MORE INFORMATION
Some protocols are aided by a transport. A transport handles the
organization of application messages, leaving the protocol to handle
only data. NetBEUI (NetBIOS Extended User Interface) is a protocol
that manages the packaging of both data and application messages.
NetBEUI and the Sliding Window Algorithm
NetBEUI, developed by IBM in 1985, is a windowing protocol. It uses an
adaptive sliding window algorithm, enabling it to determine how to
balance the load between sending and receiving data based on the size
of the network.
For example, when a workstation sends information over the network, an
acknowledgment is sent back telling the workstation the data reached
its destination. By utilizing adaptive windowing provided by NetBEUI,
the workstation can send additional information before it receives an
acknowledgment. With NetBEUI, it is possible to send more data overall
and increase throughput. This must be done on a controlled level so no
data is lost.
NetBEUI and Data Loss
NetBEUI controls data loss in two ways:
- By limiting the amount of data a receiver can store before it sends
an acknowledgment. This is called the "receive window."
- By limiting the amount of data that can be sent by the sending
workstation before it must send an acknowledgment. This is referred
to as a "send window."
To increase network throughput, the send window is sized greater than
or equal to the receive window. Having a larger send window increases
the chance of losing data. The lost data causes the sender to resend
the data and could potentially slow down the network. NetBEUI adjusts
these parameters dynamically so that the network load is optimized for
the size of the network.
Reliable Connection-Oriented vs. Unreliable Connectionless-Oriented
The sliding window algorithm provides increased performance for a type
of network traffic called "reliable connection-oriented," or session
meaning that data is sent to a specific address on the network and it
must be acknowledged.
The other kind of traffic, "unreliable connectionless-oriented" or
datagram, does not require an acknowledgment. It is not sent to an
individual address. A datagram is broadcast to either a group of
workstations on the network (multicast address), or to everyone on the
network that understands the data (broadcast address). The benefit of
using a datagram is that it provides the ability to communicate with a
workstation without knowing its exact address. It can be used as a
precursor to the reliable connection-oriented method of exchanging
data.
NetBEUI and Routing
NetBEUI is not a routable protocol. A routable protocol has the
ability to communicate with servers on networks in different cities,
or on different networks within the same city via routers or bridges.
NetBEUI can be routable only if it is on a Token Ring network. It can
accomplish this due to built-in Token Ring source routing. Not as
sophisticated as conventional routing, Token Ring source routing does
not include the ability to control the path that the data takes to
other networks. NetBEUI is optimized for the local area network
environment.
NetBEUI in Microsoft LAN Manager and Windows for Workgroups
If you load NetBEUI services from MS-DOS in Windows for Workgroups,
the functionality is the same as with LAN Manager.
In each implementation of the NetBEUI protocol, the services are
achieved through NetBIOS services provided to the presentation layer
of the OSI model, they all provide windowing services to optimize
throughput, and they all implement their session layer services via
DLC CCB2 and CCB3 protocols. They are functionally equivalent.
Loading NetBEUI at MS-DOS (before Windows)
NetBEUI is limited by static resources when loaded at the MS-DOS
level. Two examples of resources are the number of network connections
that can be made and the number of drives that can be shared. These
network resources are controlled by the SESSIONS= entry in the
PROTOCOL.INI file. If the line reads SESSIONS=6, no more than six
connections can be established.
Within windows, there are two settings which also must be pre-
allocated. They are NETHEAPSIZE=, for both standard and enhanced mode,
and NETDMASIZE= for enhanced mode only. These settings are in the
SYSTEM.INI file. The NETHEAPSIZE= setting controls the buffers windows
sets up in conventional memory for transferring data over the network.
Increasing this value decreases available conventional memory in MS-
DOS sessions. The NETDMASIZE= setting specifies the direct memory
access buffer size for NetBIOS transport software.
Loading NetBEUI from Windows
Loading NetBEUI from Windows is also referred to as loading the
protected mode protocol. VNB.386 is the virtual device driver that is
responsible for providing NetBEUI services in enhanced mode. VNB.386
allocates resources dynamically, so the static settings discussed
above are ignored. This means that if you have six network
connections, and you want a seventh it is possible even though your
SESSIONS= entry is set to 6.
One additional advantage is memory. VNB.386 starts with Windows, and,
as a result, does not use conventional memory; it loads into protected
mode. This means more memory at the MS-DOS command prompt in Windows.