Ref: 99980076
Title: Internetworking Technology
Date: 12/1/87

Copyright 3Com Corporation, 1991.  All rights reserved.

Internetwork products play an important role in LAN technology
because they allow LANs to be interconnected to extend their
length, to combine two physical networks into one logical
network, or to connect networks with different technologies.

Internetworking products can be divided into four categories:
Repeaters, Bridges, Routers, and Gateways.  The following
sections describe in more detail the functionality of these
products.

.h1;Repeaters

Repeaters are used to extend the length of a single network.
Repeaters are the least sophisticated of the internetworking
products in that they operate at the physical layer.  They
receive all data and repeat or retransmit the data on the other
segment.  Their operating speeds are dependent on the local
network speed.  Repeaters are typically hardware devices and do
not contain any network management functions.  There are
generally hardware limitations on the number of repeaters allowed
in a system.  For example, in a standard IEEE 802.3 Ethernet
network, a maximum of four repeaters can be installed to form a
network.

.h1;Bridges

Bridges operate at the second level of the OSI network
architecture (Data Link Layer) and are totally transparent to
network users.

Bridges interconnect LANs of similar or dissimilar type to create
an "Extended LAN."  Because bridges are "protocol independent,"
protocols such as XNS, TCP/IP, DECNet, or ISO can simultaneously
run on the extended LAN.

Bridges provide much more than just protocol independence; they
feature localized traffic, auto configuration, integrated network
management, generalized filtering capability, and high-
performance throughput.

Bridges do not retransmit all traffic like a repeater.  Bridges
receive all packets on an attached network.  They filter
(discard) any packets whose destination is on that network and
forward (regenerate) only the packets destined for the remote
side.  In this way, bridges route packets based strictly on the
Destination Address in the data link frame; they do not depend on
any routing protocol.  Network devices on each side of a bridge
simply address the packets to other devices as if they were on
the same LAN; the process of forwarding packets to the remote LAN
are transparent to the devices.  Bridges do not depend on any
other protocol beyond the link layer addresses (i.e., they are
"protocol independent").

A smart bridge "learns" which devices are on which side by
monitoring the Source Addresses of all packets.  The bridge
builds a routing table based on the packet's Source Address and
the network from which it was received.  Thus, whenever a device
transmits a packet, its location is detected by the bridge and an
entry is made in the routing table.  This "learned" table is then
used for the filtering and forwarding operations.  Some bridges
may not support "learning", in which case the table is configured
manually by a network administrator.

.h1;Routers

Routers are also used in local and remote networks to form an
extended LAN.  They allow the user to build complex internets.
Routers operate on the third OSI layer (Network Layer), which
provides information for routing packets through an internetwork.
Network devices with different protocols at layer three cannot
communicate via routers.  So, whereas devices of different
architecture (XNS, TCP/IP, DECNet) may share the transmission
capacity of a LAN, they may not be able to share the service
provided by a router.  The key advantage of routers is that the
network layer protocol allows more sophisticated routing (e.g.,
alternate routing and load balancing between parallel paths).
Routers require a different network ID between them to
distinguish a local from a remote network.

.h1;Gateways

Gateways are used to interconnect networks with different
protocol architectures.  Gateways operate at the higher layers
(above the network layer) of the OSI model.  They translate
protocols from one architecture to the other.  Gateways are
complex products and are required to perform substantial protocol
processing.  Examples of Gateways are XNS to SNA (Bridge
Communications' CS/1-SNA) or XNS to X.25 (Bridge Communications'
GS/1-X.25).

.h1;OSI Model and Internetworking Products

In the OSI model, the first layer is the Physical Layer, which
defines the rules (such as electrical level and physical
characteristics) for communicating across the actual transmission
media.  It also specifies the means of joining two able segments
together by a repeater.  A repeater merely accepts databits on
one side and retransmits them to the other side.  In the process,
it enables the original signals to travel a longer distance.  In
the Ethernet specification, a single segment is limited to 500
meters.  Using repeaters, Ethernet can be extended to 1,500
meters (using a maximum or two repeaters according to Ethernet
version 1.0) or 2,800 meters (using a maximum of two repeaters
according to Ethernet Version 1.0) or 2,800 meters (using a
maximum of four repeaters, per Ethernet Version 2.0 or IEEE
802.3).  However, the connected segments are one physical LAN and
all traffic is present on each segment.

OSI layer two is the Data Link Layer.  In the data link layer,
databits are grouped as packets.  Each packet contains a Source
Address and a Destination Address.  The rules at this level
define how these packets can access the physical media (i.e.,
packet type information and Logical Link Control information).
Bridges operate at this layer to interconnect two physically
distinct LANs to form an extended LAN.

OSI layer three is the Network Layer.  The network layer contains
the rules defined for interconnecting networks (LANs are a subset
of general networks) to form an internet.  In particular, network
addresses such as IP addresses and XNS network addresses are used
to distinguish one LAN from another.  Network routers join one
LAN to another.  LANs connectd by a router are physically and
logically separated networks, whereas LANs connected by a bridge
are physically different networks but logically the same network
(they share the same network address).

The network layer also contains the rules for passing routing
information and making routing decisions (i.e., the requirements
for internetworking and routing protocols such as XNS's IDP and
RIP).  Each network device wanting to use a router must
participate in the same internet protocol and explicitly address
the router to forward any internet packets.  A router simply
forwards any incoming packets without monitoring or filtering
traffic.  Routers can be used to build an internet with a complex
network topology containing many LANs with multiple communication
paths and loops.  Each network or LAN within an internet can
support its own network management functions without interfering
with others.  Thus, routers are used in different applications
from bridges; just as bridges are used differently from
repeaters.  They provide interconnection of LANs at different
levels and provide different functions and capabilities.

COMPARISON AMONG REPEATERS, BRIDGES, ROUTERS, AND GATEWAYS

Functions     Repeater      Bridge        Router             Gateway
------------------------------------------------------------------------------
OSI Layer     Physical      Data Link     Network            Above network

Filter        No            Yes           Not required       Not required
Capability                                (receives intenet
                                          traffic only)

Forward       Forward bits  Forwards      Forwards           Forwards messages
Capability                  data link     internet packets
                            packets

Forwarding    10M bps       1000 pkts     100 pkts           10 to 100 pkts
Performance   or (network   per second    per second         per second
              speed)

Addressing    No address    MAC address   Network address    Network address
Function

Ethernet      One physical  One logical   Multiple logical   One physical LAN
Extension     LAN w/ max    LAN w/ multi  LAN w/ unlimited
              2,800m        Ethernets     distance

Configuration Linear        Tree type     Arbitrary          Arbitrary
              topology      (no loops)    topology           topology

Network       No            Yes           Yes                Yes
Management

Transparent   Yes           Yes           No                 No
to Upper Layer
Protocol

Cost          Low           Medium        Medium             Medium
              $1,000-2,500  ~$10,000      $5,000-10,000      $10,000-20,000

Applications  Interconnects Localizes     Complex internet   Protocol
              Ethernet      traffic,                         Conversion
              segments      mixed media




