LAB 1 . Exercise .
Yes, as I have done this before, but with Ubuntu-based distros connected to Windows Vista. However this should still work with Windows 10. This is called a direct ethernet connection. There are a few steps to this:
check current IP for example Start, cmd to open a terminal, run ipconfig
write down the current IP(s) to compare later
attach the Ethernet cable to both machines so they are now physically connected to each other
get the new IP: Start, cmd to open a command prompt, run ipconfig
comparing with your previously copied IPs, see which new IP appears, and copy it down for example it may resemble: 169.254.123.101.
get to the network manager, for example click status bar network icon
choose Wired type
create a new wired connection, naming it something you’ll recognize such as direct-ether
under iPv4, use these settings
Method: Manual. Otherwise default Automatic (DHCP) does not let you set an IP
address: 169.254.123.105. The point is to use same IP except for last segment to be on the same subnet so if one is a.b.c.101 then you should be a.b.c.105 for example
gateway: leave blank
It is at this point, on Lubuntu for example there is weirdness where, when typing address numbers, values “disappear” when typing. Just keep typing and when you Save, it seems the values just appear.
Now choose your new direct-ether network, for example status bar click it
So now you should have, for example:
Test the connectivity for example using software that you can access by IP. For example on Windows I had Xampp Portable running which runs an Apache web server. So to test whether Ubuntu could see that web server, I simply opened a browser to http://169.254.123.101 which is the Windows’s IP in this example, and could see the Windows’ Xampp Portable default page, thus confirming the connectivity.
(Probably a better question for serverfault, but…)
If you do an nslookup on bestbuy.com:
Note that there are multiple ip addresses that correspond to this ip name. The primary purpose of this is to provide a failover. If one of their web farms goes DOA, then the other will continue to accept inbound traffic. The positive side effect of this is that when everything is operating normally, they can share the traffic load between two farms.
The reason for this is because the zonefile for the bestbuy.com zone contains multiple A records for the entry @ eg:
@ IN A 188.8.131.52 @ IN A 184.108.40.206
This is done for load balancing purposes.
# dig bestbuy.com ; <<>> DiG 9.5.0-P2.1 <<>> bestbuy.com ;; global options: printcmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 395 ;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION: ;bestbuy.com. IN A ;; ANSWER SECTION: bestbuy.com. 20 IN A 220.127.116.11 bestbuy.com. 20 IN A 18.104.22.168 ;; Query time: 52 msec ;; SERVER: 22.214.171.124#53(126.96.36.199) ;; WHEN: Sat Aug 14 14:26:35 2010 ;; MSG SIZE rcvd: 61
As pointed above, they do it for load balancing, fail over, etc… But it’s a pretty unsafe way of doing it because DNS load balancing works close to round robin: every time you ask for a hostname, the DNS server answers with one of the pool (like you see happening in your ping). Unfortunately, it means that in case of one of the “servers” goes down, half of the requests will fail.
It’s better to implement that using a true load balancer, or a VIP (virtual ip).
Using the Ping Command
Applies To: Windows Server 2008
The ping command helps to verify IP-level connectivity. When troubleshooting, you can use ping to send an ICMP echo request to a target host name or IP address. Use ping whenever you need to verify that a host computer can connect to the TCP/IP network and network resources. You can also use ping to isolate network hardware problems and incompatible configurations.
Follow this sequence to diagnose network connectivity:
- Ping the loopback address to verify that TCP/IP is configured correctly on the local computer.ping 127.0.0.1
- Ping the IP address of the local computer to verify that it was added to the network correctly.pingIP_address_of_local_host
- Ping the IP address of the default gateway to verify that the default gateway is functioning and that you can communicate with a local host on the local network.pingIP_address_of_default_gateway
- Ping the IP address of a remote host to verify that you can communicate through a router.pingIP_address_of_remote_host
The following table shows some useful ping command options.
|-nCount||Determines the number of echo requests to send. The default is 4 requests.|
|-wTimeout||Enables you to adjust the timeout (in milliseconds). The default is 4,000 (a 4-second timeout).|
|-lSize||Enables you to adjust the size of the ping packet. The default size is 32 bytes.|
|-f||Sets the Do Not Fragment bit on the ping packet. By default, the ping packet allows fragmentation.|
|/?||Provides command Help.|
By default, ping waits 4,000 milliseconds (4 seconds) for each response to be returned before displaying the “Request Timed Out” message. If the remote system being pinged is across a high-delay link, such as a satellite link, responses might take longer to be returned. You can use the -w (wait) option to specify a longer timeout.
A response of “Destination net unreachable” means there was no route to the destination. You need to check the routing table on the router listed in the “Reply from” address in the “Destination net unreachable” message. For more information about the routing table, see Manage the IPv4 Routing Table.
A response of “Request timed out” means that there was no response to the ping in the default time period (1 second). You can check for the following:
- The ping command is blocked at the corporate or personal firewall level.Configure the corporate firewall to allow the ping command network access. Configure the personal firewall to allow ICMP Echo and Echo Reply packets.
- A router is down.To check the routers in the path between the source and the destination, use the tracert command.
- The destination host is down.Physically verify that the host is running or check connectivity through another protocol.
- There is no route back to your computer.If the host is running, you can check for a return route by viewing the default gateway and local routing table on the destination host.
- The latency of the response is more than one second.Use the -w option on the ping command to increase the timeout. For example, to allow responses within 5 seconds, use ping-w 5000.
The ping command uses Windows Sockets-style name resolution to resolve a computer name to an IP address, so if pinging by address succeeds, but pinging by name fails, then the problem lies in address or name resolution, not network connectivity.
Provides information about network latency and network loss at intermediate hops between a source and destination. Pathping sends multiple Echo Request messages to each router between a source and destination over a period of time and then computes results based on the packets returned from each router. Because pathping displays the degree of packet loss at any given router or link, you can determine which routers or subnets might be having network problems. Pathping performs the equivalent of the tracert command by identifying which routers are on the path. It then sends pings periodically to all of the routers over a specified time period and computes statistics based on the number returned from each. Used without parameters, pathping displays help.
You can use TRACERT to find out where a packet stopped on the network. In the following example, the default gateway has found that there is no valid path for the host on 188.8.131.52. Probably, either the router has a configuration problem, or the 184.108.40.206 network does not exist, reflecting a bad IP address.
Pathping is a TCP/IP based utility (command-line tool) that provides useful information about network latency and network loss at intermediate hops between a source address and a destination address. It does this by sending echo requests via ICMP and analyzing the results. ICMP stands for Internet Control Message Protocol. ICMP is an extension to the Internet Protocol (IP – part of the TCP/IP protocol suite) defined by RFC 792. ICMP supports packets containing error, control and informational messages. Pathping will send multiple echo request messages to each router between what you are attempting to ping – the source address. If your destination is across a WAN link then it’s certain that you will be using some form of router, most likely two, which would mean that you could test pathping across a two hop network – two router hops. A typical network diagram is seen in the following illustration.
The term time-to-live is also used to describe the time for which a DNS record can be returned from cache. In this context, TTL is a numerical value, set in a DNS record on the authoritative DNS server for the domain, defining the number of seconds for which a caching server can provide its cached value for the record. When that many seconds have elapsed since the last refresh, the caching server will reach out to the authoritative server again and receive the current (and possibly changed) value for the record.
Question 1.1: What is the internal design of a “thinwire” Ethernet cable (10BASE2 standard)?
Question 1.2: What is the maximum transmission rate and maximum cable length specified for the 10BASE2 standard? Are there further limitations?
On the other hand, the 10Base2 also supports 10 Mbps baseband transmission, the standard specifies zero point two five inch coaxial cable known as cheapernet or thin Ethernet. So here the coaxial cable is of cheaper variety which is used in cable TV which is 0.25 inches in diameter and that’s why it is also called chapernet because of its lower cost and also it is called thin Ethernet because this diameter is thinner than the standard 10Base5. So here as I have mentioned actually 185 m is the maximum segment length and up to five cable segments can be connected using repeaters with
So here as I have mentioned actually 185 m is the maximum segment length and up to five cable segments can be connected using repeaters with maximum length of 925 m. So in this way with five repeaters you can have 925 m and total number of computers is the same which is 1024. This is connected in this manner (Refer Slide Time: 24:05).
Whenever a computer has to be connected the coaxial cable is cut and a BNC T type connector is attached at both ends of the cut cable and that can be connected to the Network Interface Card which is available in the form of T. So this T connector is essentially connected to the network interface card and then these two which are connected to ends of the cable can be connected to the network interface card.
You may be asking why we have deviated from using coaxial cable. Actually in case of 10Base5 or 10Base2 there is always a problem of loose connection, cut and other problems and for that purpose time domain reflectometry is used for detection of fault which is very time consuming. So, that problem can be avoided in case of this hub based 10BaseT Ethernet network wherein it is very easy to maintain and diagnose a fault. That’s why this particular topology has become very popular.
UTP AND STP (NPTEL )
Typical characteristics: Twisted-pair can be used for both analog and digital communication. The data rate that can be supported over a twisted-pair is inversely proportional to the square of the line length. Maximum transmission distance of 1 Km can be achieved for data rates up to 1 Mb/s. For analog voice signals, amplifiers are required about every 6 Km and for digital signals, repeaters are needed for about 2 Km. To reduce interference, the twisted pair can be shielded with metallic braid. This type of wire is known as Shielded Twisted-Pair (STP) and the other form is known as Unshielded Twisted-Pair (UTP).
Question 1.3: What is a UTP cable and how is it constructed?
Question 1.4: What UTP standards are there and which maximum transmission rates and cable lengths do they specify?
Now the twisted pair of wire is available in two different types. One is know as unshielded twisted pair and another is shielded twisted pair. Let us look at the difference.
As we have already seen in case of unshielded twisted pair apart from these two conductors and insulators there is a protective plastic cover, there is no other conductor or shielding
Now this is sufficient for ordinary telephone wire telephone network but however as there is no protective shield it is subjected to external electromagnetic interference. That means whenever this twisted pair of wire is taken through some industrial environment where lots of sparks and other things are going on then it creates a problem. So it is induced and that’s why whenever lightning occurs or there is some spark or a car is moving close by that signal is induced in this unshielded twisted pair of wire. However, this can be reduced by using shielded twisted pair where as you can see we have got an additional conductor in the form of braided mesh.
So it is induced and that’s why whenever lightning occurs or there is some spark or a car is moving close by that signal is induced in this unshielded twisted pair of wire. However, this can be reduced by using shielded twisted pair where as you can see we have got an additional conductor in the form of braided mesh.
It is not a solid conductor but it is available in the form of braided mesh and this metal shield can be connected to the ground and after that there is a plastic cover. That means if this particular shield is connected to ground then this particular metal shield will rotate both the wires from electromagnetic interference. So whenever we want lesser electromagnetic interference we should use shielded twisted pair. However, shielded twisted pair is definitely costlier than the unshielded twisted pair. Thus shielded twisted pairs are not that popular in our day to day common applications in telephone networks as well as in Local Area Networks therefore usually UTP or Unshielded Twisted Pair of cables are used.
Here are the attenuation characteristics of different types of UTP cables. Here you see that in the inner conductor that is used there is an insulator. This inner conductor diameter is specified here 18 gauge, 22 gauge, 24 gauge and twenty six gauge and here the diameters are given in inches 18 gauge corresponds to 0.0403 inch.
On the other hand 26 gauge has diameter 0.0159 gauge. So it is quite obvious that higher the diameter of the conductor lesser will be the attenuation. So 18 gauge wire gives you lesser attenuation compared to 26 gauge and obviously the 18 gauge wire will be costlier than 26 gauge. These are the common wire diameters and the gauge of wire given here and as we can see attenuation is given in dB per meter and here is the frequency. So at higher frequencies the attenuation increases for UTP.
Apart from increase in attenuation at higher frequencies as the length of the wire increases the attenuation also increases. It has been found that the attenuation is proportional to 1 by distance square so attenuation is proportional to proportional to distance square. Therefore as distance increases the attenuation becomes more and in such a situation as you know we can use repeater to amplify the signal and regenerate it and resend it if necessary. The twisted pair UTP is the simplest and possibly the cheapest guided media used in many application.
One of the most common application is local loop in telephone lines. That means in a telephone network the connection that you are getting to your home from the local exchange the wire that is used is your UTP and it is also used in Digital Subscriber Line DSL. Nowadays DSL is becoming more popular. We shall discuss about it in more detail. In DSL also the UTP is used.
Question 1.5: What is the difference between STP and UTP?
1. STP cables are shielded while UTP cables are unshielded
2. STP cables are more immune to interference and noise than UTP cables
3. STP cables are better at maximizing bandwidth compared to UTP cables
4. STP cables cost more per meter compared to UTP cables
5. STP cables are heavier per meter compared to UTP cables
6. UTP cables are more prevalent in SOHO networks while STP is used in more high-end applications
Termination and Bias Resistor Information
Question 1.6: Why are termination resistors needed for data cables e.g. Ethernet cables?
- Because each differential pair of wires is a transmission line, you must properly terminate the line to prevent reflections. A common method of terminating a two-wire multidrop RS-485 network is to install terminating resistors at each end of the multidrop network. If you daisy-chained multiple instruments together, you need a terminating resistor at only the first and last instruments. The terminating resistor should match the characteristic impedance of the transmission line (typically 100–120 Ohms). National Instruments offers an optional DB-9 RS-485 termination connector that contains embedded terminating resistors for easy termination.
- Question 1.7: What does the reflected signal look like when compared with the input signal?
- youtube link
Exercise 1.2 Transmit data via a serial link
1. Boot two computers from the Windows partition and connect them with a RS232 (V.24) serial link cable. Use the serial link patch field for this purpose. Logon as user rnlabor with password student123!
You may change the language since Windows supports multiple languages: Start | type in “Anzeigesprache ändern” and press return. Select the language after “Wählen Sie eine Anzeigesprache aus” and confirm to log out. Log in again and the language has changed.
Question 1.9: What kind of cable do you need and what is the difference between a null modem cable without handshaking and a null modem cable with partial handshaking?
Hint: Search on the internet for an introduction to RS232
(for example https://en.wikibooks.org/wiki/Serial_Programming/RS-232_Connections or https://www.lammertbies.nl/comm/info/RS-232_null_modem.html
What kind of cable do you need and what is the difference between a null modem cable without handshaking and a null modem cable with partial handshaking?
Link for answer this question .
What is software flow control and what advantages / disadvantages does software flow control have in comparison to hardware flow control (Internet research)?
Software using only the RTS/CTS protocol for flow control cannot take advantage of the partial handshaking null modem cable. Most software however will also check the DSR line and in that case—when using the null modem cable with partial handshaking—the best possible hardware flow control can be achieved which is still compatible with the original use with modems.
Flow Control is a technique so that transmitter and receiver with different speed characteristics can communicate with each other. Flow control ensures that a transmitting station, such as a server with higher processing capability, does not overwhelm a receiving station, such as a desktop system, with lesser processing capability. This is where there is an orderly flow of transmitted data between the source and the destination.(source nptel)
3.3.2 Flow Control Modern data networks are designed to support a diverse range of hosts and communication mediums. Consider a 933 MHz Pentium-based host transmitting data to a 90 MHz 80486/SX. Obviously, the Pentium will be able to drown the slower processor with data. Likewise, consider two hosts, each using an Ethernet LAN, but with the two Ethernets connected by a 56 Kbps modem link. If one host begins transmitting to the other at Ethernet speeds, the modem link will quickly become overwhelmed. In both cases, flow control is needed to pace the data transfer at an acceptable speed.
Flow Control is a set of procedures that tells the sender how much data it can transmit before it must wait for an acknowledgment from the receiver. The flow of data should not be allowed to overwhelm the receiver. Receiver should also be able to inform the transmitter before its limits (this limit may be amount of memory used to store the incoming data or the processing power at the receiver end) are reached and the sender must send fewer frames. Hence, Flow control refers to the set of procedures used to restrict the amount of data the transmitter can send before waiting for acknowledgment.
There are two methods developed for flow control namely Stop-and-wait and Sliding-window. Stop-and-wait is also known as Request/reply sometimes. Request/reply (Stop-and-wait) flow control requires each data packet to be acknowledged by the remote host before the next packet is sent. This is discussed in detail in the following subsection. Sliding window algorithms, used by TCP, permit multiple data packets to be in simultaneous transit, making more efficient use of network bandwidth as discussed in subsection 220.127.116.11.
Question 1.11: How is the signal on the monitor re-coded to the respective ASCII character?
Question 1.13: Search on the internet for a description of the Domain Name Service, DNS. What is DNS for, how does it work?
A DNS server is any computer registered to join the Domain Name System. A DNS server runs special-purpose networking software, features a public IP address, and contains a database of network names and addresses for other Internet hosts.
Step 1: Request information
The process begins when you ask your computer to resolve a hostname, such as visiting http://www.prashant.win The first place your computer looks is its local DNS cache, which stores information that your computer has recently retrieved.
If your computer doesn’t already know the answer, it needs to perform a DNS query to find out.
Step 2: Ask the recursive DNS servers
If the information is not stored locally, your computer queries (contacts) your ISP’s recursive DNS servers. These specialized computers perform the legwork of a DNS query on your behalf. Recursive servers have their own caches, so the process usually ends here and the information is returned to the user.
Step 3: Ask the root nameservers
If the recursive servers don’t have the answer, they query the root nameservers. A nameserveris a computer that answers questions about domain names, such as IP addresses. The thirteen root nameservers act as a kind of telephone switchboard for DNS. They don’t know the answer, but they can direct our query to someone that knows where to find it.
Step 4: Ask the TLD nameservers
The root nameservers will look at the first part of our request, reading from right to left — http://www.prashant.win— and direct our query to the Top-Level Domain (TLD) nameservers for .com. Each TLD, such as .com, .org, and .us, have their own set of nameservers, which act like a receptionist for each TLD. These servers don’t have the information we need, but they can refer us directly to the servers that do have the information.
Step 5: Ask the authoritative DNS servers
The TLD nameservers review the next part of our request — http://www.prashant.win — and direct our query to the nameservers responsible for this specific domain. These authoritative nameservers are responsible for knowing all the information about a specific domain, which are stored in DNS records. There are many types of records, which each contain a different kind of information. In this example, we want to know the IP address for http://www.prashant.win, so we ask the authoritative nameserver for the Address Record (A).
Step 6: Retrieve the record
The recursive server retrieves the A record for http://www.prashant.win from the authoritative nameservers and stores the record in its local cache. If anyone else requests the host record for dyn.com, the recursive servers will already have the answer and will not need to go through the lookup process again. All records have a time-to-live value, which is like an expiration date. After a while, the recursive server will need to ask for a new copy of the record to make sure the information doesn’t become out-of-date.
Step 7: Receive the answer
Armed with the answer, recursive server returns the A record back to your computer. Your computer stores the record in its cache, reads the IP address from the record, then passes this information to your browser. The browser then opens a connection to the webserver and receives the website.
This entire process, from start to finish, takes only milliseconds to complete.
Question 1.14: What does the computed TTL value reflect when considering the network connection between Ulm and Munich?
Pinging www.some-domain.com [XXX.XXX.XXX.XXX] with 32 bytes of
Reply from XXX.XXX.XXX.XXX: bytes=32 time=564ms TTL=237
Reply from XXX.XXX.XXX.XXX: bytes=32 time=555ms TTL=237
Reply from XXX.XXX.XXX.XXX: bytes=32 time=554ms TTL=237
Reply from XXX.XXX.XXX.XXX: bytes=32 time=548ms TTL=237
Ping statistics for XXX.XXX.XXX.XXX:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)
Approximate round trip times in milli-seconds:
Minimum = 548ms, Maximum = 564ms, Average = 555ms
It counts no of hops between router how many router is involved between connection so if we if we ping from different place it may be possible to have there might be more no of router involved between the route .
A timer value included in packets sent over TCP/IP-based networks that tells the recipients how long to hold or use the packet or any of its included data before expiring and discarding the packet or data. For DNS, TTL values are used in resource records within a zone to determine how long requesting clients should cache and use this information when it appears in a query response answered by a DNS server for the zone.