Networking Equipment - II

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We continue now with more networking equipment that you will be expected to know on the Network+ examination. Remember, the exam likes to test on the differences between pieces of equipment as well as their specific application and place in the OSI model.

[edit section] Equipment

CSU/DSU: A CSU/DSU (Channel Service Unit/Data Service Unit) is a special type of “bridge” that operates between the WAN (wide) and LAN (local) networks. It is typically found in devices such as “cable modems,” which are not modems in the true sense of the word, but rather, converters from one digital signal to another. CSU/DSU devices operate in the physical layer of the OSI model.

Modem: A modem (short for modulator demodulator) acts like a sort of CSU/DSU between digital/analog networks. That is, a modem can translate a physical analog signal to a digital one, and vice-versa. It typically acts as the intermediary between the analog phone system and digital networks. Modems operate in the physical layer of the OSI model.

ISDN Adapter: ISDN is a somewhat archaic technology that allows connection via a special “digital” phone line. An ISDN Adapter is a CSU/DSU for ISDN connections.

Firewall: A firewall is a device that can “filter” traffic coming into and out of a network. There are different types of firewalls that will be tested on the Network+ exam:

Packet Filtering – This firewall operates at the network layer of the OSI model, and filters traffic based on the headers (destination/source) of the individual packets.

Circuit Level – Circuit level firewalls filter traffic based on whether or not a session has been established between the destination and source using TCP handshaking. You can think of a circuit level firewall as a protective father who will not let his daughter date a boy until he “gets to know him.” In a similar way, circuit level firewalls regulate traffic based on whether or not a trusted connection has been established. These operate in the Session layer of the OSI model.

Application Level – Application level firewalls inspect the contents of packets, rather than the source/destination or connection between the two. Application level firewalls are similar to proxies in that they operate and regulate between two segments of the network. Remember that an Application Level firewall operates in the 7th layer of the OSI model (Application Layer) and can inspect the actual contents of packets.

Stateful Inspection – This firewall combines the circuit level and the application level firewall techniques and is most commonly employed today. It assures the connection (session) between the two parties is valid (like the circuit level firewall) and inspects packets from this connection to assure the packets are not malicious (application level). So, the stateful inspection firewalloperates in the network, session, and application layers of the OSI model.

Proxy: Proxy Servers operate at the Application layer of the OSI model and serve as filters of client-Internet traffic. Instead of establishing direct connections between the clients and servers on the Internet, clients connect to the proxy server, which can filter their request and then forward it to the Internet. The information sent back is first filtered and then sent back to the client. In this way, a proxy server is something like the propaganda office of an oppressive government that only presents to its citizens information that has gone through the office. The citizens never directly know what is going on, but instead, what the government (the proxy server) has told them.

[edit section] And Now for Some Quick Review

So, you should now have a command of all of the tested Network+ equipment. Below are a few questions that should help stretch your mind a bit.

1. You have a small network connected to the Internet via phone line. Which of the following pieces of equipment will you need?

a. CSU/DSU

b. Modem

c. ISDN Adapter

d. Bridge

e. Token Ring

2. A client reports that he is unable to connect to a few pages on the Internet, but can connect to almost all others. Which of the following would prevent him from connecting to these pages?

a. Gateway

b. Router

c. Firewall

d. Proxy Server

e. All of the Above

[edit section] Answers

1. B – A modem modulates/demodulates signals between analog (phone) networks and digital (local area) networks.

2. D – Only the Proxy Server is able to filter individual pages in this way.