| [[osmux]] |
| = OSmux: reduce of SAT uplink costs by protocol optimizations |
| |
| == Problem |
| |
| In case of satellite based GSM systems, the transmission cost on the back-haul |
| is relatively expensive. The billing for such SAT uplink is usually done in a |
| pay-per-byte basis. Thus, reducing the amount of bytes transferred would |
| significantly reduce the cost of such uplinks. In such environment, even |
| seemingly small protocol optimizations, eg. message batching and trunking, can |
| result in significant cost reduction. |
| |
| This is true not only for speech codec frames, but also for the constant |
| background load caused by the signalling link (A protocol). Optimizations in |
| this protocol are applicable to both VSAT back-haul (best-effort background IP) |
| as well as Inmarsat based links (QoS with guaranteed bandwidth). |
| |
| == Proposed solution |
| |
| In order to reduce the bandwidth consumption, this document proposes to develop |
| a multiplex protocol that will be used to proxy voice and signalling traffic |
| through the SAT links. |
| |
| === Voice |
| |
| For the voice case, we propose a protocol that provides: |
| |
| * Batching: that consists of putting multiple codec frames on the sender side |
| into one single packet to reduce the protocol header overhead. This batch |
| is then sent as one RTP/UDP/IP packet at the same time. Currently, AMR 5.9 |
| codec frames are transported in a RTP/UDP/IP protocol stacking. This means |
| there are 15 bytes of speech codec frame, plus a 2 byte RTP payload header, |
| plus the RTP (12 bytes), UDP (8 bytes) and IP (20 bytes) overhead. This means |
| we have 40 byte overhead for 17 byte payload. |
| |
| * Trunking: in case of multiple concurrent voice calls, each of them will |
| generate one speech codec frame every 20ms. Instead of sending only codec |
| frames of one voice call in a given IP packet, we can 'interleave' or trunk |
| the codec frames of multiple calls into one IP. This further increases the |
| IP packet size and thus improves the payload/overhead ratio. |
| |
| Both techniques should be applied without noticeable impact in terms of user |
| experience. As the satellite back-haul has very high round trip time (several |
| hundred milliseconds), adding some more delay is not going to make things |
| significantly worse. |
| |
| For the batching, the idea consists of batching multiple codec frames on the |
| sender side, A batching factor (B) of '4' means that we will send 4 codec |
| frames in one underlying protocol packet. The additional delay of the batching |
| can be computed as (B-1)*20ms as 20ms is the duration of one codec frame. |
| Existing experimentation has shown that a batching factor of 4 to 8 (causing a |
| delay of 60ms to 140ms) is acceptable and does not cause significant quality |
| degradation. |
| |
| The main requirements for such voice RTP proxy are: |
| |
| * Always batch codec frames of multiple simultaneous calls into single UDP |
| message. |
| |
| * Batch configurable number codec frames of the same call into one UDP |
| message. |
| |
| * Make sure to properly reconstruct timing at receiver (non-bursty but |
| one codec frame every 20ms). |
| |
| * Implementation in libosmo-netif to make sure it can be used |
| in osmo-bts (towards osmo-bsc), osmo-bsc (towards osmo-bts and |
| osmo-bsc_nat) and osmo-bsc_nat (towards osmo-bsc) |
| |
| * Primary application will be with osmo-bsc connected via satellite link to |
| osmo-bsc_nat. |
| |
| * Make sure to properly deal with SID (silence detection) frames in case |
| of DTX. |
| |
| * Make sure to transmit and properly re-construct the M (marker) bit of |
| the RTP header, as it is used in AMR. |
| |
| * Primary use case for AMR codec, probably not worth to waste extra |
| payload byte on indicating codec type (amr/hr/fr/efr). If we can add |
| the codec type somewhere without growing the packet, we'll do it. |
| Otherwise, we'll skip this. |
| |
| === Signalling |
| |
| Signalling uses SCCP/IPA/TCP/IP stacking. Considering SCCP as payload, this |
| adds 3 (IPA) + 20 (TCP) + 20 (IP) = 43 bytes overhead for every signalling |
| message, plus of course the 40-byte-sized TCP ACK sent in the opposite |
| direction. |
| |
| While trying to look for alternatives, we consider that none of the standard IP |
| layer 4 protocols are suitable for this application. We detail the reasons |
| why: |
| |
| * TCP is a streaming protocol aimed at maximizing the throughput of a stream |
| within the constraints of the underlying transport layer. This feature is |
| not really required for the low-bandwidth and low-pps GSM signalling. |
| Moreover, TCP is stream oriented and does not conserve message boundaries. |
| As such, the IPA header has to serve as a boundary between messages in the |
| stream. Moreover, assuming a generally quite idle signalling link, the |
| assumption of a pure TCP ACK (without any data segment) is very likely to |
| happen. |
| |
| * Raw IP or UDP as alternative is not a real option, as it does not recover |
| lost packets. |
| |
| * SCTP preserves message boundaries and allows for multiple streams |
| (multiplexing) within one connection, but it has too much overhead. |
| |
| For that reason, we propose the use of LAPD for this task. This protocol was |
| originally specified to be used on top of E1 links for the A interface, who |
| do not expose any kind of noticeable latency. LAPD resolves (albeit not as |
| good as TCP does) packet loss and copes with packet re-ordering. |
| |
| LAPD has a very small header (3-5 octets) compared to TCPs 20 bytes. Even if |
| LAPD is put inside UDP, the combination of 11 to 13 octets still saves a |
| noticeable number of bytes per packet. Moreover, LAPD has been modified for less |
| reliable interfaces such as the GSM Um interface (LAPDm), as well as for the |
| use in satellite systems (LAPsat in ETSI GMR). |
| |
| == OSmux protocol |
| |
| The OSmux protocol is the core of our proposed solution. This protocol operates |
| over UDP or, alternatively, over raw IP. The designated default UDP port number |
| and IP protocol type have not been yet decided. |
| |
| Every OSmux message starts with a control octet. The control octet contains a |
| 2-bit Field Type (FT) and its location starts on the 2nd bit for backward |
| compatibility with older versions (used to be 3 bits). The FT defines the |
| structure of the remaining header as well as the payload. |
| |
| The following FT values are assigned: |
| |
| * FT == 0: LAPD Signalling |
| * FT == 1: AMR Codec |
| * FT == 2: Dummy |
| * FT == 3: Reserved for Fture Use |
| |
| There can be any number of OSmux messages batched up in one underlying packet. |
| In this case, the multiple OSmux messages are simply concatenated, i.e. the |
| OSmux header control octet directly follows the last octet of the payload of the |
| previous OSmux message. |
| |
| |
| === LAPD Signalling (0) |
| |
| [packetdiag] |
| ---- |
| { |
| colwidth = 32 |
| node_height = 40 |
| |
| 0: - |
| 1-2: FT |
| 3-7: ---- |
| 8-15: PL-LENGTH |
| 16-31: LAPD header + payload |
| } |
| ---- |
| |
| Field Type (FT): 2 bits:: |
| The Field Type allocated for LAPD Signalling frames is "0". |
| |
| This frame type is not yet supported inside OsmoCom and may be subject to |
| change in future versions of the protocol. |
| |
| |
| === AMR Codec (1) |
| |
| This OSmux packet header is used to transport one or more RTP-AMR packets for a |
| specific RTP stream identified by the Circuit ID field. |
| |
| [packetdiag] |
| ---- |
| { |
| colwidth = 32 |
| node_height = 40 |
| |
| 0: M |
| 1-2: FT |
| 3-5: CTR |
| 6: F |
| 7: Q |
| 8-15: Red. TS/SeqNR |
| 16-23: Circuit ID |
| 24-27: AMR FT |
| 28-31: AMR CMR |
| } |
| ---- |
| |
| Marker (M): 1 bit:: |
| This is a 1:1 mapping from the RTP Marker (M) bit as specified in RFC3550 |
| Section 5.1 (RTP) as well as RFC3267 Section 4.1 (RTP-AMR). In AMR, the Marker |
| is used to indicate the beginning of a talk-spurt, i.e. the end of a silence |
| period. In case more than one AMR frame from the specific stream is batched into |
| this OSmux header, it is guaranteed that the first AMR frame is the first in the |
| talkspurt. |
| |
| Field Type (FT): 2 bits:: |
| The Field Type allocated for AMR Codec frames is "1". |
| |
| Frame Counter (CTR): 2 bits:: |
| Provides the number of batched AMR payloads (starting 0) after the header. For |
| instance, if there are 2 AMR payloads batched, CTR will be "1". |
| |
| AMR-F (F): 1 bit:: |
| This is a 1:1 mapping from the AMR F field in RFC3267 Section 4.3.2. In case |
| there are multiple AMR codec frames with different F bit batched together, we |
| only use the last F and ignore any previous F. |
| |
| AMR-Q (Q): 1 bit:: |
| This is a 1:1 mapping from the AMR Q field (Frame quality indicator) in RFC3267 |
| Section 4.3.2. In case there are multiple AMR codec frames with different Q bit |
| batched together, we only use the last Q and ignore any previous Q. |
| |
| Circuit ID Code (CIC): 8 bits:: |
| Identifies the Circuit (Voice call), which in RTP is identified by {srcip, |
| srcport, dstip, dstport, ssrc}. |
| |
| Reduced/Combined Timestamp and Sequence Number (RCTS): 8 bits:: |
| Resembles a combination of the RTP timestamp and sequence number. In the GSM |
| system, speech codec frames are generated at a rate of 20ms. Thus, there is no |
| need to have independent timestamp and sequence numbers (related to a 8kHz |
| clock) as specified in AMR-RTP. |
| |
| AMR Codec Mode Request (AMR-FT): 4 bits:: |
| This is a mapping from the AMR FT field (Frame type index) in RFC3267 Section |
| 4.3.2. The length of each codec frame needs to be determined from this field. It |
| is thus guaranteed that all frames for a specific stream in an OSmux batch are |
| of the same AMR type. |
| |
| AMR Codec Mode Request (AMR-CMR): 4 bits:: |
| The RTP AMR payload header as specified in RFC3267 contains a 4-bit CMR field. |
| Rather than transporting it in a separate octet, we squeeze it in the lower four |
| bits of the clast octet. In case there are multiple AMR codec frames with |
| different CMR, we only use the last CMR and ignore any previous CMR. |
| |
| ==== Additional considerations |
| |
| * It can be assumed that all OSmux frames of type AMR Codec contain at least 1 |
| AMR frame. |
| * Given a batch factor of N frames (N>1), it can not be assumed that the amount |
| of AMR frames in any OSmux frame will always be N, due to some restrictions |
| mentioned above. For instance, a sender can decide to send before queueing the |
| expected N frames due to timing issues, or to conform with the restriction |
| that the first AMR frame in the batch must be the first in the talkspurt |
| (Marker M bit). |
| |
| |
| === Dummy (2) |
| |
| This kind of frame is used for NAT traversal. If a peer is behind a NAT, its |
| source port specified in SDP will be a private port not accessible from the |
| outside. Before other peers are able to send any packet to it, they require the |
| mapping between the private and the public port to be set by the firewall, |
| otherwise the firewall will most probably drop the incoming messages or send it |
| to a wrong destination. The firewall in most cases won't create a mapping until |
| the peer behind the NAT sends a packet to the peer residing outside. |
| |
| In this scenario, if the peer behind the nat is expecting to receive but never |
| transmit audio, no packets will ever reach him. To solve this, the peer sends |
| dummy packets to let the firewall create the port mapping. When the other peers |
| receive this dummy packet, they can infer the relation between the original |
| private port and the public port and start sending packets to it. |
| |
| When opening a connection, the peer is expected to send dummy packets until it |
| starts sending real audio, at which point dummy packets are not needed anymore. |
| |
| [packetdiag] |
| ---- |
| { |
| colwidth = 32 |
| node_height = 40 |
| |
| 0: - |
| 1-2: FT |
| 3-5: CTR |
| 6-7: -- |
| 8-15: ---- |
| 16-23: Circuit ID |
| 24-27: AMR FT |
| 28-31: ---- |
| } |
| ---- |
| |
| Field Type (FT): 2 bits:: |
| The Field Type allocated for Dummy frames is "2". |
| |
| Frame Counter (CTR): 2 bits:: |
| Provides the number of dummy batched AMR payloads (starting 0) after the header. |
| For instance, if there are 2 AMR payloads batched, CTR will be "1". |
| |
| Circuit ID Code (CIC): 8 bits:: |
| Identifies the Circuit (Voice call), which in RTP is identified by {srcip, |
| srcport, dstip, dstport, ssrc}. |
| |
| AMR Codec Mode Request (AMR-FT): 4 bits:: |
| This field must contain any valid value described in the AMR FT field (Frame |
| type index) in RFC3267 Section 4.3.2. |
| |
| ==== Additional considerations |
| |
| * After the header, additional padding needs to be allocated to conform with CTR |
| and AMR FT fields. For instance, if CTR is 0 and AMR FT is AMR 6.9, a padding |
| of 17 bytes is to be allocated after the header. |
| |
| * On receival of this kind of OSmux frame, it's usually enough for the reader to |
| discard the header plus the calculated padding and keep operating. |
| |
| == Sequence Charts |
| |
| === Trunking |
| |
| Following chart shows how trunking works for 3 concurrent calls from different |
| MS on a given BTS. In this case only uplink data is shown, but downlink follows |
| the same idea. Batching factor is set to 1 to easily illustrate trunking mechanism. |
| |
| It can be seen how 3 RTP packets from 3 different Ms (a, b, and c) arrive to the |
| BSC from the BTS. The BSC generates 3 OSmux frames and stores and sends them |
| together in one UDP packet to the BSC-NAT. The BSC-NAT decodes the three OSmux |
| frames, identifies each of them through CID values and transform them back to |
| RTP before sending them to the MGW. |
| |
| ["mscgen"] |
| ---- |
| msc { |
| hscale = 2; |
| bts [label="BTS"], bsc [label="BSC"], bscnat [label="BSC-NAT"], mgw [label="MGW"]; |
| |
| ...; |
| --- [label="3 Regular RTP-AMR calls using OSmux (has been ongoing for some time)"]; |
| |
| bts => bsc [label="RTP-AMR[seq=y,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=z,ssrc=MSc]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m,AMR(y)],Osmux[ft=2,cid=i+1,seq=n,AMR(x)],Osmux[ft=2,cid=i+2,seq=l,AMR(z)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o,ssrc=r] (originally seq=y,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p,ssrc=s] (originally seq=x,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=q,ssrc=t] (originally seq=z,ssrc=MSc)"]; |
| bts => bsc [label="RTP-AMR[seq=y+1,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x+1,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=z+1,ssrc=MSc]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m+1,AMR(y+1)],Osmux[ft=2,cid=i+1,seq=n+1,AMR(x+1)],Osmux[ft=2,cid=i+2,seq=l+1,AMR(z+1)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+1,ssrc=r] (originally seq=y+1,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+1,ssrc=s] (originally seq=x+1,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=q+1,ssrc=t] (originally seq=z+1,ssrc=MSc)"]; |
| bts => bsc [label="RTP-AMR[seq=y+2,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x+2,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=z+2,ssrc=MSc]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m+2,AMR(y+2)],Osmux[ft=2,cid=i+1,seq=n+2,AMR(x+2)],Osmux[ft=2,cid=i+2,seq=l+2,AMR(z+2)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+2,ssrc=r] (originally seq=y+2,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+2,ssrc=s] (originally seq=x+2,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=q+2,ssrc=t] (originally seq=z+2,ssrc=MSc)"]; |
| } |
| ---- |
| |
| === Batching |
| |
| Following chart shows how batching with a factor of 3 works. To easily |
| illustrate batching, only uplink and one concurrent call is considered. |
| |
| It can be seen how 3 RTP packets from MSa arrive to the BSC from the BTS. The |
| BSC queues the 3 RTP packets and once the batchfactor is reached, an OSmux frame |
| is generated and sent to the BSC-NAT. The BSC-NAT decodes the OSmux frames, |
| transforms each AMR payload into an RTP packet and each RTP packet is scheduled |
| for delivery according to expected proportional time delay (and timestamp field |
| is set accordingly). |
| |
| ["mscgen"] |
| ---- |
| msc { |
| hscale = 2; |
| bts [label="BTS"], bsc [label="BSC"], bscnat [label="BSC-NAT"], mgw [label="MGW"]; |
| |
| ...; |
| --- [label="Regular RTP-AMR call using OSmux with batch factor 3 (has been ongoing for some time)"]; |
| |
| bts => bsc [label="RTP-AMR[seq=x,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x+1,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x+2,ssrc=MSa]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m,AMR(x),AMR(x+1),AMR(x+2)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o,ssrc=r] (originally seq=x,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+1,ssrc=r] (originally seq=x+1,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+2,ssrc=r] (originally seq=x+2,ssrc=MSa)"]; |
| bts => bsc [label="RTP-AMR[seq=x+3,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x+4,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x+5,ssrc=MSa]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m+1,AMR(x+3),AMR(x+4),AMR(x+5)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+3,ssrc=r] (originally seq=x+3,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+4,ssrc=r] (originally seq=x+4,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+5,ssrc=r] (originally seq=x+5,ssrc=MSa)"]; |
| bts => bsc [label="RTP-AMR[seq=x+6,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x+7,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=x+8,ssrc=MSa]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m+2,AMR(x+6),AMR(x+7),AMR(x+8)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+6,ssrc=r] (originally seq=x+6,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+7,ssrc=r] (originally seq=x+7,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+8,ssrc=r] (originally seq=x+8,ssrc=MSa)"]; |
| } |
| ---- |
| |
| === Trunking and Batching |
| |
| Following chart shows how trunking and batching work together. The chart shows 2 |
| concurrent calls from different MS on a given BTS, and BSC is configured with a |
| batch factor of 3. Again only uplink data is shown, but downlink follows the |
| same idea. Batching factor is set to 1 to easily illustrate trunking mechanism. |
| |
| ["mscgen"] |
| ---- |
| msc { |
| hscale = 2; |
| bts [label="BTS"], bsc [label="BSC"], bscnat [label="BSC-NAT"], mgw [label="MGW"]; |
| |
| ...; |
| --- [label="2 Regular RTP-AMR call using OSmux with batch factor 3 (has been ongoing for some time)"]; |
| |
| bts => bsc [label="RTP-AMR[seq=x,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=x+1,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+1,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=x+2,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+2,ssrc=MSb]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m,AMR(x),AMR(x+1),AMR(x+2)],Osmux[ft=2,cid=i+1,seq=n,AMR(y),AMR(y+1),AMR(y+2)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o,ssrc=r] (originally seq=x,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p,ssrc=s] (originally seq=y,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+1,ssrc=r] (originally seq=x+1,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+1,ssrc=s] (originally seq=y+1,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+2,ssrc=r] (originally seq=x+2,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+2,ssrc=s] (originally seq=y+2,ssrc=MSb)"]; |
| bts => bsc [label="RTP-AMR[seq=x+3,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+3,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=x+4,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+4,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=x+5,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+5,ssrc=MSb]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m+1,AMR(x+3),AMR(x+4),AMR(x+5)],Osmux[ft=2,cid=i+1,seq=n+1,AMR(y+3),AMR(y+4),AMR(y+5)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+3,ssrc=r] (originally seq=x+3,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+3,ssrc=s] (originally seq=y+3,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+4,ssrc=r] (originally seq=x+4,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+4,ssrc=s] (originally seq=y+4,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+5,ssrc=r] (originally seq=x+5,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+5,ssrc=s] (originally seq=y+5,ssrc=MSb)"]; |
| } |
| ---- |
| |
| === Marker bit |
| |
| As described earlier, the Marker bit is always expected to relate to the first |
| AMR payload of an OSmux frame. Thus, special considerations may be followed when |
| the OSmux encoder receives an RTP packet with a marker bit. For instance, |
| previously enqueued RTP packets may be sent even if the configured batch factor |
| is not reached. |
| |
| We again use the scenario with 2 concurrent calls and a batch factor of 3. |
| |
| ["mscgen"] |
| ---- |
| msc { |
| hscale = 2; |
| bts [label="BTS"], bsc [label="BSC"], bscnat [label="BSC-NAT"], mgw [label="MGW"]; |
| |
| ...; |
| --- [label="2 Regular RTP-AMR call using OSmux with batch factor 3 (has been ongoing for some time)"]; |
| |
| bts => bsc [label="RTP-AMR[seq=x,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=x+1,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+1,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=x+2,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+2,ssrc=MSb]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m,AMR(x),AMR(x+1),AMR(x+2)],Osmux[ft=2,cid=i+1,seq=n,AMR(y),AMR(y+1),AMR(y+2)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o,ssrc=r] (originally seq=x,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p,ssrc=r] (originally seq=y,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+1,ssrc=r] (originally seq=x+1,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+1,ssrc=s] (originally seq=y+1,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+2,ssrc=r] (originally seq=x+2,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+2,ssrc=s] (originally seq=y+2,ssrc=MSb)"]; |
| bts => bsc [label="RTP-AMR[seq=x+3,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+3,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=x+4,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+4,ssrc=MSb] with Marker bit set M=1"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m+1,AMR(x+3),AMR(x+4)],Osmux[ft=2,cid=i+1,seq=n+1,AMR(y+3)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+3,ssrc=r] (originally seq=x+3,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+3,ssrc=s] (originally seq=y+3,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+4,ssrc=r] (originally seq=x+4,ssrc=MSa)"]; |
| bts => bsc [label="RTP-AMR[seq=x+5,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+5,ssrc=MSb]"]; |
| bts => bsc [label="RTP-AMR[seq=x+6,ssrc=MSa]"]; |
| bts => bsc [label="RTP-AMR[seq=y+6,ssrc=MSb]"]; |
| bsc => bscnat [label="UDP[Osmux[ft=2,cid=i,seq=m+2,AMR(x+5),AMR(x+6)],Osmux[ft=2,cid=i+1,seq=n+2,AMR(y+4),AMR(y+5),AMR(y+6)]]"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+4,ssrc=s] (originally seq=y+4,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+5,ssrc=r] (originally seq=x+5,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+5,ssrc=s] (originally seq=y+5,ssrc=MSb)"]; |
| bscnat => mgw [label="RTP-AMR[seq=o+6,ssrc=r] (originally seq=x+6,ssrc=MSa)"]; |
| bscnat => mgw [label="RTP-AMR[seq=p+6,ssrc=s] (originally seq=y+6,ssrc=MSb)"]; |
| } |
| ---- |
| |
| == Evaluation: Expected traffic savings |
| |
| The following figure shows the growth in traffic saving (in %) depending on the |
| number of concurrent numbers of callings for a given set of batching factor |
| values: |
| |
| // Original python2 pychart code replaced with generated svg in I36b721f895caee9766528e14d854b6aa2a2fac85 |
| image::images/osmux-expected-traffic-savings.svg[] |
| |
| The results show a saving of 15.79% with only one concurrent call and with |
| batching disabled (bfactor 1), that quickly improves with more concurrent calls |
| (due to trunking). |
| |
| By increasing the batching of messages to 4, the results show a saving of 56.68% |
| with only one concurrent call. Trunking slightly improves the situation with |
| more concurrent calls. |
| |
| A batching factor of 8 provides very little improvement with regards to batching |
| 4 messages. Still, we risk to degrade user experience. Thus, we consider a |
| batching factor of 3 and 4 is adequate. |
| |
| == Other proposed follow-up works |
| |
| The following sections describe features that can be considered in the mid-run |
| to be included in the OSmux infrastructure. They will be considered for future |
| proposals as extensions to this work. Therefore, they are NOT included in |
| this proposal. |
| |
| === Encryption |
| |
| Voice streams within OSmux can be encrypted in a similar manner to SRTP |
| (RFC3711). The only potential problem is the use of a reduced sequence number, |
| as it wraps in (20ms * 2^256 * B), i.e. 5.12s to 40.96s. However, as the |
| receiver knows at which rate the codec frames are generated at the sender, he |
| should be able to compute how much time has passed using his own timebase. |
| |
| Another alternative can be the use of DTLS (RFC 6347) that can be used to |
| secure datagram traffic using TLS facilities (libraries like openssl and |
| gnutls already support this). |
| |
| === Multiple OSmux messages in one packet |
| |
| In case there is already at least one active voice call, there will be |
| regular transmissions of voice codec frames. Depending on the batching |
| factor, they will be sent every 70ms to 140ms. The size even of a |
| batched (and/or trunked) codec message is still much lower than the MTU. |
| |
| Thus, any signalling (related or unrelated to the call causing the codec |
| stream) can just be piggy-backed to the packets containing the voice |
| codec frames. |