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Network connecting devices Backbone networks Virtual LANs



1. Network Connecting Devices

We divide connecting devices into five different categories based on

the layer in which they operate in a network.



The five categories contain devices which can be defined as: 1. Those which operate below the physical layer such as a passive hub. 2. Those which operate at the physical layer (a repeater or an active hub).

Fig.1: Connecting Devices and the OSI Model

3. Those which operate at the physical and data link layers (a bridge or a

two-layer switch). 4. Those which operate at the physical, data link, and network layers (a router or a three-layer switch). 5. Those which can operate at all five layers (a gateway).
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Passive Hubs:- A passive hub is just a connector. It connects the wires coming from different branches. In a star-topology Ethernet LAN, a passive hub is just a point where the signals coming from different stations collide; the hub is the collision point. This type of a hub is part of the media; its location in the Internet model is below the physical layer. Repeaters:- A repeater is a device that operates only in the physical layer. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates the original bit pattern. The repeater then sends the refreshed signal. A repeater can extend the physical length of a LAN, as shown in Figure 2.
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Figure 2 : A repeater connecting two segments of a LAN

A repeater does not actually connect two LANs; it connects two

segments of the same LAN. The segments connected are still part of one single LAN. A repeater is not a device that can connect two LANs of different protocols. A repeater can overcome the 10Base5 Ethernet length restriction. In this standard, the length of the cable is limited to 500 m. To extend this length, we divide the cable into segments and install repeaters between segments.
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Note that- The whole network is still considered one LAN, but the

portions of the network separated by repeaters are called segments. The repeater acts as a two-port node, but operates only in the physical layer. When it receives a frame from any of the ports, it regenerates and forwards it to the other port. A repeater forwards every frame; it has no filtering capability. A repeater does not amplify the signal; it regenerates the signal. When it receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength. A repeater is a regenerator, not an amplifier. The location of a repeater on a link is vital. A repeater must be placed so that a signal reaches it before any noise changes the meaning of any of its bits. A little noise can alter the precision of a bit's voltage without destroying its identity (see Figure 3).
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Figure 3 :Function of a repeater

If the corrupted bit travels much farther, however, accumulated noise

can change its meaning completely. At that point, the original voltage is not recoverable, and the error needs to be corrected. A repeater placed on the line before the legibility of the signal becomes lost can still read the signal well enough to determine the intended voltages and replicate them in their original form.
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Active Hubs- An active hub is actually a multipart repeater.

It is normally used to create connections between stations in a

physical star topology. We have seen examples of hubs in some Ethernet implementations (lOBase-T, for example). However, hubs can also be used to create multiple levels of hierarchy, as shown in Figure 4. The hierarchical use of hubs removes the length limitation of 10BaseT (100 m).


Figure 4: A hierarchy of hubs


Bridges:- A bridge operates in both the physical and the data link layer.
As a physical layer device, it regenerates the signal it receives. As a data link

layer device, the bridge can check the physical (MAC) addresses (source and destination) contained in the frame. Filtering- One may ask, What is the difference in functionality between a bridge and a repeater? A bridge has filtering capability. It can check the destination address of a frame and decide if the frame should be forwarded or dropped. If the frame is to be forwarded, the decision must specify the port. A bridge has a table that maps addresses to ports.



Figure 5: A bridge connecting two LANs 10/12/2013

Bridges:-Connecting LANs while separating collision domains MAC addresses are used for filtering Connected segments form a single network (same broadcast domain)

Figure 5 : A bridge connecting two LANs

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(i) Transparent Bridges:- A transparent bridge is a bridge in which the

stations are completely unaware of the bridge's existence. If a bridge is added or deleted from the system, reconfiguration of the stations is unnecessary. According to the IEEE 802.1 d specification, a system equipped with transparent bridges must meet three criteria: I. Frames must be forwarded from one station to another. 2.The forwarding table is automatically made by learning frame movements in the network. 3. Loops in the system must be prevented.




(ii) Source Routing Bridges

In source routing, a sending station defines the bridges that the frame

must visit. The addresses of these bridges are included in the frame. In other words, the frame contains not only the source and destination addresses, but also the addresses of all bridges to be visited. The source gets these bridge addresses through the exchange of special frames with the destination prior to sending the data frame. Source routing bridges were designed by IEEE to be used with Token Ring LANs. These LANs are not very common today.




Switches: Similar to multiport bridges

Also run Spanning-Tree Protocol

A three-layer switch is used at the network layer; it is a kind of router. The twolayer switch performs at the physical and data link layers. A two-layer switch is a bridge, a bridge with many ports and a design that allows better (faster) performance. A bridge with a few ports can connect a few LANs together. A bridge with many ports may be able to allocate a unique port to each station, with each station on its own independent entity. This means no competing traffic (no collision, as we saw in Ethernet).
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Capable of connecting networks of different types
Routers separate networks into different broadcast domains. A router is a three-layer device that routes packets based on their

logical addresses (host-to-host addressing). A router normally connects LANs and WANs in the Internet and has a routing table that is used for making decisions about the route. The routing tables are normally dynamic and are updated using routing protocols.

Figure 7: Routers connecting independent LANs and WANs

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2. Backbone Networks
A backbone network allows several LANs to be connected.
In a backbone network, no station is directly connected to the backbone; o


the stations are part of a LAN, and the backbone connects the LANs. The backbone is itself a LAN that uses a LAN protocol such as Ethernet; each connection to the backbone is itself another LAN. Although many different architectures can be used for a backbone, the two most common: the bus and the star. Bus Backbone- In a bus backbone, the topology of the backbone is a bus. The backbone itself can use one of the protocols that support a bus topology such as lOBase5 or lOBase2. Bus backbones are normally used as a distribution backbone to connect different buildings in an organization. Each building can comprise either a single LAN or another backbone (normally a star backbone).
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A good example of a bus backbone is one that connects single- or

multiple-floor buildings on a campus. Each single-floor building usually has a single LAN. Each multiplefloor building has a backbone (usually a star) that connects each LAN on a floor. A bus backbone can interconnect these LANs and backbones.



Figure 8: Bus backbone


In Figure 8, if a station in a LAN needs to send a frame to another station in

the same LAN, the corresponding bridge blocks the frame; the frame never reaches the backbone. However, if a station needs to send a frame to a station in another LAN, the bridge passes the frame to the backbone, which is received by the appropriate bridge and is delivered to the destination LAN. Each bridge connected to the backbone has a table that shows the stations on the LAN side of the bridge. The blocking or delivery of a frame is based on the contents of this table. Star Backbone- Sometimes called a collapsed or switched backbone, the topology of the backbone is a star. In this configuration, the backbone is just one switch (that is why it is called, erroneously, a collapsed backbone) that connects the LANs. Figure 9 shows a star backbone. Note that, in this configuration, the switch does the job of the backbone and at the same time connects the LANs.
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Figure 9: Star backbone

Star backbones are mostly used as a distribution backbone inside a building.

In a multi floor building, we usually find one LAN that serves each particular floor. A star backbone connects these LANs. The backbone network, which is just a switch, can be installed in the basement or the first floor, and separate cables can run from the switch to each LAN.
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3. Connecting Remote LANs

Another common application for a backbone network is to connect

remote LANs. This type of backbone network is useful when a company has several offices with LANs and needs to connect them. The connection can be done through bridges, sometimes called remote bridges. The bridges act as connecting devices connecting LANs and point-topoint networks, such as leased telephone lines or ADSL lines. The point-to-point network in this case is considered a LAN without stations. The point-to-point link can use a protocol such as PPP.
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Figure 10: Connecting remote IANs with bridges



A station is considered part of a LAN if it physically belongs to that

LAN. The criterion of membership is geographic. What happens if we need a virtual connection between two stations belonging to two different physical LANs? We can roughly define a virtual local area network (VLAN) as a local area network configured by software, not by physical wiring.

Figure 11: A switch connecting three LANs

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4.1 Switch with VLAN Feature

Different VLANs belong to different broadcast domains They are considered isolated networks

Figure 12: A switch using VLAN software

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4.2 Multiple VLAN Switches

Figure 13: Two switches in a backbone using VLAN software

VLANs create broadcast domains.

VLANs group stations belonging to one or more physical LANs into

broadcast domains. The stations in a VLAN communicate with one another as though they belonged to a physical segment.
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We can define a VLAN characteristic:

(i) Membership- Port Numbers

Some VLAN vendors use switch port numbers as a membership

characteristic. For example, the administrator can define that stations connecting to ports 1, 2, 3, and 7 belong to VLAN 1; stations connecting to ports 4, 10, and 12 belong to VLAN 2; and so on. MAC Addresses- Some VLAN vendors use the 48-bit MAC address as a membership characteristic. For example, the administrator can stipulate that stations having MAC addresses E21342A12334 and F2A123BCD341belong to VLAN 1. IP Addresses-Some VLAN vendors use the 32-bit IP address as a membership characteristic. For example, the administrator can stipulate that stations having IP addresses,,, and belong to VLAN 1.
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(ii) Configuration- Manual Configuration- In a manual configuration, the

network administrator uses the VLAN software to manually assign the stations into different VLANs at setup. Later migration from one VLAN to another is also done manually. The term manually here means that the administrator types the port numbers, the IP addresses, or other characteristics, using the VLAN software.
Automatic Configuration - In an automatic configuration, the stations are

automatically connected or disconnected from a VLAN using criteria defined by the administrator.
Semiautomatic Configuration- A semiautomatic configuration is somewhere

between a manual configuration and an automatic configuration.




3. Communication Between Switches

In a multi switched backbone, each switch must know not only which

station belongs to which VLAN, but also the membership of stations connected to other switches.
Advantages Cost and Time Reduction- VLANs can reduce the migration cost of stations

going from one group to another.

Creating Virtual Work Groups- VLANs can be used to create virtual work

Security- VLANs provide an extra measure of security.
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Thank you for Listening !!

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