All answers have been held as short as possible and require an understanding and study of the corresponding chapter of the book.
Answers for the LTE and VoLTE chapters:
LTE Answer 1:
A 10 MHz FDD LTE channel is split into 600 sub-carriers.
LTE Answer 2:
X2 Handovers are performed directly between two eNodeBs while an S1 handover requires the help of the MME. S1 handovers are only used when two eNodeBs are unable to communicate with each other which should happen rarely in practice.
LTE Answer 3:
The MME performs the subscriber and connection management such as user authentication, location management, bearer establishment, etc. while the Serving-Gateway handles the user data (i.e. the IP packets) that are exchanged between a device and the Internet.
LTE Answer 4:
7 symbols on the time axis and 12 sub-carriers on the frequency access are grouped into a Resource Block which takes 0.5 ms to transmit. Two Resource Blocks are bundled together to form the smallest unit that can be scheduled to a user.
LTE Answer 5:
If the UE is in Idle state it has to use the random access channel to establish a connection to the network. When the network receives the request it assigns uplink resources to the device via the Physical Downlink Control Channel (PDCCH). The UE receives the assignment and can then make use of the assigned resources on the Physical Uplink Shared channel.
LTE Answer 6:
HARQ is a mechanism on Layer 2 of the radio protocol stack and ensures that faulty resource blocks are immediately retransmitted. ARQ works on the RLC layer further up in the protocol stack and ensures that larger chunks of data are repeated in case HARQ fails. While HARQ is always used, ARQ is not used for voice bearers (only IMS VoIP) as there is no time to retransmit complete VoIP packages due to the requirement to have low jitter and delay values for voice packets. For VoIP it is preferable to drop missing packets instead of repeating them.
LTE Answer 7:
A default bearer is usually assigned when the device is switched on. It serves as a bearer for Internet connectivity. A device can have several default bearer simultaneously, e.g. one for Internet connectivity and one bearer for IMS. In practice, devices usually only have one default bearer.
Dedicated bearers are established by services alongside default bearers such as the IMS to ensure a certain quality of service (minimal bandwidth, jitter, delay, precedence over other bearers) for voice, video and other time critical and delay sensitive streams.
LTE Answer 8:
Discontinuous Reception (and Transmission) is very useful in RRC connected state to reduce power consumption. Without DRX a UE has to observe the Downlink Control Channel frequently as resources could be assigned at any time. When DRX is activated the UE can turn off the receiver most of the time and only listen occasionally. This significantly reduces power consumption at the expense of slightly higher latency when data is only transmitted infrequently. Typical DRX values are a few hundred milliseconds for the activation time for DRX (after the reception of the last data frame) and activity times of a few milliseconds during an interval of several hundred milliseconds.
LTE Answer 9:
In Idle state no bearer is established to the network and the UE controls cell changes and changes to other radio networks when running out of LTE coverage autonomously.
LTE Answer 10:
When running out of LTE coverage there are several methods to guide the UE to another radio network (e.g. UMTS). An easy solution is to use a Cell Change Order which tells the UE to which UMTS cell to go. The connection is then interrupted and the UE searches the given cell, reads the system information and performs the required procedure to establish a connection in the other radio technology. This process typically takes a few seconds during which no data can be exchanged. This method is simple for implementing in the network and the mobile device but not suitable for applications such as VoLTE, for which the data bearer should be handed over between radio networks quickly. This is possible with LTE to UMTS (or GSM) handovers as the cell in the target network can be prepared for the incoming UE. The UE is then given specific instructions of how the target cell can be accessed which reduces the outage time to a few hundred milliseconds.
LTE Answer 11:
MME and S-GWs usually have the Gn GPRS interface implemented and can thus act as SGSNs and GGSNs towards 2G/3G PS core network equipment. In other words, they emulate behavior those network nodes understand so no software modifications are required in existing networks. It should be noted that in practice today, most network operators have merged corresponding 2G, 3G and LTE core networks into a single physical node and the interfaces between the logical components are handled internally.
LTE Answer 12:
This is done via the SGs interface which connects the MME to Mobile Switching Centers in the 2G/3G networks to deliver SMS over LTE and to perform a (CS = circuit switched) fallback to GSM or UMTS for incoming (and outgoing) voice calls.
LTE Answer 13:
Internet based voice services can not request special quality of service settings from the mobile core and access network (i.e. dedicated bearers, see above). Depending on the network load this can result in bad voice quality if voice packets are not preferred over other packets (e.g. from web browsing from the same or another UE) in loaded cells. Also, Internet based VoIP services can’t interact with the mobile network to perform handover to GSM or a circuit switched UMTS channel when the edge of a broadband wireless network has been reached. Operator voice services can hand over a call to GSM (Single Radio Voice Call Continuity).
LTE Answer 14:
eNodeBs have to have a high speed link to the core network to accommodate the high speed air interface data traffic. The best option is to use a fiber optic cable. If not available, other options are Ethernet based microwave links or VDSL links.
VoLTE Answer 1:
The main components of the IMS are the Serving Call Session Control Function (S-CSCF) that is the central node that handles all SIP messages. Usually, SIP messages are forwarded to an Application Server (AS) such as the Telephony Application Server that implements telephony functionality. The Proxy-CSCF sits between the S-CSCF and the mobile device and is used for tasks such as generating SIP messages for the UE when the UE is unable to do so (e.g. loss of coverage). The Interrogating CSCF (I-CSCF) is contacted when the UE sends an initial Register. It contacts the HSS (Home Subscriber Server) to get information on the user and then assignes a S-CSCF that will handle all subsequent communication.
VoLTE Answer 2:
During SIP registration an IPSec tunnel is established between the UE and P-CSCF. While encryption is optional, IPSec authentication ensures that only messages from the UE are accepted.
VoLTE Answer 3:
Preconditions are used to inform devices that a dedicated bearer has to be established for the speech path on one or both ends of a connection before a call can be further processed. In the core network, precondition messaging is used to trigger the establishment of the dedicated bearer.
VoLTE Answer 4:
The P-CSCF, which is a mobile network component, inserts the ‘asserted identity’, which is the device’s phone number (MSISDN), into SIP messages sent by the UE and then forwards those enriched SIP messages to the S-CSCF. This prevents the UE from forging its phone number.
VoLTE Answer 5:
As the payload of voice packets are small the IP, UDP, RTP header information makes up a large part of the overall packet. Therefore, header compression is used to significantly reduce this overhead which increases the number of simultaneous calls per cell.
VoLTE Answer 6:
Call forwarding settings are managed via the XCAP protocol between the UE and the network. XCAP is an XML protocol and different call forwarding options such all call forward no reply, call forward not reachable, etc. are XML encoded.
VoLTE Answer 7:
For emergency calls an IMS emergency bearer is established that is independent from the standard IMS bearer that is used for ordinary voice calls. The IMS emergency bearer is established with the highest priority in the radio and core network to guarantee emergency calls a high quality speech and signaling path even in fully loaded networks.
VoLTE Answer 8:
Unlike typical handovers that are controlled by the network, VoLTE to VoWifi handovers are controlled by the mobile device. When the device senses that LTE coverage is about to be lost, it establishes an IPSec tunnel to the evolved Packet Data Gateway (ePDG) and includes information during the tunnel establishment that allows the network to move the existing IMS bearer away from the current MME and S-GW to the ePDG. All IP packets of the connection are then automatically redirected to the ePDG and an ongoing voice call continues with only a short interruption during the redirection process.
VoLTE Answer 9:
VoWifi cellular preferred means that the UE will only connect to the ePDG and move the IMS bearer when no cellular coverage (LTE, 3G, 2G) is available. VoWifi Wifi preferred means that the IMS bearer is moved to Wifi as soon as a suitable Wifi connection is available.
VoLTE Answer 10:
MC-PTT only allows one person in a communication group to speak at a time. A central instance is required to control who is allowed to talk and deny requests from other parties if there is already another speaker in the call.
