Makidis Michael mmakidis05@aueb.gr
RLC protocol overview procedures Simulation setup Performance of applications without contention Performance of applications with contention Performance of CBR Media Distribution application SDU discard policy impact Conclusion mmakidis05@aueb.gr 2
The RLC protocol Functions Frame types SUFIs for the Status PDU Windows Protocol States mmakidis05@aueb.gr 3
Radio Link Control protocol: link layer protocol for the UMTS Transparent Mode: No services provided by the RLC e.g. voice calls Unacknowledged Mode: Duplicate avoidance & reordering only Low delay and higher reliability than the raw link e.g. video streaming using UDP Acknowledged Mode: Reliable data transfer (mostly) e.g. web browsing, file transfer, Sliding window protocol (like, GBN) Uses sequence numbers, performs retransmissions All logos and trademarks are property of their respective owners. mmakidis05@aueb.gr 4
Transfer of user data Mostly reliable data transfer Discards data after time period or number of (re)transmissions Error correction Sequence number check, duplicate detection, in-sequence delivery Protocol error detection & recovery Flow control Segmentation & reassembly, concatenation, padding Ciphering Not implemented mmakidis05@aueb.gr 5
D/C Sequence number Sequence number P HE Byte 1 Byte 2 Length Indicator (LI) E Byte 3 (opt) Data (AMD) PDU Transfers the user s data Has a sequence number in [0, 96) Contains segments of SDUs from upper layers Has a polling bit Can have a piggybacked Status PDU in the padding space Status PDU Has one or more SUFIs (Super Fields) Used for reporting ACKs/NACKs Used by the SDU Discard function Used by the Flow Control function RESET & RESET ACK PDU Used by the RLC Reset procedure D/C D/C Length Indicator (LI) DATA Padding or Piggybacked Status PDU PDU type PDU type HFNI SUFI 1 SUFI k Padding RSN HFNI HFNI Padding SUFI 1 R1 E Byte N Byte 1 Byte 2 Byte N Byte 1 Byte N Data PDU Status PDU Reset & Reset ACK PDU mmakidis05@aueb.gr 6
ACK/NACK SUFIs ACK: Acknowledges all PDUs up to the ACKed SN BITMAP: Contains bitmap indicating ACKed/NACKed PDUs of the Receiver s window LIST: List of SN/length pairs indicating NACKed PDUs RLIST: List of NACKed PDUs in encoded, relative form The spec does not specify which SUFIs to use (other than ACK). We used ACK & BITMAP. Move Receiving Window SUFIs MRW: Request from the Sender to discard SDUs and Move the Receiver s Window MRW_ACK: ACK of an MRW; indicates the Receiver s new window position Other SUFIs WINDOW: Request from the Receiver to change the Sender s window size (used for Flow Control) NO_MORE: Indicates the end of the Status PDU mmakidis05@aueb.gr 7
Transmission window [VT(WS)] Receive window VT(A) VT(S) VT(MS) VR(R) VR(H) VR(MR) Acknowledged PDU Usable slot, PDU not sent Received PDU (in-sequence) Acceptable (not received) PDU Send but unacknowledged PDU PDU outside the window (will be rejected) Received PDU (out-of-sequence) PDU outside the window (will be rejected) Sender Receiver Flow Control: The Receiver can change the Sender s window size (by using the WINDOW SUFI). The Flow Control algorithm is not specified. mmakidis05@aueb.gr 8
NULL: The RLC entity is not created (or is terminated) DATA_TRANSFER_READY RESET_PENDING: RLC Reset procedure initiated LOCAL SUSPEND: RLC entity suspended (cannot send PDUs higher than a SN) RESET_AND_SUSPEND: RLC entity suspended and RLC Reset procedure initiated NULL Rq. from upper layers Rq. from upper layers Rq. from upper layers Rq. from upper layers Reset trigger RESET RESET_ PENDING RESET RESET ACK RESET RESET ACK DATA_ TRANSFER_ READY RESET ACK RESET ACK Rq. from upper layers Conf. to upper layers Rq. from upper layers Conf. to upper layers Rq. from upper layers Conf. to upper layers Rq. from upper layers Conf. to upper layers RESET Rq. from upper layers RESET_AND_ SUSPEND LOCAL_ SUSPEND RESET ACK mmakidis05@aueb.gr 9
Data transfer Polling Status Report transmission Limited reliability of the RLC Reset procedure Other procedures mmakidis05@aueb.gr 10
Sender Segmentation, Concatenation, Padding Increase sequence number (ensure it s within window) Start discard timer/count (re)transmission Set polling bit, if poll triggered Attach piggybacked Status PDU (optional) Sender Receiver Receiver Advance window Reassembly, Deliver SDUs in sequence Send a Status report, if polled Abnormal cases Receive PDUs outside the reception window Duplicate PDUs Invalid PDU format/size Full buffer (for the UE) AMD PDU mmakidis05@aueb.gr 11
Poll: Request from the Sender to the Receiver for a Status Report (i.e. ACKs & NACKs) Poll triggers (any combination may be used) Every last PDU in buffer (not impl.) Every last PDU in Retransmission buffer Poll Timer Every x PDUs Every x SDUs Window Based (poll triggered if a percentage of Sender s window is used) Timer Based (periodic polling) Poll Prohibit (optional) After the transmission of a Poll, a timer is started Polling is prohibited (delayed) while the timer is active Sender AMD PDU with polling bit set Status PDU with ACKs/NACKs Receiver mmakidis05@aueb.gr 12
Status Report: Status PDU from the Receiver to the Sender with ACKs/NACKs Uses the ACK SUFI and one or more of the BITMAP, LIST, RLIST SUFIs Sender Receiver Status Report triggers Polling Detection of missing PDU(s) (optional) Timer based (periodic transmission) (optional) Request from lower layers (not impl.) AMD PDU with polling bit set Status PDU with ACKs/NACKs Status Prohibit (optional) After the transmission of a Status Report, a timer is started Status Report transmission is prohibited (delayed) while the timer is active mmakidis05@aueb.gr 13
The Sender discards PDUs if they have not been successfully sent Within a certain time period, or Within a number of (re)transmissions Sender Receiver SDU Discard Function operation modes Discard SDUs (and PDUs) after a period of time, or Discard PDUs (and SDUs) after x transmissions, or Reset the RLC entities after x transmissions The Receiver is informed of the discarded SDUs in order to Move (advance) its Reception Window (MRW) The MRW request is periodically send until it is ACKed If the MRW request is not ACKed within a number of attempts, the RLC Reset procedure is initiated Status PDU with MRW SUFI Status PDU with MRW_ACK SUFI mmakidis05@aueb.gr 14
Reset triggers Erroneous sequence numbers in a Status PDU Too many attempts to send a MRW command Using the Reset after x transmissions SDU Discard function and the limit is reached RLC Reset: clear buffers, stop timers, reset variables Procedure Sender sends a RESET PDU (periodically) Receiver gets the RESET PDU, resets itself, responds with a RESET ACK Sender receives RESET ACK, resets itself Sender RESET PDU RESET ACK PDU Receiver Abnormal cases RESET PDU sent too many times without ACK: unrecoverable error signaled; protocol stalled Duplicate RESET PDU is received: the Receiver responds with a RESET ACK without resetting itself RESET PDU received by the Sender: the Sender acts as the Receiver mmakidis05@aueb.gr 15
Local Suspend & Resume The Sender allowed to send only N more PDUs PDUs after the N th are delayed until the entity is resumed Stop & Continue All transmissions & receptions are delayed until the upper layers initiate the Continue procedure Timers are not affected Re-establishment The RLC entity is re-established by upper layers All buffers cleared All parameters & variables reset Reconfiguration of RLC parameters by upper layers The upper layers change one or more parameters of the RLC entity while it is active Variable values may be changed because of the parameters Window sizes can be changed as well A number of buffered PDUs may be dropped as a result (especially in the UE). mmakidis05@aueb.gr 16
Applications & topology variants & RLC parameters mmakidis05@aueb.gr 17
ns-2 simulator with extra link layer protocols, wireless links & error models Web Browsing Metric: Server-to-client data throughput Measured completed transactions only File transfer (FTP) Transfer of a 10 MB file (unidirectional) Metric: Application data throughput CBR Media Distribution Audio or Video distribution (56 kbps CBR) Alternates between talking and silent state Uses UDP (bypasses reliable LL protocol) Metric: CBR stream delay Contention with TCP application (CBR stream consumes 37,5% of bandwidth) Each test performed 30 times Tested frame loss at rates 0%, 1.5%, 2.5%, 5.4% and 9.8% TCP/UDP IP LL PHY TCP/UDP LAN: 10 Mb data rate, 1 ms delay WAN: 2 Mb data rate, 50 ms delay IP LL PHY TCP/UDP Wired server Wired host Wireless client Simulation topology 64 Kbps data rate, 50 ms delay, 250 bytes frame payload, Uniform or Two State error model Uniform error model IP LL PHY Loss probability same for each pkt Two state error model Good state of the channel (10-6 error rate) Bad state of the channel (10-2 error rate) mmakidis05@aueb.gr 18
Sliding window protocol Receiver uses ACK and NACK frames to inform the Sender of its status Sender retransmits NACKed frames Delayed, piggybacked ACKs in data frames Timer for each not ACKed data frame (in case its ACK/NACK gets lost) Supports multireject (a frame can be NACKed multiple times) Offers good performance SR variants Simple SR with fixed timeout value 1.1s Adaptive SR with dynamic timeout value (self-cloacking, like TCP) Uniform model: 3 srtt+2 srttvar Two State model: 4 srtt+0 srttvar Parameter Window size (Sender & Receiver) parameters Uniform model ( Cellular link) Two State model ( PCS link) 128 frames 128 frames Poll triggers Timer_Poll, Window Based Timer_Poll, Window Based, Every last PDU in retransmission buffer Status report triggers (other than polling) Detection of missing PDU, Periodic status reporting Poll Prohibit enabled Yes Yes Status Prohibit enabled No Yes RESET enabled No No Poll Window 70% 80% SDU Discard Mode SDU discard after x number of transmissions MaxDAT 3 2 Timer_Status_Prohibit timeout Timer_Status_Periodic timeout - 90 ms 0 ms 500 ms Timer_Poll timeout 200 ms 200 ms Timer_Poll_Prohibit timeout 100 ms 100 ms Timer_MRW timeout 500 ms 110 ms Status report piggybacking enabled No Detection of missing PDU, Periodic status reporting SDU discard after x number of transmissions No mmakidis05@aueb.gr 19
Application impact (Uniform mdl.) Application impact (Two State mdl.) Topology impact mmakidis05@aueb.gr 20
Throughput (Kbps) Throughput (Kbps) Web browsing File transfer 50 45 HTTP w/o contention, LAN/Cellular links Adapt. 70 60 FTP w/o contention, LAN/Cellular links Adapt. 35 50 30 25 20 30 15 10 20 5 10 performance depends on application Poor performance for web browsing Adaptive SR has high throughput Almost identical results for WAN topology mmakidis05@aueb.gr 21
Throughput (Kbps) Throughput (Kbps) Web browsing File transfer 50 45 HTTP w/o contention, LAN/PCS links Adapt. 65 60 FTP w/o contention, LAN/PCS links Adapt. 55 50 35 45 30 25 Lower throughput in Two State model performance depends on application Poor performance for web browsing, OK for FTP but not for high frame loss rates Adaptive SR has high throughput Almost identical results for WAN topology It has been shown that TCP performs worse in the Two State model 35 mmakidis05@aueb.gr 22
File transfer Web browsing Throughput (Kbps) Throughput (Kbps) Throughput (Kbps) Throughput (Kbps) Almost the same performance for both topologies HTTP w/o contention, LAN/Cellular links HTTP w/o contention, WAN/Cellular links 50 45 35 30 25 20 15 10 LAN Adapt. 35 30 25 20 15 10 WAN Adapt. Lower throughput in WAN topology due to higher delay in interactive application 5 5 FTP w/o contention, LAN/Cellular links FTP w/o contention, WAN/Cellular links 70 60 50 Adapt. 70 60 50 Adapt. Similar results for the Two State model as well. 30 30 20 LAN 20 WAN 10 10 mmakidis05@aueb.gr 23
CBR cont. impact (HTTP) (Un. mdl.) CBR cont. impact (HTTP) (TS mdl.) CBR cont. impact (FTP) (Un. mdl.) CBR cont. impact (FTP) (TS mdl.) mmakidis05@aueb.gr 24
Throughput (Kbps) Throughput (Kbps) Without CBR contention With CBR contention 50 45 HTTP w/o contention, LAN/Cellular links Adapt. 35 30 HTTP with contention, LAN/Cellular links Adapt. 35 25 30 25 20 20 15 15 10 10 5 5 Similar relative performance for all protocols Identical results for the WAN topology as well Adaptive SR has the highest throughput mmakidis05@aueb.gr 25
Throughput (Kbps) Throughput (Kbps) 50 45 Without CBR contention HTTP w/o contention, LAN/PCS links Adapt. 34 32 30 With CBR contention HTTP with contention, LAN/PCS links Adapt. 28 26 35 24 30 22 20 25 18 Similar relative performance for and Adaptive SR Fixed SR performance drops slightly in the Two State model Similar results for the WAN topology Adaptive SR has the highest throughput mmakidis05@aueb.gr 26
Throughput (Kbps) Throughput (Kbps) Without CBR contention With CBR contention FTP w/o contention, LAN/Cellular links FTP with contention, LAN/Cellular links 70 60 Adapt. 45 Adapt. 50 35 30 25 30 20 20 15 10 10 Similar relative performance for all protocols Identical results for the WAN topology as well mmakidis05@aueb.gr 27
Throughput (Kbps) Throughput (Kbps) Without CBR contention With CBR contention 65 60 FTP w/o contention, LAN/PCS links Adapt. 42 38 FTP with contention, LAN/PCS links Adapt. 55 36 34 50 32 30 45 28 26 24 22 35 20 Similar relative performance for and Adaptive SR Fixed SR performance drops significantly in the Two State model Similar results with the WAN topology mmakidis05@aueb.gr 28
CBR with HTTP contention CBR with FTP contention mmakidis05@aueb.gr 29
Packet delay (seconds) Packet delay (seconds) Uniform error model Two State error model 3 2.5 CBR with HTTP contention, LAN/Cellular links Adapt. 4 3.5 CBR with HTTP contention, LAN/PCS links Adapt. 2 3 1.5 2.5 1 2 0.5 1.5 0 1 causes the lowest delay (but has low throughput as well); fixed SR causes the highest (esp. in the Two State model) Adaptive SR causes low delay SR variants perform more retransmissions as the frame loss rate increases, thus causing more delay Similar results for WAN topology mmakidis05@aueb.gr 30
Packet delay (seconds) Packet delay (seconds) Uniform error model Two State error model 3.5 3 CBR with FTP contention, LAN/Cellular links Adapt. 5.5 5 CBR with FTP contention, LAN/PCS links Adapt. 2.5 4.5 2 4 3.5 1.5 3 1 2.5 0.5 2 0 1.5 causes the lowest delay; fixed SR the highest (esp. in the Two State model) Adaptive SR causes low delay, almost as low as Similar results for WAN topology mmakidis05@aueb.gr 31
FTP throughput, Uniform model FTP throughput, Two State model Web Browsing throughput mmakidis05@aueb.gr 32
Throughput (Kbps) Throughput (Kbps) Discard after x transmissions Discard after x time 70 60 50 FTP w/o contention, LAN/Cellular links MaxDAT=2 MaxDAT=3 MaxDAT=4 MaxDAT=5 MaxDAT=6 MaxDAT=7 70 60 50 FTP w/o contention, LAN/Cellular links disc. to=0.1s disc. to=0.2s disc. to=0.3s disc. to=0.4s disc. to=0.5s disc. to=0.6s disc. to=0.7s 30 30 20 20 10 10 Discard after x transm.: Higher throughput Highest throughput for MaxDAT=3 for the Uniform error model Timer-based discard: Lower throughput Higher than raw link only in high frame loss rates Timeout values 300-700 ms have almost the same throughput mmakidis05@aueb.gr 33
Throughput (Kbps) Throughput (Kbps) Discard after x transmissions Discard after x time 65 60 55 FTP w/o contention, LAN/PCS links MaxDAT=2 MaxDAT=3 MaxDAT=4 MaxDAT=5 MaxDAT=6 MaxDAT=7 65 60 55 FTP w/o contention, LAN/PCS links disc. to=0.1s disc. to=0.2s disc. to=0.3s disc. to=0.4s disc. to=0.5s disc. to=0.6s disc. to=0.7s 50 50 45 45 35 Discard after x transm.: Higher throughput Highest throughput for MaxDAT=2 for the Two State error model Timer-based discard: Lower throughput Almost the same as the raw link Timeout values 300-700 ms have almost the same throughput 35 mmakidis05@aueb.gr 34
Throughput (Kbps) Throughput (Kbps) Discard after x transmissions Discard after x time 50 45 35 HTTP w/o contention, LAN/Cellular links MaxDAT=2 MaxDAT=3 MaxDAT=4 MaxDAT=5 MaxDAT=6 MaxDAT=7 50 45 35 HTTP w/o contention, LAN/Cellular links disc. to=0.1s disc. to=0.2s disc. to=0.3s disc. to=0.4s disc. to=0.5s disc. to=0.6s disc. to=0.7s 30 30 25 25 20 20 15 15 10 10 5 Discard after x transm.: Higher throughput Timer-based discard: Lower throughput Higher than raw link only in high frame loss rates Timeout values 300-700 ms have almost the same throughput Similar results for the Two State model 5 mmakidis05@aueb.gr 35
throughput is dependent on the application offers better throughput than the raw link in all cases Relative throughput is generally not affected by the topology (LAN/WAN) Contention from the CBR stream has minimal impact on relative and Adaptive SR throughput causes the lowest delay to the CBR stream Adaptive SR generally has the highest throughput in TCP applications and causes low delay on the CBR stream Fixed SR has higher throughput in some apps/models and lower throughput in others compared to the ; it causes the highest delay on the CBR stream In, SDU Discard after x retransmissions policy offers better performance than the timer-based one mmakidis05@aueb.gr 36
Makidis Michael mmakidis05@aueb.gr