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Internetworking Technology

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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



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



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.


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.


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'


.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.


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


Forwarding    10M bps       1000 pkts     100 pkts           10 to 100 pkts

Performance   or (network   per second    per second         per second


Addressing    No address    MAC address   Network address    Network address


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


Transparent   Yes           Yes           No                 No

to Upper Layer


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