Computer Networks 15-1

Connecting Devices

Connecting Device

• We divide connecting devices into five different categories based on

the layer in which they operate in a network.

• Passive Hubs (below physical layer)

• Repeater/Active Hubs (at physical layer)

• Bridges/Two-Layer Switches (at physical and data link layer)

• Routers/Three-Layer Switches (at network layer)

• Gateways (at all layers)

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Five Categories of Connecting Devices

• Below the physical layer: passive hub

• At the physical layer: repeater or active hub

• At the physical and data link layers: bridge or two-layer switch

• At the physical, data link, network layers: router or three-layer switch

• At all five layers: gateway

Devices that interconnect LANs are known as relays and operate at one layer of OSI model.

There are different types of relays

Devices

• Connecting Devices

• Networking Devices

– Repeaters – Bridges

• InternetworkingDevices

– Routers – Gateways

Passive and Active hubs• A passive hub is just a connector. It connects the wires coming from different

branches.

• A central connecting device in a network that joins wires from several stations

in a star configuration. It does not provide any processing or regeneration of signals.

• These hubs are nothing more than point contacts for the wires that make up the physical network. An example of this would be a punchdown block that is a simple plastic, unpowered box used to plug network cables into it.

• Hubs are most commonly used in Ethernet 10BASE-T networks, although there are other network architectures that use them.

Some people use the terms concentrator when referring to a passive hub and multiport repeater when referring to an active hub.

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Repeater• A repeater operates only in the physical layers

• A repeater connects segments of a LAN

• Recieves a signal and regenerates the original bit pattern (signals). A repeater forwards every frame; it has no filtering capability

• The repeater is a two-port device that extends the LANs’ physical length

• It does not connects actually two LANs. It connects the same segments of the two LANs

.

Computer Networks

Use of Repeaters for a Multi-segment LAN

Repeater

Segment A

Segment B

Station

PrinterStation

Stations

File Server

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Function of a Repeater

• A repeater is a regenerator, not an amplifier

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Active Hubs• An active hub is actually a multiport repeater

• It is normally used to create connections between stations in a star topology

• Transmission from any station is received by all other station.– If two stations transmit at same time, there will be collision.

• Hubs can also be used to create multiple levels of hierarchy.

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Repeaters/Hubs

Bridges• When a frame (or packet) enters a bridge, the bridge not only regenerates the

signal but checks the destination address and forwards the new copy only to the segment the address belong.

– Controlling the congestion

– Used to isolate network traffic and computers

– Also provide the security through the partitioning the traffic

– Has the intelligent to examine incoming packet source and destination addresses

– Extracts destination address from the frame

– Looks up the destination in a table

– Forwards the frame to the appropriate LAN segment• The basic functionality of the bridge resides in its ability to make intelligent

decisions about whether to pass signals on to the next segment of a network

• Working

• Bridges work at the Media Access Control Sub-layer of the OSI model• Routing table is built to record the segment no. of address• If destination address is in the same segment as the source address, stop transmit• Otherwise, forward to the other segment

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Bridges…• A bridge operates in both physical and data link layers

• Physical Layer : regenerates the signal.• Data Link Layer : check MAC address.

• A bridge has filtering capability: Having a table used in filtering decisions• A bridge can check, does not change the physical (MAC) addresses in a frame

Types of bridges

• Simple

• Multiport

• Transparent

Simple

• Least expensive

• Links two segments

• The address table must be entered manually, before a simple bridge can

be used.

• Whenever a new station is added or removed, the table must modified.

• Installation and maintenance of simple bridges are time-consuming and

potentially more trouble than the cost savings are worth.

• If the station is removed the newly invalid address must be deleted.

•To select between segments, a bridge must have a look-up table that contains the physical addresses of every station connect to it. The table indicate to which segment each station belongs.

Mutiport Bridge

• Connects more than two LANs

• The bridge has three tables, each one holding the physical address of

stations reachable through the corresponding port

Transparent bridges– Transparent bridges are devices which connects more than one network segments

with other bridges to make all routing decisions. A transparent bridge is essentially used to learn the MAC addresses of all nodes and their associated port, to filter incoming frames whose destination MAC addresses are located on the same incoming port, and to forward incoming frames to the destination MAC.

– Criteria for a transparent bridge– Learning (actively listen to traffic on each segment on which it is attached. This is

why transparent bridges are often called 'learning bridges'. A transparent bridge is listening to see where each MAC has been connected to the network )

– Forwarding (When a transparent bridge encounters a frame that is to be forwarded to a destination MAC it forwards it out a specific port that it has associated with that MAC address. )

– Filtering (Frames are never forwarded out the port they are received on. When a bridge learns which segment a particular MAC address is on, it will ignore any frames transmitted by hosts ON that segment to the destination MAC that is on the same segment. All communication on the same segment between end nodes )

– Flooding If a bridge does not 'know' that MAC address (has no port associated with that MAC), it sends the frame out all the other ports on the bridge. Flooding is also performed when a multicast or broadcast frame is received.)

– Avoiding Loops

Learning Bridge

ROUTERS

• Routers are the hardware device and are more sophisticated than bridge

• They have access to network layer addresses and contain s/w which of the several possible path is best for transmission.

• It acts like a station in a network.

• Route packets based on IP addresses.

• Connects LANs and WANs.

• Use routing tables for making routing decisions.

• Routers have access to network layer addresses and contain software that enables them to determine which of several possible paths between those addresses is the best for a particular transmission.

• Routers operate in the physical, data link, and network layers of the OSI model.

A router in the OSI Model

• Routers relay packets among multiple interconnected networks. They route packets from one network to any of a number of potential destination networks on an internet.

Figure: Routers in an Internet

Routers connecting independent LANs and WANs

Routers

An Institutional Network Using Hubs, Ethernet Switches, and a Router

Interconnecting networks: Bridges versus Routers

Routers

• Each host’s IP address must be configured

• If network is reconfigured, IP addresses may need to be reassigned

• Routing done via RIP or OSPF

• Each router manipulates packet header (e.g., reduces TTL field)

Bridges/LAN switches

• MAC addresses of hosts are hardwired

• No network configuration needed

• Routing done by

– learning bridge algorithm

– spanning tree algorithm• Bridges do not manipulate frames

A Gateway in the OSI Model

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A network gateway is an internetworking system capable of joining together two networks that use different base protocols. A network gateway can be implemented completely in software, completely in hardware, or as a combination of both. Depending on the types of protocols they support, network gateways can operate at any level of the OSI model.

A Gateway

A gateway is a protocol converter. A router by itself transfers, accepts, and relays packets only across networks using similar protocols. A gateway can accept a packet formatted for one protocol (e.g. AppleTalk) and convert it to a packet for another protocol (e.g. TCP/IP).

•A gateway is generally software installed within a router. The gateway understands the

protocols used by each network linked into the router and is therefore able to translate from one

to another.

…

• Connects end-systems whose host protocols have varying degrees of

difference

• Transport gateways make a connection between two networks at the

transport layer.

• Application gateways connect two parts of an application in the application layer, e.g., sending email between two machines using different mail formats.

• Connect two networks above the network layer of OSI model.

• Are capable of converting data frames and network protocols into the

format needed by another network.

• Provide for translation services between different computer protocols.