telecom study notes

Adaptive Modulation and Coding: modulation and code rates are changed adaptively, depending on channel conditions (typically, based on SNR reported by UEs)

• Automatic Repeat reQuest (ARQ): Errors are detected and requests for retransmission of erroneous data are sent
• Forward Error Correction (FEC): Data are encoded using an error-correcting code (ECC) prior to transmission. The additional information (redundancy) added by the code is used by the receiver to recover the original data at the expense of lower data rate (e.g., 1/2, 2/3, 5/6, etc).
• Hybrid automatic repeat-request (HARQ): ARQ and FEC are combined; packets with errors that cannot be corrected but only detected are retransmitted, otherwise only error correction is applied.

MIMO:

BF is used for low SNR

Spatial Multiplexing is used for high SNR

Open loop MIMO: no feedback from receiver, good for high speed of mobile device

Closed loop MIMO: feedback from receiver, can be used for specific UE. robust for low speed of mobile device

Fading:

• Slow fading (shadow): caused by major obstructions within the propagation environment (coherence time is larger than the delay of the channel)
• Fast fading (multipath; small-scale): small movements of a mobile or obstacle and media changes. (coherence time is smaller than the delay)

(FDD and TDD) for up link and down link

for multiple users (and CSMA collision detection)

compared with FDM, OFDM saves bandwidth

cyclic prefix: a repetition of data used to fix interference

for LTE: 

UE: mobile terminal (with sim card) and terminal equipment

• MT: radio interface and wireless network access and data transmission in general. Therefore, it supports the following features:

1. Transmission and reception of data and signalling over the radio interface.
2. Authentication and registration to the UMTS network.
3. Management (including creation, de-activation and modification) of PDP (Packet Data Protocol) contexts on request from a TE.
4. Session control.
5. Support of radio mobility functions, such as the handover.

• TE: the part the end-user has access to, as it supports all the functions related to user applications and interfaces. It contains the following features:

1. Control of application-related hardware functions, such as speaker, microphones, video cameras, displays, etc.
2. Support of user applications and services, such as email client, Web-browsing client, instant-messaging client, etc.
3. Support of application-related protocol and session-management functions (for IMS-based applications, this includes protocol stacks like SIP, SDP and RTP).

types of messaging

1. email
email to SMTP server (forward) to receiver's SMTP server to POP3 server (store) to receiver

2. paging

Paging typically involves a caller dialing a telephone number associated with the intended recipient of the page. Once connected to the paging terminal, the person sending the page can enter a message that will be sent to the pager. When the message is complete, the paging terminal converts the message into a pager code and sends it to a series of transmitters to which it is connected. These transmitters then send out the message as a radio signal throughout the entire coverage area. Every pager within this area on the particular frequency will receive the message, but only the pager with the proper code (the intended recipient) will be alerted. In essence, the pager works much like an FM receiver.

3. SMS

4. EMS

enhanced SMS, can send sounds, animations and pictures

5. MMS

6. instant messaging

7. HDML Notifications

alerts

two channels: the push channel and the pull channel. On packet-switched networks, all data transmissions are treated the same, allowing for push delivery of information, regardless of the size. Circuit-switched networks, on the other hand, use SMS to deliver asynchronous messages, preventing the HDML gateway from delivering messages that exceed the SMS message size limits. In all cases, the push channel is meant for delivering time sensitive material, using only alert or cache operations. The pull channel is better suited for data that is not critical, and for preloading content into the microbrowser.

Once an alert is sent to the HDML gateway, it is queued for delivery. The length of time it spends in the gateway's queue depends on the following information:

• For all push notifications and for pull notifications on packet-switched networks, the gateway will attempt to deliver the message immediately. If the destination phone is unavailable, the gateway will keep the message in its queue and reattempt to deliver it periodically. If the message TTL is exceeded, it will be removed from the queue.
• For pull notifications on circuit-switched networks, the message will remain in the queue until the destination phone opens up a circuit. At this time, the message will be sent to the user for viewing. If the message TTL is exceeded, it will be removed from the queue.

HDML notifications provide a powerful way to push content to wireless users. In North America, where HDML is still widely used, these notifications are often the only option available for push services. However, because HDML notifications are a proprietary messaging technology developed by Openwave, they are only supported in Openwave microbrowsers. As HDML gateways are replaced by WAP gateways, and HDML handsets are replaced by WAP handsets, HDML notifications will gradually give way to WAP Push for push messaging capabilities.

8. WAP push

 

• Push Initiator (PI). The PI is an application that pushes the content and delivery instructions to the Push Proxy Gateway (PPG). It typically runs on a standard Web server and communicates with the PPG using the Push Access Protocol (PAP).
• Push Proxy Gateway (PPG). The PPG does most of the work in the push framework. Its main responsibility is delivering push content to the WAP client. In doing so, it may have to translate the client address into a format understood by the wireless carrier. The PPG is also the location where messages are stored when they cannot be immediately delivered to the client. It also maintains the status of each message, allowing the PI to cancel, replace, and request the current status of a message. The PPG uses the Push Over-the-Air (OTA) Protocol to deliver push content over a wireless network.
  To send a WAP Push message, the PI must have two sets of information about the destination: the domain of the PPG and the client address. An arbitrary text string, such as an email address, can be used to identify the client. The PPG is then responsible for translating the string into a format that is understood by the mobile network.
• WAP client. The WAP client is typically a wireless device that contains a WAP microbrowser capable of receiving WAP Push content. This is where the user is able to view the content that was pushed from the PPG using the Push OTA Protocol.

9. application to application messaging

This client application communicates directly back to the messaging server, without going through a gateway from the wireless carrier. This eliminates the requirement of having to communicate with SMSC or MMSC servers. Instead, the client application communicates directly to the messaging server—hence the name application-to-application messaging.

The vendor can choose the communication protocols to use, along with the compression techniques and security features. It is recommended that you select a solution that has addressed all of these issues and that has created a solution that is well suited for wireless communication networks. This involves using a suitable protocol, such as the User Datagram Protocol (UDP), that provides built-in message compression and integrated security, including data encryption and user authentication.

Interesting: sniffing packets

add a hub so that we can see the packets of the entire network

SMS

The SMC (Short Message Center) is the entity which does the job of store and forward of messages to and from the mobile station. The SME (Short Message Entity) which can be located in the fixed network or a mobile station, receives and sends short messages.

The SMS GWMS (SMS gateway MSC) is a gateway MSC that can also receive short messages. The gateway MSC is a mobile network抯 point of contact with other networks. On receiving the short message from the short message center, GMSC uses the SS7 network to interrogate the current position of the mobile station form the HLR, the home location register.

HLR is the main database in a mobile network. It holds information of the subscription profile of the mobile and also about the routing information for the subscriber, i.e. the area (covered by a MSC) where the mobile is currently situated. The GMSC is thus able to pass on the message to the correct MSC.

MSC (Mobile Switching Center) is the entity in a GSM network which does the job of switching connections between mobile stations or between mobile stations and the fixed network.

A VLR (Visitor Location Register) corresponds to each MSC and contains temporary information about the mobile, information like mobile identification and the cell (or a group of cells) where the mobile is currently situated. Using information form the VLR the MSC is able to switch the information (short message) to the corresponding BSS (Base Station System, BSC + BTSs), which transmits the short message to the mobile. The BSS consists of transceivers, which send and receive information over the air interface, to and from the mobile station. This information is passed over the signaling channels so the mobile can receive messages even if a voice or data call is going on.