Hardware Elements of Network Design

A network can be defined as the grouping of hardware devices and software components

which are necessary to connect devices within a geographical location, and to connect
them to other locations and the Internet.
Basic elements of a computer network include hardware, software, and protocols. The
interrelationship of these basic elements constitutes the infrastructure of the network.
A network infrastructure is the topology in which the nodes of a local area network (LAN) or
a wide area network (WAN) are connected to each other. These connections involve
equipment like routers, switches, bridges and hubs using cables (copper, fiber, and so on) or
wireless technologies (Wi-Fi).
If we think of a network as roads, highways, rails, and other means of transport, the network
protocols are the "traffic rules." The network protocols define how two devices in the
network communicate. The specification of the network protocols starts with the electrical
specifications of how a networking device is connected to the infrastructure. For example,
line voltage levels, carrier signals and the designation of which line might be used for what
types of signals all must be specified. Building up from there, network protocols include such
specifications as the methods that can be used to control congestion in the network and how
application programs will communicate and exchange data.
Network Connecting Devices
Hardware elements of network design: terminals, modems, multiplexers and
concentrators;
Various hardware components are used in a networking environment, prominent among them
are network interface cards (NIC), computers, routers, hubs, switches, printers, and cabling
and phone lines.
A variety of special devices are used to extend the size or improve the performance of
networks. Brief descriptions of the devices you may encounter are:
Repeaters
Repeaters are physical layer amplifying devices which allow the range of a LAN to be
extended by regenerating weak digital signals, see Figure 1. They may also allow a
change transmission media e.g. UTP to fibre.

Figure 1: Repeater function

In digital communication systems, a repeater is a device that receives a digital signal on

an electromagnetic or optical transmission medium and regenerates the signal along the
next leg of the medium. In electromagnetic media, repeaters overcome
the attenuation caused by free-space electromagnetic-field divergence or cable loss. A
series of repeaters make possible the extension of a signal over a distance but suffer from
propagation delay which can affect network communication when there are several
repeaters in a row. This is because repeaters require a small amount of time to regenerate
a signal.
A repeater connects two segments of your network cable. It retimes and regenerates the
signals to proper amplitudes and sends them to the other segments. When talking about,
ethernet topology, you are probably talking about using a hub as a repeater. Many
network architectures limit the number of repeaters that can be used in a row. Repeaters
work only at the physical layer of the OSI network model.
Due digital signals’ dependence on the presence or absence of voltage, they tend to
attenuate faster than analog signals thus requiring more frequent repeating. Whereas
analog signal amplifiers are spaced at 18,000 meter intervals, digital signal repeaters are
typically placed at 2,000 to 6,000 meter intervals.
In computer networking, because repeaters work with the actual physical signal, and do
not attempt to interpret the data being transmitted, they operate on the physical layer, the
first layer of the OSI model.
NB. In 10Base2 and 10Base 5 Ethernet only 4 in-line repeaters are allowed and the 3-4-5
rule applies i.e. only 3 segments occupied, see Figure 2.

Figure 2: Connecting segments with repeaters

Hubs
Hubs are really multi-port repeaters used in 10/100BaseT networks to allow UTP cable to
be wired in the STAR-BUS configuration, see Figure 3. Hybrid Hubs also enable media
conversion e.g. they may have BNC, RJ45, AUI and ST connections.

Figure 3: Hub network

Hub is a network hardware device for connecting multiple Ethernet devices together and

making them act as a single network segment. It has multiple input/output (I/O) ports, in
which a signal introduced at the input of any port appears at the output of every port
except the original incoming. A hub works at the physical layer (layer 1) of the OSI
model. Hubs are now largely obsolete, having been replaced by network switches except
in very old installations or specialized applications.
As a multiport repeater it works by repeating bits (symbols) received from one of its ports
to all other ports. It is aware of physical layer packets, that is it can detect their start
(preamble), an idle line (interpacket gap) and sense a collision which it also propagates by
sending a jam signal. A hub cannot further examine or manage any of the traffic that
comes through it: any packet entering any port is rebroadcast on all other ports. A
hub/repeater has no memory to store any data in – a packet must be transmitted while it is
received or is lost when a collision occurs (the sender should detect this and retry the
transmission). Due to this, hubs can only run in half duplex mode. Consequently, due to a
larger collision domain, packet collisions are more frequent in networks connected using
hubs than in networks connected using more sophisticated devices
The need for hosts to be able to detect collisions limits the number of hubs and the total
size of a network built using hubs (a network built using switches does not have these
limitations). For 10 Mbit/s networks built using repeater hubs, the 5-4-3 rule must be
followed.
Hub falls in two categories:
 Active Hub: They are smarter than the passive hubs. They not only provide the path
for the data signals in-fact they regenerate, concentrate and strengthen the signals
before sending them to their destinations. Active hubs are also termed as ‘repeaters’.
 Passive Hub: They are more like point contact for the wires to built in the physical
network. They have nothing to do with modifying the signals.
Bridges

Figure 4: Bridging between networks

Bridges are data link devices that switch frames between different layer 2 segments or cables.
They perform their switching in software, and their switching decisions are based on the
destination MAC address in the header of the data link layer frames. They perform three main
functions:

 They learn where devices are located by placing the MAC address of a NIC and the
identifier of the bridge port to which it is connected in a port address table.
 They forward traffic intelligently, drawing on information they have in their port
address table.
 They remove layer 2 loops by running the Spanning Tree Protocol (STP).
Learning Function
One of the three functions of a bridge is to learn which devices are connected to which port of
the bridge. The bridge then uses this information to switch frames intelligently. When bridge
receives a frame, it reads the source MAC address in the frame and compares it to a local
MAC address table, called a port address table. If the address is not already in this table, the
bridge adds the address and the port identifier or number on which the frame was received. If
the address is already in the table, the bridge resets the timer for the table entry. Entries in the
table remain there as long as the bridge sees traffic from them; otherwise, the bridge ages out
the old entries to allow room for newer ones.
Forwarding Function
To forward traffic intelligently, the bridge uses the port address table to help it find where
destinations are located. When a frame is received on a port, the bridge first performs its
learning function and then performs its forwarding function. The bridge examines the
destination MAC address in the frame header and looks for a corresponding entry in the port
address table. If it finds a matching entry, the frame is forwarded out of the specified port. If
the port is the same port on which the frame was received (the source and destination are
connected to the same port), the bridge drops the frame. If the bridge doesn’t find an entry, or
if the destination MAC address is a broadcast or multicast address, the bridge floods the
frame out all of the remaining ports.
Removing Loops

Consider Figure 5 to see the problem that layer 2 loops can cause.

Figure 5

An advantage of using two bridges to connect two segments together is that it creates
redundancy.
These loops do however create problems. We know that a bridge always floods traffic that
has a destination address that is an unknown unicast, a broadcast or a multicast address. This
traffic will continuously circle around the loop – may be forever. In Figure 5, assume a PC
generates a broadcast on Segment 1. When Bridge A and Bridge B receive the broadcast,
they flood it out all their remaining ports. This means that the same broadcast will appear
twice on Segment 2. Each bridge sees the other’s broadcast on Segment 2 and forwards this
back to Segment 1. This process will continue forever, wasting not only the bandwidth on
your LAN segments but also affects the CPU cycles of all devices on these segments, since
all NICs will accept the broadcast and pass it up the protocol stack for further processing.
With STP, one of the ports of the bridges in a loop is disabled in software. In Figure 5, this is
the port on Bridge B that is connected to Segment 2. Any user traffic is ignored if it is
received on this port and is not forwarded out of this port. In our assumption, if a PC on
Segment 1 generates a broadcast, both bridges, again, would receive it. Bridge A would flood
the broadcast on Segment 2, but Bridge B would not, since the port is in a blocked state.
Switches (Ethernet)
The main functions of bridges and switches is to solve bandwidth, or collision, problems.
Although both bridges and switches operate at the data link layer, there are many
differences between them with switches having many advantages, including the
following:
 Supports full-duplex to allow a device to send and receive simultaneously.
 Support for different Ethernet speeds on different switch ports, such as 10BaseT,
100BaseTX and Gigabit Ethernet.
 Dedicated connections between a router, PC or server to a port on a switch.
 Multiple, simultaneous session transmission between different switch ports.

Table 1: Bridge and Switch Comparison

Methods of Switching
The switching method affects how a layer 2 device receives, processes, and forwards a
frame. Bridges support only one switching method, store-and-forward, while switches
might support one, two, or three different switching methods. These methods include:
Store-and-forward
This is the most basic form of switching where the layer 2 device must pull in the entire
frame into the buffer of the inbound port and check the Field Check Sum (FCS) of the
frame before the layer 2 device will perform any additional processing of the frame. In
checking the FCS, commonly called the Cyclic redundancy Check (CRC), the layer 2
device will calculate a CRC value, just as the source did, and compare this value to what
was included in the frame. If they are the same, the frame is considered as good and the
layer 2 device can start processing the frame, including forwarding the frame out the
correct destination port of the layer 2 device. If the FCS value in the frame and the frame
value it computes are different, the layer 2 device will drop the frame.
Cut-Through
The switch reads only the first part of the frame before making switching decision. Once
the switch device reads the destination MAC address (8-byte preamble and 6-byte MAC
address), it begins forwarding the frame (even though the frame may still be coming into
the interface). This method is faster than store-and-forward. However, it may be
switching bad frames since the header could be legible, but the rest of the frame corrupted
from late collision.
Fragment-Free
This is a modified version of store-and-forward by ensuing that the frame is at least 64
bytes long before switching it (64 bytes is the minimum legal size of an Ethernet frame).
The goal is to reduce the number of Ethernet runt frames (frames less than 64 bytes) that
are switched. Corrupt frames could still be switched since it is just checking the first 64
bytes and the FCS is at the end of the frame.
Switching functions
Learning
A transparent bridge learns which device is connected to each of its active ports. As a
frame comes into the port of a switch, the switch examines the source MAC address of
the frame and compares it to its switch table commonly called content addressable
memory (CAM) table ( also called port or MAC address table).
When the switch receives a frame on a port, and as it examines the source MAC address
in the frame and doesn’t see a corresponding entry in the CAM table, the switch will add
the address to the table, including the source port identifier or number. If the address is
already in the table, the switch compares the incoming port with port already in the table.
If they are different, it updates the CAM table with the new port information. This is
important as the device might have been moved from one port to another on the same
switch.
Aging is used to age out old information in the CAM table to make room for new
connected devices.
Forwarding Function
Anytime a frame comes into a port on the switch, its destination MAC address is also
examined in order for it to perform its forwarding function. The destination MAC address
is compared to the addresses in the switch’s CAM table to determine which interface to
use in forwarding the frame to its destination.
If the destination address is found in the CAM table, the switch forwards the frame out
the port for the corresponding CAM entry. If the destination address is associated with the
same port as the source of the frame, it drops the frame. In this case, a hub might be
connected to this port of the switch, and both the source and the destination are connected
to this hub. In this scenario, the switch doesn’t forward any frames between these two
machines to other switch segments, since this would be wasting bandwidth in the
network. The switch is therefore intelligently forwarding traffic, thus creating a separate
bandwidth domain per port.
Three different Mac address destination types are available:
 Broadcast address: Destination MAC address of FFFF.FFFF.FFFF.
 Multicast address: Destination MAC addresses between 0100.5E00.0000 and
0100.5E7F.FFFF
 Unknown unicast destination MAC addresses: The destination MAC address is not
found in the CAM table
Routers
Routers are network layer devices that require a protocol (such as IP) to operate, see
Figure 6. They only pass packets of data to networks with the correct network address.
 Figure 6: Routing

A router basically has two functions:

1. To find a layer 3 path to a destination network
2. To move packets from one interface to another to get a packet to its destination
To accomplish the first function, a router will need to do the following:
 Learn about the routers to which it is connected to determine the networks that are
reachable
 Find locations of destination network numbers
 Choose a best path to each destination
 Maintain the most up-to-date routing information about how to reach destination
networks
To accomplish the second function, a router will need to examine the destination IP
address in an incoming IP packet, determine the network number of the destination, look
in its routing table, and switch the packets to an outgoing interface.
Types of Routes
A router can learn a route using one of two methods: static and dynamic.

Gateways
The term gateway is applied to any device, system, or software application that can perform
the function of translating data from one format to another. The key feature of a gateway is
that it converts the format of the data, not the data itself.
You can use gateway functionality in many ways. For example, a router that can route data
from an IPX network to an IP network is, technically, a gateway. The same can be said of a
translational bridge that converts from an Ethernet network to a Token Ring network and
back again.
Software gateways can be found everywhere. Many companies use an email system such as
Microsoft Exchange or Novell GroupWise. These systems transmit mail internally in a
certain format. When email needs to be sent across the Internet to users using a different
email system, the email must be converted to another format, usually to Simple Mail Transfer
Protocol (SMTP). This conversion process is performed by a software gateway.
Another good (and often used) example of a gateway involves the Systems Network
Architecture (SNA) gateway, which converts the data format used on a PC to that used on an
IBM mainframe or minicomputer. A system that acts as an SNA gateway sits between the
client PC and the mainframe and translates requests and replies from both directions.

Modems
Modem is a contraction of the terms modulator and demodulator. Modems perform a simple
function: They translate digital signals from a computer into analog signals that can travel
across conventional phone lines. The modem modulates the signal at the sending end and
demodulates at the receiving end.
Modems provide a relatively slow method of communication. In fact, the fastest modem
available on the market today has a maximum speed of 56Kbps. Compare that to the speed of
a 10Mbps network connection, and you’ll find that the modem is approximately 180 times
slower. That makes modems okay for browsing web pages or occasionally downloading
small files but wholly unsuitable for downloading large files. As a result, many people prefer
to use other remote access methods.
Modems are available as internal devices that plug into expansion slots in a system; external
devices that plug into serial or USB ports; PCMCIA cards designed for use in laptops; and
specialized devices designed for use in systems such as handheld computers. In addition,
many laptops now come with integrated modems. For large-scale modem implementations,
such as at an ISP, rack-mounted modems are also available.

Wireless Access Point (WAP)

Wireless access points, referred to as either WAPs or wireless APs, are a transmitter and
receiver (transceiver) device used for wireless LAN (WLAN) radio signals. A WAP is
typically a separate network device with a built-in antenna, transmitter, and adapter. WAPs
use the wireless infrastructure network mode to provide a connection point between WLANs
and a wired Ethernet LAN. WAPs also typically have several ports allowing a way to expand
the network to support additional clients.
Depending on the size of the network, one or more WAPs may be required. Additional WAPs
are used to allow access to more wireless clients and to expand the range of the wireless
network. Each WAP is limited by a transmissions range, the distance a client can be from a
WAP and still get a useable signal. The actual distance depends on the wireless standard
being used and the obstructions and environmental conditions between the client and the
WAP. Figure 7 shows an example of a WAP in a network configuration.
 Figure 7: An infrastructure wireless network uses a WAP

Firewalls
Today, firewalls are an essential part of a network’s design. A firewall is a networking
device, either hardware or software based, that controls access to your organization’s
network. This controlled access is designed to protect data and resources from outside threat.
To do this, firewalls are typically placed at entry/exit points of a network. For example, a
firewall might be placed between an internal network and the Internet. After the firewall is in
place, it can control access in and out of that point.
Although firewalls typically protect internal networks from public networks, they are also
used to control access between specific network segments within a network. For example,
you might place a firewall between the Accounts Department and the Sales Department.
As mentioned, firewalls can be implemented through software or through a dedicated
hardware device. Organizations implement software firewalls through network operating
systems (NOS) such as Linux/Unix, Windows servers, and Mac OS servers. The firewall is
configured on the server to allow or permit certain types of network traffic. In small offices
and for regular home use, a firewall is commonly installed on the local system and configured
to control traffic. Many third-party firewalls are available.
Hardware firewalls are used in networks of all sizes today. Hardware firewalls are often
dedicated network devices and can be implemented with very little configuration and protect
all system behind it from outside sources. Hardware firewalls are readily available and often
combined with other devices today. For example, many broadband routers and wireless
access points have firewall functionality built in. In such a case, the router or WAP may have
a number of ports available to plug systems into.