralized or
distributed fashion.
Contention For bursty traffic, contention techniques are usually appropriate.With
these techniques, no control is exercised to determine whose turn it is; all stations
contend for time in a way that can be, as we shall see, rather rough and tumble.These
techniques are of necessity distributed in nature. Their principal advantage is that
they are simple to implement and, under light to moderate load, efficient. For some
of these techniques, however, performance tends to collapse under heavy load.
Although both centralized and distributed reservation techniques have been
implemented in some LAN products, round-robin and contention techniques are
the most common.
MAC Frame Format The MAC layer receives a block of data from the LLC
layer and is responsible for performing functions related to medium access and for
transmitting the data. As with other protocol layers, MAC implements these functions
making use of a protocol data unit at its layer. In this case, the PDU is referred
to as a MAC frame.
The exact format of the MAC frame differs somewhat for the various MAC
protocols in use. In general, all of the MAC frames have a format similar to that of
Figure 15.7. The fields of this frame are
• MAC Control: This field contains any protocol control information needed for
the functioning of the MAC protocol. For example, a priority level could be
indicated here.
• Destination MAC Address: The destination physical attachment point on the
LAN for this frame.
15.4 / BRIDGES 465
• Source MAC Address: The source physical attachment point on the LAN for
this frame.
• LLC: The LLC data from the next higher layer.
• CRC: The Cyclic Redundancy Check field (also known as the frame check
sequence, FCS, field). This is an error-detecting code, as we have seen in
HDLC and other data link control protocols (Chapter 7).
In most data link control protocols, the data link protocol entity is responsible
not only for detecting errors using the CRC, but for recovering from those errors by
retransmitting damaged frames. In the LAN protocol architecture, these two functions
are split between the MAC and LLC layers. The MAC layer is responsible for
detecting errors and discarding any frames that are in error. The LLC layer optionally
keeps track of which frames have been successfully received and retransmits
unsuccessful frames.
15.4 BRIDGES
In virtually all cases, there is a need to expand beyond the confines of a single LAN,
to provide interconnection to other LANs and to wide area networks. Two general
approaches are used for this purpose: bridges and routers. The bridge is the simpler
of the two devices and provides a means of interconnecting similar LANs. The
router is a more general-purpose device, capable of interconnecting a variety of
LANs and WANs. We explore bridges in this section and look at routers in Part
Five.
The bridge is designed for use between local area networks (LANs) that use
identical protocols for the physical and link layers (e.g., all conforming to IEEE
802.3). Because the devices all use the same protocols, the amount of processing
required at the bridge is minimal. More sophisticated bridges are capable of mapping
from one MAC format to another (e.g., to interconnect an Ethernet and a
token ring LAN).
Because the bridge is used in a situation in which all the LANs have the same
characteristics, the reader may ask, why not simply have one large LAN? Depending
on circumstance, there are several reasons for the use of multiple LANs connected
by bridges:
• Reliability: The danger in connecting all data processing devices in an organization
to one network is that a fault on the network may disable communication
for all devices. By using bridges, the network can be partitioned into
self-contained units.
• Performance: In general, performance on a LAN declines with an increase in
the number of devices or the length of the wire. A number of smaller LANs
will often give improved performance if devices can be clustered so that
intranetwork traffic significantly exceeds internetwork traffic.
• Security: The establishment of multiple LANs may improve security of communications.
It is desirable to keep different types of traffic (e.g., accounting,
466 CHAPTER 15 / LOCAL AREA NETWORK OVERVIEW
personnel, strategic planning) that have different security needs on physically
separate media. At the same time, the different types of users with different
levels of security need to communicate through controlled and monitored
mechanisms.
• Geography: Clearly, two separate LANs are needed to support devices clustered
in two geographically distant locations. Even in the case of two buildings
separated by a highway, it may be far easier to use a microwave bridge link
than to attempt to string coaxial cable between the two buildings.
Functions of a Bridge
Figure 15.8 illustrates the action of a bridge connecting two LANs, A and B, using
the same MAC protocol. In this example, a single bridge attaches to both LANs; frequently,
the bridge function is performed by two “half-bridges,” one on each LAN.
The functions of the bridge are few and simple:
• Read all frames transmitted on A and accept those addressed to any station on B.
• Using the medium access control protocol for B, retransmit each frame on B.
• Do the sam