LinkMeUp. Release 0
It all started with a very long way home.
This time it was necessary to take something. Once it was ESL (English is a Second Language) - very useful in terms of language learning, but the usual issues get boring fast, and interesting - English Cafe - do not go out often enough. Then I discovered CBT-Nuggets - the most interesting thing, but the course is rather short. And finally, Radio-T, which, it seems, is also IT, but somehow very much apple and programmer.
There was a tangible hunger on telecom topics. There was at least a demand for several people, but there was no supply - not a single podcast in this area. There were near-connectivity things, but it's not that.
And where there is demand, there will be supply, even if I will create it.
So, we are pleased to present a zero release podksta for LinkMiAp operators.
In this issue we discuss:
1) Mobile number portability between operators and regions. MNP - Mobile Number Portability.
2) Technology for making voice calls in LTE networks: CSFB, IMS.
3) Practical topic: how the router behaves, if you configure the interface as Next Hop, instead of the IP address.
You can listen to us on our website or on rpod .
')
Below is a brief brief and schemes in order to understand the topics was easier.
If you have suggestions, what topics would be interesting to discuss in the next issues, we are happy to accept them in the comments.
Schematically, the GSM network works like this: every cellular operator has a certain base of all-all its numbers. It contains a lot of all kinds of information (services allowed to a subscriber, for example), but the main thing is that it is recorded there, where each specific subscriber is currently located. It is clear that the base is huge, so for large operators it is divided into parts, for example, one base serves the first 10,000 operator numbers, the other - the next 10,000.
How does this base work? When a subscriber calls someone, the equipment that processes the call, looks to which database to access, makes a request to it, finds out which switch and within which base station the destination is located, and routes the call there. No problem, because we know who this range belongs to and what to do with the challenge.
This is how it works before the introduction of MNP. With it, everything becomes more complicated: the subscriber goes with his number to another operator, and now the information necessary to communicate with the subscriber is stored with the latter. But somehow you need to route. There is a need for an additional database that would store the numbers of subscribers who changed the operator, which would say that such a subscriber with a number from operator A is actually a client of operator B, therefore all information needs to be asked B. There is 3 types of such "forwarding":
In any case, we need some kind of external base of the transferred subscribers, and this is the whole intrigue: who will do this, because it is a serious matter, in case of failure of this base, the transferred subscribers of all operators remain without communication.
Useful links:
pro-gsm.info/mnp.html
pro-gsm.info/cap-roaming.html
Soon after the development and the beginning of the active implementation of the 3G standard, the engineers of the 3GPP organization (the main one in the field of mobile communication standards) realized that the need for mobile subscribers to increase data transfer rates would inevitably increase, and the traffic volume would increase almost exponentially (for example, data traffic in Megafon networks is 99%, and the voice is only 1%). Therefore, some of the developers of the village have been finalizing the W-CDMA (3G) standard - the HSDPA, HSDPA +, HSUPA protocols have resulted in their efforts, which have increased the peak speed to 42 Mbps in DL, while the other part of 3GPP has begun a completely new communication standard - LTE ( Long Term Evolution), which was introduced in 2009, and in 2010 the launch of the first commercial networks in Sweden, the USA and South Korea began.
From the point of view of the division of radio channels, LTE is divided into 2 sub-standards that have their pros and cons.
1. FDD - frequency division of channels, this sub-standard uses two different frequency spectrum bands for DL ​​and UL, i.e. subscriber radio channel is symmetrical and is fully duplex
2. TDD - time division of channels, this substandard uses one frequency (or a set of several asymmetrical frequencies) for transmitting to DL and UL, alternating time intervals for transferring from / to the subscriber, i.e. is half duplex.
FDD and TDD LTE networks have their advantages and disadvantages. FDD is generally more suitable for videoconferencing-type applications that have symmetrical traffic. This is due to the fact that traffic in both directions is continuous, and the use of TDD would be a waste of bandwidth while constantly switching from one mode to another. TDD is good for applications that have unbalanced traffic, an example of this would be online viewing. TDD can give more time to portions of data that require more bandwidth, thus balancing the load. With FDD, bandwidth cannot be dynamically reallocated and unused bandwidth is wasted.
Another advantage of FDD LTE communications is site planning for base stations. Due to the fact that base stations use different frequencies for receiving and transmitting data, this is effective, since different frequencies do not interrupt each other and special scheduling is not required, however, this requires strict eNodeB synchronization (BS in LTE) over time, which complicates the construction of the network.
FDD was implemented before TDD, since most of the operators have at their disposal precisely paired frequencies in different regions of the spectrum, besides, there is no problem of synchronizing the BS in time, however, now, in conditions of lack of frequencies, it becomes more logical for operators to use TDD, since it allows to get a full-fledged network with scattered frequency bands available (rather than two 10 MHz bands, as in FDD), the synchronization problem is solved using GPS and clock synchronization from other network elements (for example, RNC - Radio Network Contro). ller).
Perhaps the most important difference of LTE from the previous communication standards is the fact that the main thing in LTE is data transmission, and voice transmission is relegated to the background and is accomplished by including additional features and interfaces.
1. IMS and SRVCC. IMS - IP Multimedia subsystem is an additional part of the mobile network that allows voice over IP based protocol - SIP (i.e., in fact, ordinary IP telephony) to be transmitted, but in the logic of the mobile network it provides the possibility of a seamless subscriber transfer handover) during a conversation, from the LTE network to the GSM / W-CDMA network - thereby realizing the SRVCC feature - Single-Radio Voice Call Continuity. Those. if the subscriber has established a voice connection in LTE using the IMS subsystem, then it will be easy to implement the continuity of the connection when handover to other networks - the type of radio access will change, and the call, as processed by IMS, will remain to be processed.
2. However, not every operator is willing to invest in the construction of an additional subsystem on its network, which is why another way of transmitting voice in LTE is more popular - CSFB - Circuit-switched fallback, the use of this feature allows you not to transmit voice over LTE network in general. Namely - when trying to activate a voice call, the subscriber “falls” into the CS Core 2G / 3G network, registering there and using its resources, this is done thanks to the additional SGs interface connecting the MME and MSC, while the subscriber does not lose the ability to transfer data during the conversation , although the speed at the same time reduced to a speed of 2G / 3G network. This method allows you to use the existing network structure of the operator, without the need for additional investments.
What happens if the interface name is specified as Next Hop, and not the IP address of the node. When it is recommended to use it than it threatens.
Short brief on the issue.
In this scheme, router A has the following configuration:
When it receives (or forms) a packet destined for router C , it must encapsulate it into an Ethernet frame. It is logical that the sender's MAC address it substitutes the address of the FE0 / 0 interface, the recipient the address of the FE0 / 1 interface of router B.
The catch is how he gets this MAC address.
In the usual case, when ip route 0.0.0.0/0 10.1.2.2 is configured , host A sends an ARP request, where it asks - “What kind of MAC is 10.1.2.2?”, B responds with sending its own MAC and everyone is happy.
Now A does not even have an IP address B , it says only where to send the packet. How to be? After all, in the ARP request, we can not put an empty IP, we can not and broadcast - it is meaningless.
The solution to this situation is provided by ARP-Proxy. It sends an ARP request, where it requests the MAC address of the device with the IP address 3.3.3.3 (despite the fact that it is on a different subnet).
B receives such an ARP request and, if the ARP-Proxy mechanism is activated on the interface, checks that there is a route to the recipient in its routing table (at least even a default), and sends an ARP reply with the expected content:
That is, it returns the MAC address of its interface.
Router A adds this entry to the ARP cache.
Thus, each address accessible via this route will be added as directly connected to the ARP cache.
Enjoy this little. A huge number of broadcast requests, crowded ARP-cache, high CPU usage.
You need to avoid such configurations. It applies essentially only to point-to-point interfaces (PPP, HDLC, FR).
Useful links:
ciscoexpert.wordpress.com/2008/06/28/proxy-arp
blog.initialdraft.com/archives/2605
This time it was necessary to take something. Once it was ESL (English is a Second Language) - very useful in terms of language learning, but the usual issues get boring fast, and interesting - English Cafe - do not go out often enough. Then I discovered CBT-Nuggets - the most interesting thing, but the course is rather short. And finally, Radio-T, which, it seems, is also IT, but somehow very much apple and programmer.
There was a tangible hunger on telecom topics. There was at least a demand for several people, but there was no supply - not a single podcast in this area. There were near-connectivity things, but it's not that.
And where there is demand, there will be supply, even if I will create it.
So, we are pleased to present a zero release podksta for LinkMiAp operators.
In this issue we discuss:
1) Mobile number portability between operators and regions. MNP - Mobile Number Portability.
2) Technology for making voice calls in LTE networks: CSFB, IMS.
3) Practical topic: how the router behaves, if you configure the interface as Next Hop, instead of the IP address.
You can listen to us on our website or on rpod .
')
Below is a brief brief and schemes in order to understand the topics was easier.
If you have suggestions, what topics would be interesting to discuss in the next issues, we are happy to accept them in the comments.
MNP
Schematically, the GSM network works like this: every cellular operator has a certain base of all-all its numbers. It contains a lot of all kinds of information (services allowed to a subscriber, for example), but the main thing is that it is recorded there, where each specific subscriber is currently located. It is clear that the base is huge, so for large operators it is divided into parts, for example, one base serves the first 10,000 operator numbers, the other - the next 10,000.
How does this base work? When a subscriber calls someone, the equipment that processes the call, looks to which database to access, makes a request to it, finds out which switch and within which base station the destination is located, and routes the call there. No problem, because we know who this range belongs to and what to do with the challenge.
This is how it works before the introduction of MNP. With it, everything becomes more complicated: the subscriber goes with his number to another operator, and now the information necessary to communicate with the subscriber is stored with the latter. But somehow you need to route. There is a need for an additional database that would store the numbers of subscribers who changed the operator, which would say that such a subscriber with a number from operator A is actually a client of operator B, therefore all information needs to be asked B. There is 3 types of such "forwarding":
- direct routing - the equipment of the network operator (for example, C), from which we are calling, requests a centralized base of switched numbers, and, having received the answer, routes the call to network B
- indirect - the equipment of the network operator from which we are calling, in the old manner transfers the call to operator A (old), and A already transfers the call to B
- the third option is a combination of the first and second - if direct routing does not work, indirect is done
In any case, we need some kind of external base of the transferred subscribers, and this is the whole intrigue: who will do this, because it is a serious matter, in case of failure of this base, the transferred subscribers of all operators remain without communication.
Useful links:
pro-gsm.info/mnp.html
pro-gsm.info/cap-roaming.html
LTE
Soon after the development and the beginning of the active implementation of the 3G standard, the engineers of the 3GPP organization (the main one in the field of mobile communication standards) realized that the need for mobile subscribers to increase data transfer rates would inevitably increase, and the traffic volume would increase almost exponentially (for example, data traffic in Megafon networks is 99%, and the voice is only 1%). Therefore, some of the developers of the village have been finalizing the W-CDMA (3G) standard - the HSDPA, HSDPA +, HSUPA protocols have resulted in their efforts, which have increased the peak speed to 42 Mbps in DL, while the other part of 3GPP has begun a completely new communication standard - LTE ( Long Term Evolution), which was introduced in 2009, and in 2010 the launch of the first commercial networks in Sweden, the USA and South Korea began.
From the point of view of the division of radio channels, LTE is divided into 2 sub-standards that have their pros and cons.
1. FDD - frequency division of channels, this sub-standard uses two different frequency spectrum bands for DL ​​and UL, i.e. subscriber radio channel is symmetrical and is fully duplex
2. TDD - time division of channels, this substandard uses one frequency (or a set of several asymmetrical frequencies) for transmitting to DL and UL, alternating time intervals for transferring from / to the subscriber, i.e. is half duplex.
FDD and TDD LTE networks have their advantages and disadvantages. FDD is generally more suitable for videoconferencing-type applications that have symmetrical traffic. This is due to the fact that traffic in both directions is continuous, and the use of TDD would be a waste of bandwidth while constantly switching from one mode to another. TDD is good for applications that have unbalanced traffic, an example of this would be online viewing. TDD can give more time to portions of data that require more bandwidth, thus balancing the load. With FDD, bandwidth cannot be dynamically reallocated and unused bandwidth is wasted.
Another advantage of FDD LTE communications is site planning for base stations. Due to the fact that base stations use different frequencies for receiving and transmitting data, this is effective, since different frequencies do not interrupt each other and special scheduling is not required, however, this requires strict eNodeB synchronization (BS in LTE) over time, which complicates the construction of the network.
FDD was implemented before TDD, since most of the operators have at their disposal precisely paired frequencies in different regions of the spectrum, besides, there is no problem of synchronizing the BS in time, however, now, in conditions of lack of frequencies, it becomes more logical for operators to use TDD, since it allows to get a full-fledged network with scattered frequency bands available (rather than two 10 MHz bands, as in FDD), the synchronization problem is solved using GPS and clock synchronization from other network elements (for example, RNC - Radio Network Contro). ller).
Perhaps the most important difference of LTE from the previous communication standards is the fact that the main thing in LTE is data transmission, and voice transmission is relegated to the background and is accomplished by including additional features and interfaces.
1. IMS and SRVCC. IMS - IP Multimedia subsystem is an additional part of the mobile network that allows voice over IP based protocol - SIP (i.e., in fact, ordinary IP telephony) to be transmitted, but in the logic of the mobile network it provides the possibility of a seamless subscriber transfer handover) during a conversation, from the LTE network to the GSM / W-CDMA network - thereby realizing the SRVCC feature - Single-Radio Voice Call Continuity. Those. if the subscriber has established a voice connection in LTE using the IMS subsystem, then it will be easy to implement the continuity of the connection when handover to other networks - the type of radio access will change, and the call, as processed by IMS, will remain to be processed.
2. However, not every operator is willing to invest in the construction of an additional subsystem on its network, which is why another way of transmitting voice in LTE is more popular - CSFB - Circuit-switched fallback, the use of this feature allows you not to transmit voice over LTE network in general. Namely - when trying to activate a voice call, the subscriber “falls” into the CS Core 2G / 3G network, registering there and using its resources, this is done thanks to the additional SGs interface connecting the MME and MSC, while the subscriber does not lose the ability to transfer data during the conversation , although the speed at the same time reduced to a speed of 2G / 3G network. This method allows you to use the existing network structure of the operator, without the need for additional investments.
IP routing
What happens if the interface name is specified as Next Hop, and not the IP address of the node. When it is recommended to use it than it threatens.
Short brief on the issue.
In this scheme, router A has the following configuration:
When it receives (or forms) a packet destined for router C , it must encapsulate it into an Ethernet frame. It is logical that the sender's MAC address it substitutes the address of the FE0 / 0 interface, the recipient the address of the FE0 / 1 interface of router B.
The catch is how he gets this MAC address.
In the usual case, when ip route 0.0.0.0/0 10.1.2.2 is configured , host A sends an ARP request, where it asks - “What kind of MAC is 10.1.2.2?”, B responds with sending its own MAC and everyone is happy.
Now A does not even have an IP address B , it says only where to send the packet. How to be? After all, in the ARP request, we can not put an empty IP, we can not and broadcast - it is meaningless.
The solution to this situation is provided by ARP-Proxy. It sends an ARP request, where it requests the MAC address of the device with the IP address 3.3.3.3 (despite the fact that it is on a different subnet).
B receives such an ARP request and, if the ARP-Proxy mechanism is activated on the interface, checks that there is a route to the recipient in its routing table (at least even a default), and sends an ARP reply with the expected content:
That is, it returns the MAC address of its interface.
Router A adds this entry to the ARP cache.
Thus, each address accessible via this route will be added as directly connected to the ARP cache.
Enjoy this little. A huge number of broadcast requests, crowded ARP-cache, high CPU usage.
You need to avoid such configurations. It applies essentially only to point-to-point interfaces (PPP, HDLC, FR).
Useful links:
ciscoexpert.wordpress.com/2008/06/28/proxy-arp
blog.initialdraft.com/archives/2605
Source: https://habr.com/ru/post/170649/
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