Lec. 4 Intermediate Devices

The network infrastructure contains three categories of network components:
• End devices
• Intermediary devices
• Network media
Devices and media are the physical elements, or hardware, of the network. Hardware comprises the components of the network platform that typically are visible, such as a laptop, PC, switch, router, wireless access point, or the cabling used to connect the devices. Occasionally, some network components may not be visible. In the case of wireless media, for example, messages are transmitted through the air using invisible radio frequency or infrared waves.
Network components are used to provide services and processes. These services and processes are the communication programs, called software, that run on the networked devices. A network service provides information in response to a request. Services include many of the common network applications people use every day, like email hosting services and web hosting services. Processes provide the functionality that directs and moves the messages through the network. Processes are less obvious to us but are critical to the operation of networks.
End Devices
The network devices that people are most familiar with are called end devices, or hosts. These devices form the interface between users and the underlying communication network.
Some examples of end devices are
• Computers (work stations, laptops, file servers, web servers)
• Network printers
• VoIP phones
• TelePresence endpoints
• Security cameras
• Mobile handheld devices (such as smartphones, tablets, PDAs, and wireless debit/credit card readers and barcode scanners)
A host device is either the source or destination of a message transmitted over the network. In order to distinguish one host from another, each host on a network is identified by an address. When a host initiates communication, it uses the address of the destination host to specify where the message should be sent.
In modern networks, a host can act as a client, a server, or both. Software installed on the host determines which role it plays on the network. Servers are hosts that have software installed that enables them to provide information and services, like email or web pages, to other hosts on the network. Clients are hosts that have software installed that enables them to request and display the information obtained from the server.
Intermediary Devices
Intermediary devices interconnect end devices. These devices provide connectivity and work behind the scenes to ensure that data flows across the network. Intermediary devices connect the individual hosts to the network and can connect multiple individual networks to form an internetwork.

Examples of intermediary network devices are
• Network access devices (switches and wireless access points)
• Internetworking devices (routers)
• Security devices (firewalls)
The management of data as it flows through the network is also a role of the intermediary devices. These devices use the destination host address, in conjunction with information about the network interconnections, to determine the path that messages should take through the network.
Processes running on the intermediary network devices perform these functions:
• Regenerate and retransmit data signals
• Maintain information about which pathways exist through the network and internetwork
• Notify other devices of errors and communication failures
• Direct data along alternate pathways when there is a link failure
• Classify and direct messages according to QoS priorities
• Permit or deny the flow of data, based on security settings
Network Media
Communication across a network is carried on a medium. The medium provides the channel over which the message travels from source to destination.
Modern networks primarily use the following three types of media to interconnect devices and to provide the pathway over which data can be transmitted:
• Metallic wires within cables
• Glass or plastic fibers (fiber-optic cable)
• Wireless transmission