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  1. 21 points
    Josh HillSprint 4G Rollout UpdatesFriday, April 5, 2019 - 3:06 AM PDT Now that VoLTE is actually rolling out on Sprint, it's a good time to dive into what exactly is VoLTE, and how is it different from Calling+ and VoWiFi (Wifi Calling). Background Terms E-UTRA or EUTRA: Stands for Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access. This is the technical name for the actual LTE airlink. QoS: Quality of Service. This is a way of tagging / flagging certain types of traffic to have priority above or below other traffic. When traffic has a QoS tag higher than other traffic, network equipment (the tower, routers, etc) will drop or ignore lower priority traffic to ensure that this traffic goes through instead. The equipment can also be configured to reserve a certain amount of bandwidth to only be used by traffic with a particular QoS tag. For example, if a router has 10 Mbps available, it can allocate 1 Mbps for a certain QoS tag. Normal traffic will only be able to use 9 Mbps, with 1 Mbps reserved for that QoS tag. The number of QoS priorities / tags varies between equipment vendors, but can be in excess of 256 priority levels. QCI: QoS Class Identifier. This is a value that an LTE / E-UTRA session can be assigned that corresponds to a particular QoS tag and specific attributes of that particular QoS queue. For example, it may or may not specify a guaranteed/dedicated bandwidth allocation (GBR). APN: The APN is the name of the gateway on a mobile network. It identifies the packet data network that should be used for that E-UTRA session. IMS: IP Multimedia Subsystem. It is a method for sending SMS over LTE, along with setting up VoLTE calls and other signaling. eCSFB: Circuit Switched Fall Back. For phones / UEs that can only listen on either LTE or CDMA rather than both simultaneously, it is a method for the LTE network to tell the device that a call is coming in, and to switch over to CDMA to process it. SRLTE: Single Radio LTE. This is a capability of newer devices that allows them to listen on both CDMA and LTE at the same time, but only transmit on one at a time. This replaces the need for eCSFB, allowing the device to see a call coming in over CDMA while it’s using LTE. It is also more reliable and reduces the number of missed calls due to failed fallback. When a call is active, the LTE session is stopped / paused. SIP: Session Initialization Protocol. This is the standard protocol for VoIP in telecom networks. How VoLTE Works While we typically think of LTE as a single connection, multiple E-UTRA “sessions” can actually be established, creating what are essentially virtual/multiple LTE interfaces, each with their own IP address, QoS level, APN, etc. Each session has a numerical QCI assigned that dictates the actual QoS priority and whether or not it has a GBR (Guaranteed Bitrate). QCI Resource Type QoS Priority Packet Delay Budget Packet Error Loss Rate Example Services 1 GBR 2 100ms 10−2 Conversational Voice 2 GBR 4 150ms 10−3 Conversational Video (Live Streaming) 3 GBR 3 50ms 10−3 Real Time Gaming, V2X messages 4 GBR 5 300ms 10−6 Non-Conversational Video (Buffered Streaming) 65 GBR 0.7 75ms 10−2 Mission Critical user plane Push To Talk voice (e.g., MCPTT) 66 GBR 2 100ms 10−2 Non-Mission-Critical user plane Push To Talk voice 75 GBR 2.5 50ms 10−2 V2X messages 5 non-GBR 1 100ms 10−6 IMS Signalling 6 non-GBR 6 300ms 10−6 Video (Buffered Streaming) TCP-Based (for example, www, email, chat, ftp, p2p and the like) 7 non-GBR 7 100ms 10−3 Voice, Video (Live Streaming), Interactive Gaming 8 non-GBR 8 300ms 10−6 Video (Buffered Streaming) TCP-Based (for example, www, email, chat, ftp, p2p and the like) 9 non-GBR 9 300ms 10−6 Video (Buffered Streaming) TCP-Based (for example, www, email, chat, ftp, p2p and the like). Typically used as default bearer 69 non-GBR 0.5 60ms 10−6 Mission Critical delay sensitive signalling (e.g., MC-PTT signalling) 70 non-GBR 5.5 200ms 10−6 Mission Critical Data (e.g. example services are the same as QCI 6/8/9) 79 non-GBR 6.5 50ms 10−2 V2X messages (source: https://en.wikipedia.org/wiki/QoS_Class_Identifier) As you can see in the above table, the QCI does not necessarily correspond to the QoS level. For example, QCI 1 has a QoS priority of 2, but QCI 5 has a QoS priority of 1, making it actually higher priority traffic. On Sprint, traditionally one E-UTRA session was used, with a QCI of 9 and QoS priority of 9. This is the lowest QoS priority, and does not have a guaranteed bitrate. On devices which use eCSFB or VoLTE, another E-UTRA session is established for the IMS APN using a QCI of 5 and QoS priority of 1, and is used for IMS. This session also does not have a guaranteed bitrate, but it has the highest QoS priority. IMS is used for SMS over LTE, along with setting up VoLTE calls. eCSFB devices use it for SMS, and likely also for triggering eCSFB. On newer device which instead use SRLTE, IMS is not used unless VoLTE is enabled, and they instead use CDMA 1x for SMS, so an IMS E-UTRA session is often not setup. When a VoLTE call is initiated, a third E-UTRA session is established, also using the IMS APN. This session has a QCI of 1 and QoS priority of 2. Unlike the other two sessions, this one does have a guaranteed bitrate. For Sprint, this bitrate is 39 Kbps. The screenshot below shows all 3 sessions: VoLTE E-UTRA sessions This is how VoLTE calls are prioritized over regular data. Normal data usage, such as loading a web page or watching a video, will still use the lower, default QoS (QCI of 9), while the data for the VoLTE call will be at the second highest priority (QCI 1), just after IMS signaling (QCI 5). The tower / eNB will ensure that the VoIP session always is able to use up to 39 Kbps by reserving that bandwidth and dedicating it to the call. This is in contrast to “Calling+”, which does not establish a separate E-UTRA session, and instead uses the normal QCI 9 session. The below screenshot shows an active Calling+ call. Note the presence of only a single E-UTRA session. Calling+ E-UTRA sessions So now that we have the airlink for VoLTE, what happens? VoLTE, Calling+, and VoWiFi are essentially standard SIP VoIP calls. The below screenshots show the SIP details for an active call, and the LTE Signaling messages that setup and then end the SIP call. VoLTE SIP details VoLTE Signaling For VoLTE, the traffic for the SIP call goes over the QCI 1 E-UTRA session instead of the normal QCI 9 session. This means that the eNB (tower) will reserve and guarantee 39 kbps for the call, but other traffic from the same device will not be prioritized and will use the normal session. So starting a VoLTE call will not make the rest of your traffic prioritized, it will apply only to the VoLTE call. So as a recap, when VoLTE is enabled, the UE / phone establishes multiple E-UTRA sessions. One is used for normal usage, one is used for texting and signaling, and one is used for the VoLTE call. Think of these like separate virtual ethernet cables. On the QoS prioritized and guaranteed bitrate VoLTE session, the UE establishes a SIP VoIP connection for a call. On Calling+ devices, the same SIP connection is used, however it runs over the default QCI 9 session instead, and therefore isn’t prioritized and doesn't have a guaranteed bandwidth. This is why Calling+ calls are more likely to cut out or not sound as good. VoLTE call Calling+ call VoWiFi (Wifi calling) operates almost the same way. Like VoLTE and Calling+, it also uses the same SIP connection for calls and presumably IMS for signaling, but instead of using an LTE E-UTRA session, the phone establishes an IKEv2 IPsec VPN connection to Sprint. This is an encrypted connection that allows data to be tunneled directly into Sprint’s network. The SIP and IMS traffic are then routed over this VPN to Sprint, but not other, normal traffic. From a QoS perspective, VoWiFi is identical to Calling+, in that neither are prioritized above other traffic. VoWiFi call Because VoLTE, Calling+, and VoWiFi all use the same SIP servers and connections, under normal conditions they sound the same and can technically hand off to one another. They can all take advantage of HD Voice codecs and should sound the same, since the call itself is identical across all three. The difference is how the data for that call makes it to Sprint. VoLTE is able to use a dedicated, guaranteed airlink to ensure that congestion on the network (LTE or WiFi) won’t adversely affect the call. One final performance benefit is that VoLTE is able to take advantage of something called RoHC (Robust Header Compression), seen in the above 3 screenshots. This compresses the IP, TCP, UDP, and RTP headers from 60 bytes to 1-3 bytes, resulting in up to 60% bandwidth savings. It’s only possible on a dedicated link, which is why VoLTE has it but Calling+ and VoWiFi do not. So not only does VoLTE have guaranteed, dedicated bandwidth, it will use potentially half as much, which matters a lot for maintaining the call in edge of cell scenarios.
  2. 1 point
    by Tim Yu Sprint 4G Rollout Updates Friday, April 8, 2016 - 3:40 PM MDT Over the past week, S4GRU members in multiple Sprint markets have discovered new EARFCNs and corresponding GCI endings that identify new Band 41 LTE carriers. The EARFCN is the center frequency of an LTE carrier that, along with the carrier bandwidth, identifies the carrier placement and occupied spectrum. As Sprint is doing intraband contiguous/adjacent carrier aggregation -- 20 MHz TDD Band 41 carriers are lined up right next to one another with no gaps -- Band 41 EARFCNs are highly predictable in a given market by knowing the location of at least one carrier. Say the ever popular EARFCN 40978 is the first carrier. You add 198 (19.8 MHz) to it to get EARFCN 41176, which is the second Band 41 carrier. Thus, it stands to reason if you add 198 to that EARFCN, you will get the third Band 41 carrier so 41176 + 198 = 41374, the EARFCN for the third Band 41 carrier. Or, in some other markets, 41078 is the first Band 41 carrier. In this case, it goes like this: 41078 + 198 = 41276 + 198 = 41474 Alternatively, say a market has EARFCNs 40056 and 40254. Adding 198 would bring us to EARFCN 40452, but that is not possible due to the BRS/EBS 2500-2600 MHz band plan -- there are spectrum gaps around 2570 MHz and 2610 MHz that Sprint cannot utilize. See the band plan: Thus, in the case of EARFCNs 40056 and 40254, not addition, but use subtraction: 40056 - 198 = 39858, which would be the third Band 41 carrier. The GCI is the unique cell sector identifier of a LTE carrier. Generally speaking, Sprint's GCI patterns are standardized market by market and network wide, making for an easy method to identify each LTE carrier within a given band. In the case of Band 41, GCIs ending in 00/01/02 (Samsung) or x1/x2/x3 (ALU/NSN) indicate a connection to the original and first Band 41 carrier. GCIs ending in 03/04/05 (STA) or x9/xA/xB (ALU/NSN) denote the second Band 41 carrier. It stands to reason that -- if this second carrier pattern were to continue to the third Band 41 carrier in Samsung markets -- we would expect to see GCIs possibly ending in 06/07/08. Long story short, this theory is supported by evidence. See below SignalCheck Pro logs and numerous in app screenshots: This log is from my Nexus 5x. I traveled for a hour around Sacramento, searching for the third Band 41 carrier. Note the GCI endings for the Sprint Band 41 entries: The following is from site member bmoses in Des Moines. Note the 07 GCI ending and EARFCN: Below is from a S4GRU member in the Colorado market: One more from the Cincinnati, Ohio Market: The following is from yours truly in Sacramento: And these below are from Fremont, CA, near San Jose: See those EARFCNs and GCI endings? Look at the calculations from the top of this PSA. Everything is as we would have predicted for a third carrier. With the end of the WiMAX injunction and the decomissioning of the last active Clearwire WiMAX markets that held up huge swaths of leased EBS and licensed BRS spectrum, Sprint finally has the capability to show off its deep spectrum pockets in numerous markets. This has been long awaited and should definitely give a huge boost to Sprint data speeds in numerous markets where spectrum for additional Band 41 carriers now is available. Of course, there still is the issue of actually connecting to and using this third Band 41 carrier. From firsthand reports and personal use, this third Band 41 carrier is not currently carrier aggregation enabled. Thus, 2x/3x CA devices may not connect to it to use data right now. By default, these devices will have CA enabled, causing them to utilize only the first and second carriers that they can aggregate. For the time being, in order to reliably connect to this third carrier, a non CA triband device or a 2x/3x CA setting disabled triband device may be required. Regardless, this appears to be just a minor issue from the initial rollouts that should be resolved soon. Source: S4GRU member reports
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