Networking Components and Devices


Introduction
All but the most basic of networks require devices to provide connectivity and functionality. understanding how these networking devices operate and identifying the functions they perform are essential skills for any network administrator and are requirements for a Network+ candidate. This chapter introduces commonly used networking devices. Although it is true that you are not likely to encounter all the devices mentioned in this chapter on the exam, you can be assured of working with at least some of them.


Networking Devices
  • Hub, Repeater, Modem, Network Interface Card (NIC), Media converters, Basic switch, Bridge, Wireless access point, Basic router, Basic firewall, Basic DHCP server, Multilayer switch, Bandwidth shaper

Hub

At the bottom of the networking food chain, so to speak, are hubs. Hubs are used in networks that use twisted-pair cabling to connect devices. Hubs also can be joined to create larger networks. Hubs are simple devices that direct data packets to all devices connected to the hub, regardless of whether the data package is destined for the device. This makes them inefficient devices and can create a performance bottleneck on busy networks.

In its most basic form, a hub does nothing except provide a pathway for the electrical signals to travel along. Such a device is called a passive hub. Far more common nowadays is an active hub, which, as well as providing a path for the data signals, regenerates the signal before it forwards it to all the connected devices. In addition, an active hub can buffer data before forwarding it. However, a hub does not perform any processing on the data it forwards, nor does it perform any error checking.

Multistation Access Unit

In a token ring network, a multistation access unit (MSAU) is used in place of the hub that is used on an Ethernet network. The MSAU performs the token circulation inside the device, giving the network a physical star appearance. It functions as a logical ring. The logical ring function is performed from within the MSAU. Each MSAU has a ring in (RI) port on the device, which is connected to the ring out (RO) port on another MSAU. The last MSAU in the ring is then connected to the first to complete the ring. Because token ring networks are few and far between nowadays, it is far more likely that you will find yourself working with Ethernet hubs and switches.

Switches



Like hubs, switches are the connectivity points of an Ethernet network. Devices connect to switches via twisted-pair cabling, one cable for each device. The difference between hubs and switches is in how the devices deal with the data they receive. Whereas a hub forwards the data it receives to all the ports on the device, a switch forwards it to only the port that connects to the destination device. It does this by learning the MAC address of the devices attached to it and then by matching the destination MAC address in the data it receives. By forwarding data to only the connection that should receive it, the switch can greatly improve network performance. By creating a direct path between two devices and controlling their communication, the switch can greatly reduce the traffic on the network and therefore the number of collisions. As you might recall, collisions occur on Ethernet networks when two devices attempt to transmit at exactly the same time. In addition, the lack of collisions enables switches to communicate with devices in full-duplex mode. In a full-duplex configuration, devices can send data to and receive data from the switch at the same time. Contrast this with half-duplex communication, in which communication can occur in only one direction at a time. Full-duplex transmission speeds are double that of a standard half-duplex connection. So, a 10Mbps connection becomes 20Mbps, and a 100Mbps connection becomes 200Mbps.

Bridges

Bridges are used to divide larger networks into smaller sections. Bridges accomplish this by sitting between two physical network segments and managing the flow of data between the two. By looking at the MAC address of the devices connected to each segment, bridges can elect to forward the data (if they believe that the destination address is on another interface) or block it from crossing (if they can verify that it is on the interface from which it came).
When bridges were introduced, the MAC  addresses of the devices on the connected networks had to be entered manually. This was a time-consuming process that had plenty of opportunity for error.  Today, almost all bridges can build a list of the MAC addresses on an interface by watching the traffic on the
network. Such devices are called learning bridges because of this functionality.

Types of Bridges


Three types of bridges are used in networks:
Transparent bridge: Derives its name from the fact that the devices on the network are unaware of its existence. A transparent bridge does nothing except block or forward data based on the MAC address.
Source route bridge: Used in token ring networks. The source route bridge derives its name from the fact that the entire path that the packet is to take through the network is embedded in the packet.
Translational bridge: Used to convert one networking data format to another, such as from token ring to Ethernet and vice versa.


Routers


In a common configuration, routers are used to create larger networks by joining two network segments. A small office, home office (SOHO) router is used to connect a user to the Internet. A SOHO router typically serves 1 to 10 users on the system. A router can be a dedicated hardware device or a computer system with more than one network interface and the appropriate routing software. All modern network operating systems include the functionality to act as a router.A router derives its name from the fact that it can route data it receives from one network to another. When a router receives a packet of data, it reads the packet’s header to determine the destination address. After the router has determined the address, it looks in its routing table to determine whether it knows how to reach the destination; if it does, it forwards the packet to the next hop on the route. The next hop might be the final destination, or it might be another router.

Gateways

Any device that translates one data format into another is called a gateway. Some examples of gateways include a router that translates data from one network protocol into another, a bridge that converts between two networking systems, and a software application that converts between two dissimilar formats. The key point about a gateway is that only the data format is translated, not the data itself. In many cases, the gateway functionality is incorporated into another device.Don’t confuse a gateway with the term default gateway. The term default gateway refers to a router to which all network transmissions not destined for the local network are sent. Don’t confuse a gateway  with the term default gateway. The term default gateway refers to a router to which all network transmissions not destined for the local network are sent.


Network Cards



A network card, also called a network interface card (NIC), is a device that enables a computer to connect to the network. When specifying or installing a NIC, you must consider the following issues:

System bus compatibility: If the network interface you are installing is an internal device, bus compatibility must be verified. The most common bus system in use is the Peripheral Component Interconnect (PCI) bus, but some older systems might still use Industry Standard Architecture (ISA) expansion cards.
System resources: Network cards, like other devices, need Interrupt Request (IRQ) and memory I/O addresses. If the network card does not operate correctly after installation, there might be a device conflict.
Media compatibility: Today, the assumption is that networks use twisted- pair cabling, so if you need a card for coaxial or fiber-optic connections, you must specify this. Wireless network cards are also available.

 Types of Network Interfaces


Network interfaces come as add-in expansion cards or as PCMCIA cards used in laptop systems. In some cases, rather than having an add-in NIC, the network interface is embedded into the motherboard.


A network interface typically has at least two LEDs that indicate certain conditions:
Link light: This LED indicates whether a network connection exists between the card and the network. An unlit link light indicates that something is awry with the network cable or connection.

Activity light: This LED indicates network activity. Under normal conditions, the light should flicker sporadically and often. Constant flickering might indicate a very busy network or a problem somewhere on the network that is worth investigating.

Speed light: This LED indicates that the interface is connected at a certain speed. This feature normally is found on Ethernet NICs that operate at 10Mbps/100Mbps—and then only on certain cards.
Wireless Access Points


Wireless access points (APs) are a transmitter and receiver (transceiver) device used to create a wireless LAN (WLAN). APs typically are a separate network device with a built-in antenna, transmitter, and adapter. APs use the wireless infrastructure network mode to provide a connection point between WLANs and a wired Ethernet LAN. APs also typically have several ports, giving you a way to expand the network to support additional clients.
Depending on the size of the network, one or more APs might be required. Additional APs are used to allow access to more wireless clients and to expand the range of the wireless network. Each AP is limited by a transmission range the distance a client can be from an AP and still get a usable signal. The actual distance depends on the wireless standard being used and the obstructions and environmental conditions between the client and the AP.


Modems


A modem, short for modulator/demodulator, is a device that converts the digital signals generated by a computer into analog signals that can travel over conventional phone lines. The modem at the receiving end converts the signal back into a format that the computer can understand. Modems can be used as a means to connect to an ISP or as a mechanism for dialing up a LAN. Modems can be internal add-in expansion cards or integrated with the motherboard, external devices that connect to a system’s serial or USB port, PCMCIA cards designed for use in laptops, or proprietary devices designed for use on other devices, such as portables and handhelds. The configuration of a modem depends on whether it is an internal or external device. For internal devices, the modem must be configured with an interrupt request (IRQ) and a memory I/O address. It is common practice, when installing an internal modem, to disable the built-in serial interfaces and assign the modem the resources of one of them (typically COM2). Table shows the resources associated with serial (COM) port assignments. Which Given Below.




Common Serial (COM) Port Resource Assignments



Port ID
IRQ
I/O Address
Associated Serial Interface Number
COM1
4
03F8
1
COM2
3
02F8
2
COM3
4
03E8
1
COM4
3
02E8
2





For external modems, you need not concern yourself directly with these port assignments, because the modem connects to the serial port and uses the resources assigned to it. This is a much more straightforward approach and one favored by those who work with modems on a regular basis. For PCMCIA and USB modems, the plug-and-play nature of these devices makes them simple to configure, and no manual resource assignment is required. After the modem is installed and recognized by the system, drivers must be configured to enable use of the device.


Two factors directly affect the speed of the modem connection the speed of the modem itself and the speed of the Universal Asynchronous Receiver/Transmitter (UART) chip in the computer that is connected to the modem. The UART chip controls a computer’s serial communication. Although modern systems have UART chips that can accommodate far greater speeds than the modem is capable of, older systems should be checked to make sure that the UART chip is of sufficient speed to support the modem speed. Normally you can determine which UART chip is installed in the system by looking at the documentation that comes with the system.


UART Chip Speeds



UART Chip
Speed (Kbps)
8250
9600
16450
9600
16550
115,200
16650
430,800
16750
921,600
16950
921,600




Firewalls


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 an outside threat. To do this, firewalls typically are placed at a network’s entry/exit points—for example, between an internal network and the Internet. After it is in place, a firewall can control access into and out of that point.


As mentioned, firewalls can be implemented through software or through a dedicated hardware device. Organizations implement software firewalls through network operating systems (NOSs) such as Linux/UNIX, Windows servers, and Mac OS servers. The firewall is configured on the server to allow or block certain types of network traffic. In small offices and for regular home use, a firewall is commonly installed on the local system and is configured to control traffic.  Many third-party firewalls are available.


DHCP Server



Without question, the easiest way to assign TCP/IP information to client systems is to use a Dynamic Host Configuration Protocol (DHCP) server. On a network running TCP/IP, each computer must have a unique IP address in order to be recognized and be part of the network. Briefly, a protocol is a method of communicating between computers.

Computers on a network using TCP/IP require specific network settings to be able to connect to the network. First among these settings is the IP address. An IP address consists of four octets, or four sets of 8 bits—for example, 192.168.2.1. Each computer on the network must have one of these numbers in order to perform network functions through TCP/IP. The number must be unique to the PC and must be within a certain range to allow the PC to connect to other systems. In larger networks, the assignment of manual addresses can be a nightmare, especially when IP addressing schemes can be changed and computers can be moved, retired, or replaced. That’s where DHCP comes in. DHCP assigns IP addresses, eliminating the need to assign IP addresses individually and making the job of network administrators considerably easier. When a DHCP server is running on a network, the workstation boots up and requests an IP address from the server. The server responds to the request and automatically assigns an IP address to the computer for a given period of time, known as a lease. The workstation acknowledges the receipt of the IP address, and the workstation has all the information it needs to become part of the network. This communication between the server and the workstation happens completely automatically and is invisible to the computer user.




Repeaters
Data signals weaken as they travel down a particular medium. This is known as attenuation. To increase the distance a signal can travel,  you can use repeaters. Repeaters increase the cable’s usable length and are commonly used with coaxial network configurations. Because coaxial networks have fallen out of favor, and because the functionality of repeaters has been built in to other devices, such as hubs and switches, repeaters are rarely used as an independent device.

Specialized Network Devices


Any network is composed of many different pieces of hardware. Some, like firewalls and DHCP servers,are in most networks. Other devices are more specialized and are not found in every network environment. CompTIA lists the following as specialized networking devices:



  • Multilayer and content switch

  • IDS and IPS

  • Load balancer

  • Multifunction network devices

  • DNS server

  • Bandwidth shaper

  • Proxy server

  • CSU/DSU

Multilayer and Content Switches


It used to be that networking devices and the functions they performed were pretty much separate. We had bridges, routers, hubs, and more, but they were separate devices. Over time, the functions of some individual network devices became integrated into a single device. This is true of multilayer switches. A multilayer switch is one that can operate at both Layer 2 and Layer 3 of the OSI model, which means that the multilayer device can operate as both a switch and a router. Also called a Layer 3 switch, the multilayer switch is a high-performance device that actually supports the same routing protocols that routers do. It is a regular switch directing traffic within the LAN; in addition, it can forward packets between subnets.





Intrusion Detection and Prevention Systems



Administrators can use several methods to help secure the network. In addition to a firewall, an intrusion detection system (IDS) and intrusion prevention system (IPS) can be used. Both are designed to help identify unwanted network access and traffic; however, they work in slightly different ways. An IDS is either a hardware- or software-based device that constantly monitors inbound and outbound network traffic. The IDS uses built-in parameters to flag and document any traffic it determines to be suspicious or potentially dangerous. But that is where the IDS stops. It does not actively try to manage the threat. Instead, it identifies the threat, and then the administrator must monitor the IDS to see what the problem might be. Although it doesn’t try to fix the potential threat, the IDS can be configured to send an alert to the administrator, notifying him or her of a potential threat and security breach.



An IDS can be deployed as a host-based (resident to a single system) or networkbased (watches all network traffic) device. In either case, an IDS cannot replace a firewall, because they have different functions. The firewall monitors secured access between two networks such as a business and the Internet and prevents unwanted traffic from entering the network. The IDS inspects an intrusion after it has taken place—that is, after it has passed the firewall. An IDS also watches for threats from within the network while the firewall operates on the network perimeter.



Load Balancer



Network servers are the workhorses of the network. They are relied on to hold and distribute data, maintain backups, secure network communications, and more. The load of servers is often a lot for a single server to maintain. This is where load balancing comes into play. Load balancing is a technique in which the workload is distributed between several servers. This feature can take networks to the next level; it increases network performance, reliability, and availability.



Multifunction Network Devices



It used to be that each device on a network (firewall, router, repeater, hub, to name a few) had its own purpose. It wasn’t long before the functions of these individual devices were combined into single units, creating multifunction network devices. Consider a high-speed cable modem used by home users or small companies to access the Internet. These are multifunction network devices that have combined functionality, including firewall, DHCP server, wireless access point, switch, and router. Networks are full of multifunction devices, including switches, routers, servers, and more.



Multifunction devices offer some advantages over multiple independent devices or software packages. Suppose an organization maintains antivirus, firewall, content filtering, and IDS/IPS software on a single server or even several servers. This organization must pay for the software on each of the servers, the operating system, and the personnel to maintain the systems. All of this can be simply replaced with a single multifunction network device.



DNS Server



A Domain Name System (DNS) server performs a relatively basic, but vital, role for many organizations. The function of a DNS server is relatively simple in that it provides name resolution from hostnames to IP addresses. The measures to which the server goes to provide a successful resolution, however, are not so simple. As well as consulting its own databases for the requested information, a DNS server contacts other DNS servers as needed to get the necessary information. This process might involve a large number of queries.



As you may know, each device on a network requires a unique IP address so that it can provide services to clients. Rather than rely on flawed human memory to remember these addresses, DNS allows us to use easy-to-remember hostnames, such as comptia.org, to access these hosts. When we type www.comptia.org into a web browser, our configured DNS server takes the request and searches through a system of servers to find the correct TCP/IP address that relates to www.comptia.org. After the DNS server has ascertained the correct TCP/IP address, that address is returned to the client, which then contacts the IP address directly. To speed up subsequent requests for the same address, the DNS server adds the address to its cache. For a workstation to send requests to the DNS server, the TCP/IP address of the DNS server must be provided to the workstations. This can be done manually, or the address can be included in the information supplied by a DHCP (Dynamic Host Configuration Protocol) server.



Before DNS was used, resolution of hostnames to IP addresses was (and still is in some cases) performed through static text files called HOSTS files. These text files quickly became too large to manage easily and therefore were replaced by DNS.



The function of DNS remains largely hidden from most users, but our reliance on the system is amazingly

high. In January 2001, a Microsoft employee made a configuration change to one of Microsoft’s DNS  ervers. The change caused an error that rendered some Microsoft-hosted websites, including the popular Hotmail system, inaccessible for a number of hours. The servers were up and running, but they simply could not be reached.



Most common operating systems provide the capability to act as a DNS server. Some implementations are more sophisticated than others, but the basic principle of hostname-to-TCP/IP address resolution remains the same.



The amount of computing power required by a DNS server is proportional to the number of DNS requests that it will handle. Within an organization, records might be configured for only a relatively small number of hosts, and there might be only a small number of client requests. In such an environment, it would be unlikely to have a server dedicated to DNS functions. In contrast, a DNS server for an Internet service provider would need to be powerful enough to accommodate perhaps millions of requests per hour.



Bandwidth Shaper



Bandwidth Shaper
The demand for bandwidth on networks has never been higher. Internet and intranet applications demand a large amount of bandwidth. Administrators have to ensure that despite all these demands, adequate bandwidth is available for mission-critical applications while few resources are dedicated to spam or peerto- peer downloads. To do this, you need to monitor network traffic to ensure that data is flowing as you need it to. The term bandwidth shaping describes the mechanisms used to control bandwidth usage on the network. With this, administrators can control who uses bandwidth, for what purpose, and what time of day bandwidth can be used. Bandwidth shaping establishes priorities for data traveling to and from the Internet and within the network. A bandwidth shaper, essentially performs two key functions— monitoring and shaping. Monitoring includes identifying where bandwidth usage is high and the time of day. After that information is obtained, administrators can customize or shape bandwidth usage for the best needs of the network.



Proxy Server



Proxy servers typically are part of a firewall system. In fact, they have become so integrated with firewalls that the distinction between the two can sometimes be lost.



However, proxy servers perform a unique role in the network environment—a role that is very separate from that of a firewall. For the purposes of this book, a proxy server is defined as a server that sits between a client computer and the Internet, looking at the web page requests the client sends. For example, if a client computer wants to access a web page, the request is sent to the proxy server rather than directly to the Internet. The proxy server first determines whether the request is intended for the Internet or for a web server locally. If the request is intended for the Internet, the proxy server sends the request as if it originated the request. When the Internet web server returns the information, the proxy server returns the information to the client. Although a delay might be induced by the extra step of going through the proxy server, the process is largely transparent to the client that originated the request. Because each request a client sends to the Internet is channeled through the proxy server, the proxy

server can provide certain functionality over and above just forwarding requests.



One of the biggest of these extra features is that proxy servers can greatly improve network performance through a process called caching. When a caching proxy server answers a request for a web page, the server makes a copy of all or part of that page in its cache. Then, when the page is requested again, the proxy server answers the request from the cache rather thangoing back to the Internet. For example, if a client on a network requests the web page www.comptia.org, the proxy server can cache the contents of that web page. When a second client computer on the network attempts to access the same site, that client can grab it from the proxy server cache, and accessing the Internet is unnecessary. This greatly increases the response time to the client and can significantly reduce the bandwidth needed to fulfill client requests.



Nowadays, speed is everything, and the ability to quickly access information from the Internet is a crucial concern for some organizations. Proxy servers and their ability to cache web content accommodate this need for speed. An example of this speed might be found in a classroom. If a teacher asks 30 students to access a specific Uniform Resource Locator (URL), without a proxy server, all 30 requests would be sent into cyberspace and subjected to delays or other issues that might arise. The classroom scene with a proxy server is quite different. Only one request of the 30 finds its way to the Internet; the other 29 are filled by the proxy server’s cache. Web page retrieval can be almost instantaneous.



CSUs/DSUs

A Channel Service Unit/Data Service Unit (CSU/DSU) acts as a translator between the LAN data format and the WAN data format. Such a conversion is necessary because the technologies used on WAN links are different from those used on LANs. Some consider a CSU/DSU a type of digital modem. But unlike
a normal modem, which changes the signal from digital to analog, a CSU/DSU changes the signal from one digital format to another.

A CSU/DSU has physical connections for the LAN equipment, normally via a serial interface, and another connection for a WAN. Traditionally, the CSU/DSU has been in a box separate from other networking equipment. However, the increasing use of WAN links means that some router manufacturers are now including CSU/DSU functionality in routers or are providing the expansion capability to do so.

Network Devices Summary
 
Device
Description
Key Points
Hub
Connects devices on an Ethernet twisted-pair network.
A hub does not perform any tasks besides signal regeneration.
Switch
Connects devices on a twisted-pair network.
A switch forwards data to its destination by using the MAC address embedded in each packet.
Repeater
Regenerates data signals.
The function a repeater provides
typically is built in to other
devices such as switches.
Bridge
Connects LANs to reduce overall network traffic.
A bridge allows data to pass through it or prevents data from passing through it by reading the MAC address.
Router
Connects networks.
A router uses the softwareconfigured
network address to make forwarding decisions.
Gateway
Translates from one data format into another.
Gateways can be hardware or softwarebased. Any device that translates data formats is called a gateway.
CSU/DSU
Translates digital signals used on a LAN into those used on a WAN
CSU/DSU functionality is sometimes incorporated into other devices, such as a router with a WAN connection.
Modem
Provides serial communication capabilities across phone lines.
Modems modulate the digital signal into analog at the sending end and perform the reverse function at the receiving end.
Network card
Enables systems to connect to the network.
Network interfaces can be add-in expansion cards, PCMCIA cards, or built-in interfaces.
Media converter
Interconnects older technology with new.
A media converter is a hardware device that connects newer Gigabit Ethernet technologies with older 100BaseT networks or older copper standards with fiber.
Firewall
Provides controlled data access between networks.
Firewalls can be hardware- or softwarebased. They are an essential part of a network’s security strategy
DHCP server
Automatically distributes information.
DHCP assigns all IP information, including IP address, subnet mask, DNS, gateway, and more.
Multilayer switch
Functions as a switch or router
Operates on Layers 2 and 3 of the OSI model as a switch and can perform router functionality
Content switch
Forwards data by application.
Content switches can identify and forward data by its port and application
Load balancer
Distributes network load.
Load balancing increases redundancy by distributing the load to multiple servers.
Multifunction devices
Combines network services
These are hardware devices that combine multiple network services into a single device reducing
cost and easing administrative difficulty.
DNS server
Provides name resolution from hostnames to IP addresses.
A DNS server answers clients’ requests to translate hostnames into IP addresses.
Bandwidth shaper
Manages network bandwidth.
The bandwidth shaper monitors and controls bandwidth usage.
Proxy server
Manages client Internet requests.
Serves two key network functions increases network performance by caching, and filters outgoing client requests.

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