Това е една интересна програмка за Мак. Идеята и е да “слуша” за безжични мрежи. От скоро, SVN версията поддържа и пасивен режим на Airport Extreme картите в най-новите лаптопи на Apple. Програмката поддържа и няколко режима на кракване на WEP и WPA защити. Направих едно Универсално байнъри на най-новата версия от вчера, можете да я дръпнете от тук. Ако си свалите последната версия от сайта им (вече билдната) няма да имате готиния режим. Ако искате сами да си го компилирате от последния svn се консултирайте с бинаерварианз.де

П.П. Лиценза е GPL и не поемам никаква отговорност за нищо!

Here are my answers to the second chapter of Computer Networks by Andrew Tanenbaum 4th edition with ISBN 0130661023.

Problem №3:
Television channels are 6 MHz wide. How many bits/sec can be sent if four-level digital signals are used? Assume a noiseless channel.

Answer:
Using Henry Nyquist’s theorem we have 2Nlog2(V) where N is 6MHz, and V is 4
gives us 24mbit/s

Problem №4:
If a binary signal is sent over a 3-kHz channel whose signal-to-noise ratio is 20 dB, what is the maximum achievable data rate?

Answer:
Using the formula Hlog2(1+S/N), and from the chapter we know that 20dB is 100S/N, we can calculate that 3000* 6.65821148 = 19 974.6344 bits per second

Problem №5:
What signal-to-noise ratio is needed to put a T1 carrier on a 50-kHz line?

Answer:
Since first grade we know that T1 is about 1.544 million bits per second theoretical speed 🙂
So using the same formula as above we have:
1544000 = 50000*log2(1+S/N)
log2(1+S/N) = 30.88
1+S/N = 1976087930
S/N = 1976087929
10log10(1976087929) = approx 92.95806263 dB

Problem №8:
It is desired to send a sequence of computer screen images over an optical fiber. The screen is 480 x 640 pixels, each pixel being 24 bits. There are 60 screen images per second. How much bandwidth is needed, and how many microns of wavelength are needed for this band at 1.30 microns?

Answer:
480 * 640 * 24 * 60 = 442 368 000 bits of bandwith is needed.

Problem №9:
Is the Nyquist theorem true for optical fiber or only for cooper wire?

Answer:
It is true for all media.

Problem №18:
A simple telephone system consists of two end offices and a single toll office to which each end office is connected by a 1-MHz full-duplex trunk. The average telephone is used to make four calls per 8-hour workday. The mean call duration is 6 min. Ten percent of the calls are long-distance (i.e., pass through the toll office). What is the maximum number of telephones an end office can support? (Assume 4 kHz per circuit.)

Question №1:
What is the point at which the responsibility of a network connection transfers from the service provider to the customer?

Answer:
DEMARC- The Demarcation Point

Question №2:
Where would you normally get a block of adresses from if you are connecting to the Internet for the first time?

Answer:
The service provider

Question №3:
What are the primary advantages of dual homing to the Internet trough the same Internet Service Provider?

Answer:
It is easy. Only one bill, one set of people to deal with for technical support, sales etc. Also it is easier to deal with load balancing between the lines with one provider. It also reduces the risk of single point of failure.

Question №4:
How many routers should you use to pear up with when dual homing to a single internet service provider?

Answer:
Two. One for each connection.

Question №5:
What is the primary disadvantage of dual homing to the same ISP rather than to two different ISPs

Answer:
The two connection can be one physical connection or be in the same pysical trench or conduct, which can cause te custumer to loose all connections to the Internet if bachhol fade occurs.

Question №6:
What other type of diversity is just as important as logical diversity?

Answer:
Physical diversity

Question №7:
What types of records can you requrest from the Internet service provider to determine weather or not you have enough diversity of the right types?

Answer:
Physical network maps

Question №8:
What are the primary considerations that would cause you to run eBGP with your Internet Service Provider?

Answer:
Optimal routing and traffic flow control.

Question №9:
How can you run BGP to an Internet Service Provider without a registert Autonomus System number?

Answer:
By using appropriate ASN assigned by the Service Provider (64512 – 65535)

Question №10:
What is the most common problem preventing traffic from flowing inbound on two links when dual homed to two different Internet Service Providers?

Answer:
The problem is in the length of the network prefix. By default routers will choose the longest (more specific) network prefix which will give them tore specific route to a network.

Question №12:
What methods could you investigate to balance inbound traffic flow trough two different connections to the Internet?

Answer:
Setting metrics and locations from which the default route is advertised. Modifying the amount of address space each edge router advertises.

I just finished reading the first chapter of Practical BGP ISBN 0321127005 and here are my answers of the questions at the end of the chapter:

Question №1:
What is a routing domain from BGP’s perspective? How is this different from a routing domain within IS-IS?

Answer:
A routing domain from BGP’s perspective is the set of routes under the same administrative control, usualy referred as an Autonomus System. The difference is that in IS-IS a routing domain is the area in which topology information is spread.
——————

Question №2:
What are the two primary differences between an interior gateway protocol and an exterior gateway protocol?

Answer:
The Interior gateway protocol is used within an AS. Exterior gateway protocol is used for routing between AS. Also IGP don’t cary policy information.
——————

Question №3:
What types of policies would you normally see implemented through BGP?

Answer:
Two possible policies are to mark routes so they cannot be advertised beyond the adjacent routing domain and to prevent leakking of information that would provide a better route to internal networks that internal routing information provides. Also policy to take the closest exit point, one to take the cheapest exit point, one that should not allow traversing of certain networks.
——————

Question №4:
For what does BGP use the path information it carries through the network?

Answer:
To determine loop-free paths between AS.
——————

Question №5:
Why does BGP threat each autonomus system as a point on the connectivity graph?

Answer:
The main reason is to hide topological details of the AS.
——————

Question №6:
What transport does BGP use to build a session to another BGP speaker? What local port number and remote port number does BGP use when initiating a connection?

Answer:
It uses TCP for sessions between speakers. The local port number is a random number above 1024 and the remote port number is 179.
——————

Question №7:
How is a collision resolved between two BGP speakers attempting to open a connection at the same time?

Answer:
Whe a collision occurs (two routers try to initiate a session) only the session by the router with the largest router ID value is preserved. The other one is dropped.
——————

Question №8:
Define prefix, NLRI, and attribute.

Answer:
A prefix is a network address. NLRI stands for Network Layer Reachability Information and consists of set of destinations. An attribute is an array of informatino that holds various pieces of informationt such as path preference and others.
——————

Question №9:
How many sets of attributes can a single BGP update contain? How many prefixes?

Answer:
One set of attributes with many prefixes, to which the attributes apply.
——————

Question №10:
What are the four primary difference between eBGP peering relationships and iBGP peering relationships?

Answer:
* Routes learned from an iBGP peer are not advertised to other iBGP peers. (Prevents routing loops within the AS)
* The attributes of paths learned from iBGP peers are not changed to impact the path selection to reach some outside network.
* The AS Path is not manipulated when advertising a route to an iBGP peer; the local AS is added to the AS Path only when advertising a route to an eBGP peer.
* The BGP next hop is normally not changed when advertising a route to an iBGP; it is always changed to the local peer termination IP address when a route is being advertised to an eBGP peer.