Tech Arsenal 1/Tech Arsenal (Arsenal Computer).ISO/tek-06/bridge.zip/BRIDGE.TXT

Text File | 1992-03-25 | 28KB | 562 lines

                            LAN Interconnection
                            -------------------


(C) 1992 ZyBel MicroSystems Inc.           March 19, 1992


This file contains technical information about using LAN bridge and router
products, and a discussion of application considerations.  It will focus on
bridges being used to extend network length beyond the limits dictated by
the type of media being utilized, and will also discuss bridging as a means
of segmenting congested networks.

This information was compiled by ZyBel MicroSystems Inc., a software
developer and reseller of Artisoft Lantastic products.  ZyBel would be
happy to work with any resellers or end users who have bridging needs.
Our address and phone number can be found at the end of this file.

The author will assume that the reader is familiar with basic LAN
terminology and wiring standards.  The first section of this file contains
detailed technical information regarding LAN interconnection.  The latter
section details appropriate solutions for specific application problems.



1.   LAN Interconnection - the technology explained.

1.1  Wiring Standards.

The Artisoft Network Interface Cards (AE1, AE1, and AE3) all follow the
Ethernet LAN standard.  These cards make available various types of network
interfaces:

   AUI:      Attachment Unit Interface.  A point to point, short distance
             digital interface which is usually used to connect to a LAN
             transceiver, for attachment to a particular type of media.
             This interface is most commonly used to access a 10Base5 (or
             thick coaxial) network.

   10Base2:  Also called Thinnet or Cheapernet.  Specifies thin coax
             (RG58), 10 Mb/s, with a maximum segment length of 185 Meters.
             There can be up to 30 nodes per segment, and a total network
             span of 925 Meters.

   10Base5:  Specifies thick coax cable, 10 Mb/s, with a maximum segment
             length of 500 Meters.  There can be up to 100 nodes per
             segment, and a total network span of 2500 Meters.  The
             network interface card connects to an external transceiver
             through an AUI interface.

   10BaseT:  Unshielded twisted pair wiring (UTP).  Runs at 10 Mb/s, with
             a maximum segment length of 100 Meters, and a total network
             span of 500 Meters.


With the Ethernet standard, each network interface card is assigned a
unique address by the manufacturer.  These addresses are used to identify
the source and destination of information which is transmitted over the
LAN.


The term LAN "segment" refers to a contiguous length of media between two
network terminations.  In the case of 10Base2 and 10Base5, many devices can
be attached to a single segment.  In the case of 10BaseT, each segment
supports only two devices - one at either end.

Segments can be joined together to form larger and more complex networks
that exceed the single segment distance limitations.  There are three basic
devices which can be used to join LAN segments - repeaters, bridges, and
routers.


1.2  Repeaters

A repeater can be thought of as two Ethernet transceivers connected back-
to-back.  Any signal received on one LAN segment is immediately transmitted
on the other segment.  The repeater amplifies and regenerates the signal,
but is transparent to both segments.  Thus the function of a repeater can
be thought of as creating one large LAN out of smaller segments.
Collisions will occur if two stations, each on a different segment,
transmit at the same time.  The only real purpose of a repeater is to
extend the physical length of the LAN, or to join two different types of
media (10Base5 to 10Base2 for example).  Ethernet specifies a maximum of 4
repeaters between any two stations (thus allowing up to 5 segments in a
single LAN).  There are also multi-port repeaters, which transmit incoming
signals on all other segments (thereby increasing the maximum cabling
length of the network, while still complying with a maximum of 4 repeaters
between any two stations).  A 10BaseT wiring hub is essentially a multi-
port repeater.

It is important that no "loops" be created when installing repeaters.
Since repeaters re-transmit everything that is received, a "loop" (in which
a circular path is formed for the data) will cause data to be repeated
forever, essentially killing the network.  This effect is known as a data
storm.


1.3  Bridging

A bridge operates quite differently from a repeater.  Before bridging is
discussed in any significant detail, a review of LAN architecture is
required.  The ISO (International Organization for Standardization) created
an overall communications structure (or architecture) in 1977 called OSI,
or Open Systems Interconnection model.  The OSI model was meant to
standardize the way in which LANs were architected, and allow for inter-
operation between various types of LANs.  The OSI model consists of 7
layers, each dealing with a different function required by the LAN system.


1.3.1     The OSI Reference Model

The Ethernet standard is based on work done by the IEEE Local Network
Standards Committee (Project 802), and specifically on the 802.3 standard
(which details CSMA/CD operation). Ethernet utilizes a layered approach
similar to the OSI Reference Model.  Although a discussion of the OSI
Reference Model is beyond the scope of this document, a brief examination
of the lower three layers is necessary to fully understand bridging and
routing.

1.3.1.1   Layer 1: The Physical Layer

This layer is concerned with the physical characteristics of the network -
for example, the signal voltage swing and timing, and mechanical,
electrical and procedural details involved in a link.

1.3.1.2   Layer 2: The Data Link Layer

Layer 2 is responsible for the reliable transfer of data across a physical
link.  It communicates blocks of data (frames or packets) with necessary
synchronization, error checking and flow control.

1.3.1.3   Layer 3: The Network Layer

Provides the higher level software with independence from the underlying
transmission and switching technologies, and is responsible for
establishing, maintaining and terminating connections between entities.  As
an example, the TCP/IP protocol is a layer 4 and layer 3 protocol (IP, or
Internetworking Protocol is layer 3).  TCP/IP can be run over Ethernet, or
on other types of networks.  Ethernet only deals with layers 1 and 2.


1.3.2     How a Bridge Works

A bridge is a device which interconnects LANs based on layer 2 information.
That is, unlike a repeater, a bridge is not transparent to the LANs
involved, and does not re-transmit all information detected on each LAN.

As an example, consider a bridge connected to two LANs, call them "A" and
"B".  The bridge will appear as a device to both "A" and "B".  It will
monitor traffic on each LAN, but only transmit packets destined for a
device on the other LAN.  For instance, the bridge will only send packets
from "A" to "B" if the packet is addressed to a device located outside of
LAN "A".  The way the bridge determines the destination for a data packet
is by examining the Layer 2 information.  Thus the bridge must "know" the
structure of Layer 2 packets, and it can only interconnect LANS that have
identical Layer 2 protocols.  Therefore, a bridge cannot be used to connect
Token Ring to Ethernet.

It is important to note that the bridge actually receives the LAN data,
buffers it, examines it, and then decides whether to forward the data or
ignore it.  Should it decide to forward the data, it transmits it on the
other LAN just as a local device would - it waits until the LAN is clear
and then sends the information.  With this structure, the bridge actually
operates on LAN "A" and LAN "B" independently - i.e. if LAN "A" is busy,
the bridge can still transmit to LAN "B".  Similarly, if devices on "A" and
"B" both transmit at the same time, there will NOT be a collision, since
the information is not immediately forwarded to the other LAN.  The effect
of this operation is to logically segment the two networks.  Information
destined for a device on the same network will NOT be forwarded to the
second LAN.  Thus, only traffic destined for the other LAN will be
forwarded, and overall congestion on each LAN will be greatly reduced.

Because a bridge buffers the data, and must wait to forward packets to a
second LAN, the data will take longer to reach its eventual destination
than if the bridge were not present.  However, since the bridge actually
receives the data, stores the information, and then re-transmits the data
on another LAN as if it was the originator, the rule about segments and
repeaters does not hold true.  There can be virtually any number of bridges
between two network devices (obviously within practical limits).

The higher software layers (layers 3 and 4, the Network and Transport
layers) of the OSI Reference Model are responsible for managing end-to-end
connections.  They ensure that data is delivered in an orderly fashion
without errors, and that flow control is implemented as required.  Because
bridges introduce delays in data reaching its destination, the higher layer
protocols must be able to tolerate these additional delays.

Now that we understand what a bridge does, we should understand why someone
would utilize a bridge.  There are basically four reasons for utilizing a
bridging device:

  a). To isolate portions of the network and therefore provide better
      reliability.  With a bus topology (10Base2 and 10Base5), a failure
      in any device on the bus can bring down the entire LAN.  By
      inserting bridges between LAN segments, other segments are not
      affected by a failure.

  b). To increase network performance by segmentation of the network.  If
      there are two LANs, and one is used primarily for an accounting
      system, and the other primarily for an order entry system, it would
      slow both LANs considerably if all packets were transmitted on both
      LANs.  A better choice would be to utilize a bridge which will only
      forward packets that are involved with the application being
      utilized, and reduce overall congestion on each LAN.  This is
      especially important in an Ethernet network because LAN performance
      will not degrade in a linear fashion with congestion - rather it
      will fall off rapidly once a critical utilization level is reached,
      and collisions become commonplace.

  c). To implement security in a network.  Some bridges can be programmed
      to only allow specific network devices to forward data to other
      LANs, thus providing network security.

  d). To expand a network beyond the physical limits imposed by the media
      being used.  This is especially important if a LAN is to be
      interconnected with another LAN located across town, or across the
      country.  A remote bridge can forward LAN traffic over various types
      of Wide Area Networks (to another bridge at the remote end), and
      thus join two networks that are physically very far apart.  This
      application will be discussed in some detail later.


Depending upon the features supported by the bridge, circular paths and
multiple paths can be supported.  By utilizing special algorithms in the
bridge (the most common of which is the Spanning Tree Algorithm), a complex
network can be created, and the bridges will "know" how to forward data
between interconnected LANs (without causing circular repetition).


1.4  Routing

A Router operates on a similar concept to a bridge, but operates on Layer 3
of the OSI Reference Model (the Network Layer).  Routers determine how to
forward or filter data based on Layer 3 information.

Most LANs can support many types of protocols carried on the same media.
For example, an IPX Ethernet based LAN might share the same LAN cable as
devices utilizing TCP/IP over Ethernet.  As long as Layer 1 and Layer 2 are
identical for both protocol stacks, they will co-exist on the same media
(although they may not interact with each other).

Because routers operate on Layer 3 information, it is possible to make
routing decisions based not only on the source and destination of the data,
but also protocol.  Thus a multi-protocol router might be configured to
only forward TCP/IP packets to a particular LAN, and filter all IPX
packets.

Routers can support very complex network topologies which include redundant
paths and path costing information.  A decision to route a packet in a
particular fashion can be made based on WAN congestion, link speed and
transmission cost.

Thus a router has more capabilities than a bridge, but is usually slower
(because it must do more processing on the data), and much more costly.  In
general, a router would be selected over a bridge if complex network
topologies are required, or more than one protocol were being used on one
or more of the interconnected networks.


1.5  The Brouter

In recent years, a new form of product known as a "brouter" has appeared.
This is essentially a combination of a bridge and router.  Unknown
protocols are handled as a bridge, while specific protocols are routed.

The term brouter is also commonly applied to bridge devices which use
proprietary techniques to handle complex topologies, rather than the
standards based Spanning Tree Algorithm.


1.6  Bridge Configuration

In the discussions above, it is apparent that enormous configuration tasks
are required of a network manager, to configure the bridge to forward data
for certain destinations, and filter data for the local LAN.

With the first generations of bridging devices, this was usually
accomplished by setting up static tables that define which network devices
are located on which LANs.  The bridge would then know when to forward and
when to filter data for a particular destination.  This task is very time
consuming, and subject to constant change as the LAN evolves.

A better strategy in recent years involves the introduction of the
"learning" bridge.  A learning bridge starts out by broadcasting all LAN
data to all interconnected LANs (similar to a repeater).  However, as
responses are received, the bridge can "learn" where devices are located by
examining the source address.  As it gathers this information, it
automatically configures its internal tables.  In this fashion, the bridge
actually dynamically re-configures itself as the LANs evolve.

A final word about bridges/routers is regarding network management.  Many
bridges and routers provide facilities for configuration and monitoring
(including the gathering of key LAN statistics).  Some devices provide this
function through the use of a separate communications interface (such as
RS-232), and some allow this function through the LAN itself.  An important
recent development is the widespread acceptance of a network management
protocol known as SNMP.  SNMP is a generic way of managing network devices,
and supports various devices according to a Management Information Base (or
MIB).  Manufacturers can produce MIBs with extended features for their own
devices, which will theoretically operate with any SNMP package.



2.   Applications

Most people become aware of LAN bridging due to a need to satisfy one of
the following applications:


2.1  LAN Segmentation

If a LAN is exhibiting unacceptable performance because of some data
intensive application, a bridge can be used to isolate users of the
problematic program, greatly reducing network congestion for the remaining
users.


2.2  LAN Extension

If a LAN must be extended beyond the length limits imposed by the
particular media being utilized, a bridge can be used to extend the overall
network length, by actually creating two distinct networks which are
interconnected.  A bridge used in this application is known as a Local
Bridge.


2.3  Remote Access to Other LANs

If an organization has LANs located some distance apart (e.g. across town),
and requires access to data on more than one network, a Remote Bridge can
be used to interconnect all LANs, and in essence, form one large network.

Before discussing this application in detail, it is wise to examine another
approach, since remote bridging can become very expensive and exhibit
performance limitations.


2.3.1     Screen Sharing Packages.

Perhaps the least costly solution is to utilize a "screen" sharing package
such as PC Anywhere.  A remote user can utilize a dial up modem to
communicate with a PC located on the other LAN, and essentially take
control of it's keyboard and screen.  This allows the use of any
applications located on the other network, and supports basic file transfer
between networks.  The benefit to this approach is that only screen data is
being transmitted over the modems, so even if the application manipulates
hundreds of megabytes of data (a large database, for example), perhaps only
a few thousand bytes of information are being sent over the modem.  In
addition, low cost dial modems and normal telephone lines can be utilized.

The problem with remote control packages is that the PC being used on the
remote LAN is unavailable while remote access is underway, and therefore
might require additional PCs to be purchased to support the application.
If more than one user wanted access between the two LANs at the same time,
multiple PCs, modems and telephone lines would be required.  Finally, data
on the remote LAN can only be accessed through application programs located
on the remote LAN, which might not be acceptable depending upon the
particular application.

A good candidate for remote control software might be financial accounting,
since the data is generally only accessed through the accounting software
program, and large amounts of data are manipulated by the application.
Thus a remote control package would perform well (only transmitting screen
changes), and local access to the raw data is not required.

If, however, access to the raw data is required, or large amounts of data
are actually to be exchanged between the two LANs, a remote bridge is
warranted.


2.3.2     Remote Bridging

A remote bridge contains one or more LAN ports, and one or more Wide Area
Network (or WAN) ports.  LAN networks are connected to the LAN ports, and
some form of wide area communication link is connected to the WAN ports.
Data destined for another LAN is packaged in a form suitable to be carried
over the WAN link, and then transmitted through the WAN to another bridge.
Once received, the second bridge recovers the original LAN data from the
WAN format, and places it on the other LAN at the remote site.  In this
manner, networks are interconnected over long distances.

An important issue to note is that WAN links are almost always slower than
LANs, and thus performance problems can occur.  In addition, since the
bridge must buffer LAN data until it can be transmitted over the slower WAN
link, it is possible for LAN data to be lost (the higher layer protocols
should detect any errors and cause the information to be re-transmitted).

The physical interface to the WAN can come in many forms; V.24 (RS-232),
X.21, V.35 and T1 to name a few.  WAN speeds generally range from 9600 baud
to 1.544 MB/S (for T1) or 2.048 MB/S (for E1).  In some cases, a bridge
will support more than one link between destinations, essentially combining
the bandwidth of these WAN links (for example, two T1 links might be
combined to provide 3.088 MB/S of bandwidth between two LANs).

Before selecting a bridging device, the user should first select a
particular WAN transmission service.  There are many different WAN links
available, from private microwave and satellite links, to telephone company
dedicated data lines and switched services.  In general, there are three
types of WAN links:

a).  Point to Point dedicated service.  A fixed bandwidth is always
     available between two specific sites.   Speeds can range from 4800 bps
     to T1 and higher.  High speed links are usually very expensive, but
     multiplexer equipment can be used to mix LAN data with voice and video
     signals, to better utilize available bandwidth (and help with cost
     justification).  Monthly cost is usually fixed regardless of
     utilization.

b).  Circuit switched services.  Bandwidth is established between sites as
     required (similar to dialing a telephone).  Generally provides slower
     speed service, at much lower cost than dedicated links.  A good
     example is Switched 56, providing "dial up" connections with 56 KB/S
     speeds.  These services are very useful if part-time LAN
     interconnection is required.  For example, a massive file transfer
     might occur every evening for 2 hours, not requiring interconnection
     for the remaining hours in the day.  Most circuit switched billing is
     primarily based on usage.

c).  Packet switched services.  Frame Relay is an emerging standard which
     is ideal for LAN interconnection.  Frame Relay is similar to X.25
     service, but without error correction facilities, thereby supporting
     much higher speeds than X.25.  It is assumed that higher layer
     protocols (layers 3 and 4) will recover from any lost or corrupted
     data.  Frame Relay billing is based on actual packet usage.


To select a particular type of WAN link, the user should examine
application requirements (amount of data to be transmitted, performance
considerations, time of day requirements), and perform a cost analysis on
all types of links which support the application.

Once a decision is reached regarding the type of link, a particular WAN
service can be selected, which will establish what type of physical
interface is required.  An appropriate bridge can then be purchased which
supports this interface.



2.4  Enterprise Networking

A large company may develop many separate LANs, reaching sizes of several
thousand nodes, and supporting many different protocol stacks (IPX, TCP/IP
etc).  It may be desirable to interconnect all company wide LANs, and
perhaps provide redundancy for mission critical network applications.

This form of network is very complex, and involves huge expenditures in
equipment and transmission services.  Many such organizations have a
private backbone network that carries data, voice and video signals.  In
the near future, many companies may choose Frame Relay as a method of
interconnecting LANs.  In any event, such networks generally make heavy use
of router products, and are far beyond the scope of this discussion.


3.   Selecting a Bridge

When selecting a bridge, certain key features should be considered:

a).  Performance.  Bridges must perform two basic functions; the filtering
     of packets, and forwarding of packets.  Performance is usually stated
     separately for each function ("X" packets/second filtering, and "Y"
     packets/second forwarding).  Ethernet defines a minimum packet size,
     which limits the maximum speed of a 10 MB/S network to 14,800
     packets/second.  Therefore, a wire-speed bridge will filter and
     forward packets at up to 14,800 packets/second (it would appear almost
     as a repeater in terms of performance).

     Be careful when evaluating performance figures.  Some bridge
     manufacturers combine the performance of each LAN port, and state one
     total number for the bridge.  Therefore, a four port LAN bridge may
     claim performance of 24,000 packets/second, when in fact it supports
     6,000 packets/second on each port.

     Some bridges also support data compression over WAN links, improving
     overall throughput for remote applications.

b).  Availability of LAN interfaces.  It is desirable for a bridge to
     support many types of LAN interfaces, so that mixed media networks are
     possible.  Many bridges have integral transceivers, supporting either
     Thinnet or UTP (10BaseT) directly.  Others will only support an AUI
     interface, and require external transceivers.

c).  Availability of WAN interfaces.  As mentioned earlier, there are many
     forms of WAN interfaces.  An ideal bridge would support all common
     connection standards.  In the case of T1 equipment, a CSU/DSU may be
     required, which may or may not be available as an integral component
     to the bridge.

d).  Configuration and Management.  The bridge should support remote
     configuration, so that field service is not required to effect network
     changes.  In addition, some form of statistics collection is
     desirable.  SNMP support is a definite plus, given the growing
     prevalence of SNMP management packages.

e).  Intelligence.  It is a definite advantage if the bridge supports
     "learning" of device locations.  Without a learning bridge, a network
     manager must configure lengthy tables, and maintain the information as
     the network changes.

f).  If a bridge is to be used in a complex topology (or redundant WAN
     links are required), it should support the Spanning Tree Algorithm.


4.   Pricing

Pricing for bridging products vary dramatically.  At the lowest end of the
market, there are local bridges available for under $2,000.  At the high
end, some bridges cost between $10,000 and $20,000 for T1 applications.
Most bridges today range between $2,000 and $10,000 in price.


5.   Summary

Bridges are devices which can be used to extend the total length of a LAN,
interconnect LANs which are located far apart, and segment LANs to reduce
congestion and improve security.

Bridges can be used in local applications (all LANs located on the same
premises), or remote applications, to interconnect LANs anywhere in the
world.

When selecting a remote bridge, the first consideration should be the
selection of a WAN transmission vehicle, which depends upon the application
being utilized on the LAN.  A suitable remote bridge can then be purchased
which supports the selected WAN link, and provides the necessary network
performance.

Care should be exercised in selecting a bridge, especially with regard to
confusing performance figures.


===========================================================================


ZyBel carries a product known as the 8230 Littlebridge, manufactured by
Newbridge Networks Inc. (a leader in the T1 multiplexer market).  This
bridge supports up to 2 LAN interfaces and 2 WAN interfaces.  LAN ports can
be AUI, Thinnet or UTP (10BaseT).  WAN support includes V.24, X.21, V.35,
T1 (with or without integral CSU), and E1.  The product is a learning
bridge, includes the Spanning Tree Algorithm, and supports SNMP management
(it comes complete with a basic SNMP package).  The Littlebridge features
full wire speed filtering AND forwarding on all ports.  List price for the
product ranges from about $2,200 for a local bridge to $4,400 for a T1
equipped box.

For further information, please feel free to contact the author at the
address listed below.  I sincerely hope the above information has proved to
be useful.


     Warren D. Belkin
     ZyBel MicroSystems Inc.
     PO Box 1284
     Weston, CT  06883-0284

     (203)-226-8800
     (203)-226-8851 (FAX)

     Compuserve: 70571,3651