Overview of basic types of Network Devices

This post gives a brief introduction to the very basic types of network devices.

Physical Layer Modems, Repeaters, Hubs, Layer 2 Switches and Routers are the main network devices typically used in a network. The role of each devices are given below:

Physical Layer Modems :

These are line encoding/decoding devices, used to send information in the form of electromagnetic/optical signals over a wide variety of wired/wireless physical media. On the sending end, they convert digital bits into signals and on the receiving end they convert back the signals into stream of bits. Examples include DSL/Cable/Optical/Wireless Modems. Modems are typically used at the end nodes or customer premises, to connect to the Internet.

Repeaters/Hubs :

These are intermediate physical layer devices, used to extend the span of a single LAN/WAN. They connect multiple segments of a LAN/WAN and basically repeat or amplify the signals that they receive on one port onto other ports. Additionally, intelligent repeaters, that understand the line encoding techniques, have the capability to generate fresh signals, instead of blindly amplifying the incoming signals, thereby enabling filtering of different types of noises introduced in the telecommunication links.

Layer 2 Switches/ Bridges :

These devices, like repeater/hubs, are used within the same LAN/WAN network. A bridge or a Layer 2 switch, operates at the data link layer and is used to bridge/switch data link layer frames between different segments of the same LAN or WAN. The bridging or switching of frames are based on the MAC address of the destination node. The different segments of a LAN/WAN would usually be of the same physical media type, though there are exceptions, where special devices known as translation bridges are used in scenarios where the same LAN/WAN has different types of physical media segments (e.g. one segment is Ethernet and another segment is Token Ring).
There are a wide variety of Layer 2 switches, based on the number of ports, the physical link type, the speed of the ports and the different types of features that they support.

Both Layer 2 Switches and Bridges are one and the same, functionally, though in earlier days,they were slightly different.  In earlier days, bridges were implemented in software and  Layer2 switches were implemented in hardware. Moreover bridges were used to connect different segments (bus) of a LAN/WAN, whereas a Layer 2 switch had individual point to point connections to each end devices. Nowadays, these distinctions are obsolete and there is just one type of device at the data link layer named as Layer 2 switch, which does bridging/switching functionality.

Routers (Layer 3 Switches):

Routers are Network layer (layer 3) devices used to interconnect two different physical networks. These two networks may either be different LAN networks or different WAN networks or a LAN and a WAN network. The whole internet can be treated logically as a collection of millions of individual networks (LANs/WANs), joined together by a huge collection of different types of routers. Each individual network is called as a subnet in internet terms and is analogous to a separate region (ZIP/PIN code) in the postal domain. So, the router is a device that connects different Layer 3 subnets. Each Layer 3 Subnet has a different range of logical address space (IPV4/IPV6 addresses). The router basically deals with layer 3 datagrams/packets and routes packets from one of its input port to another output port, based on the packet’s destination IP address. If the routing is entirely done in hardware, without the intervention of software, then such a router is termed as a Layer 3 switch.

Just like Layer 2 switches, routers also can be of a wide variety of types like low-end, mid-end, high-end, access end, core end routers. The classification can be based on the number of ports, the type of ports (ethernet, optical etc. ), the speed of the ports, the processing capacity of the router (number of packets per second or pps), the type of features supported by the router (e.g. different types of routing protocols like RIP, OSPF, BGP etc., support for QOS, support for Security etc. ).

Other special network devices :

Apart from Routers and Layer 2 switches, we can have different special  types of  hardware devices like Dedicated Firewall/intrusion Detection/Intrusion Prevention boxes (used for protecting networks), Layer 4 Switches (if the transport layer processing can be done in hardware), Layer 7 switches (if the application layer processing itself can be done in hardware e.g. HTTP gateway), Voice over IP (VOIP) boxes (that sends analog voice calls inside IP packets), SAN/NAS boxes (that understands storage area networking protocols and connects end nodes to storage devices), IPSEC boxes (implementing IP Security/encryption in hardware) etc.

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