Computer Networks Chapter 7 Connectionoriented Networks X 25

Computer Networks Chapter 7 – Connection-oriented Networks: X. 25 and ATM CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 1

Generic Connection-oriented Network • Call Admission Control (CAC) • Call Setup – Routing choices – Resource reservation – Connection Identifier (CI) – local significance – Confirmation • Routing by CI • Call Teardown CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 2

Connection Identifiers (CIs) A X 57 a, 57, c, 33, a, 57 a R 1 c b d a, 33, d, 79, a, 33 a R 2 d 33 b c R 4 b 79 22 R 5 c a b, 79, c, 22, b, 79 B R 3 A has a connection to B via R 1, R 2, and R 5 The Connection Identifier (CI) only has local significance – it changes along the route Each router has a table with in-port, CI-in, out-port, CI-out CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 3

Generic Connection Setup b A X a R 2 d b c Setup a R 1 c b d R 4 R 5 c a B R 3 Application at A wants to talk to application at B with some Qo. S A checks local resources, if OK, proceeds, else rejects A creates table entry for A-B connection via R 1 using CI=57 A sends setup packet to R 1 Setup Pkt = (A wants to talk to B, use CI=57) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 4

Generic Connection Setup b A X 57 a, 57, c, 33 Setup c, 33, a, 57 a R 1 c b d R 4 a R 2 d b c R 5 c a B R 3 R 1 checks local resources, if OK, proceeds, else rejects R 1 makes routing decision to forward to B via R 2 using CI=33 R 1 creates table entries for A-B connection R 1 forwards setup packet to R 2 Setup Pkt = (A wants to talk to B, use CI=33) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 5

Generic Connection Setup A X 57 a, 57, c, 33, a, 57 a R 1 c b d a, 33, d, 79, a, 33 a R 2 d 33 b c R 4 Setup b R 5 c a B R 3 R 2 checks local resources, if OK, proceeds, else rejects R 2 makes routing decision to forward to B via R 5 using CI=79 R 2 creates table entries for A-B connection R 2 forwards setup packet to R 5 Setup Pkt = (A wants to talk to B, use CI=79) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 6

Generic Connection Setup A X 57 a, 57, c, 33, a, 57 a R 1 c b d a, 33, d, 79, a, 33 a R 2 d 33 b c R 4 b 79 Setup R 5 c a b, 79, c, 22, b, 79 B R 3 R 5 checks local resources, if OK, proceeds, else rejects R 5 makes routing decision to forward to B directly using CI=22 R 5 creates table entries for A-B connection R 5 forwards setup packet to B Setup Pkt = (A wants to talk to B, use CI=22) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 7

Generic Connection Setup A X 57 a, 57, c, 33, a, 57 a R 1 c b d a, 33, d, 79, a, 33 a R 2 d 33 b c R 4 b 79 22 R 5 c Setup ACK a B c, 22, b, 79, c, 22 R 3 B checks local resources, if OK, proceeds, else rejects B creates table entries for A-B connection B replies with setup ACK to R 5 Setup ACK = (B accepts call from A, using CI=22) R 5, R 2, and R 1 forward Setup ACK, changing CI as it goes, and marking table entries as confirmed CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 8

Connection-oriented Routing a, 57, c, 33 (57, M 1) A a R 1 c b d X b, 79, c, 22 b R 5 c a, 33, d, 79 a R 2 d b c R 4 a B R 3 A sends M 1 to B, CI=57, routed to R 1 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 9

Connection-oriented Routing A X b, 79, c, 22 b R 5 c a, 33, d, 79 a, 57, c, 33 a R 2 d ) 1 (33, M b c R 1 a c b d R 4 a B R 3 R 1 forwards M 1 from port a CI=57 to port c, changes CI to 33 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 10

Connection-oriented Routing a, 57, c, 33 A X a R 1 c b d b, 79, c, 22 b a, 33, d, 79 (79, M 1) R 5 c a R 2 d a b c R 4 B R 3 R 2 forwards M 1 from port a CI=33 to port d, changes CI to 79 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 11

Connection-oriented Routing a, 57, c, 33 A X a R 1 c b d b, 79, c, 22 b R 5 c (22 a, 33, d, 79 a R 2 d b c R 4 a , M 1 ) B R 3 R 5 forwards M 1 from port b CI=79 to port c, changes CI to 22 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 12

Connection-oriented Routing c, 33, a, 57 A X a R 1 c b d c, 22, b, 79 b R 5 c (22, M 2) a d, 79, a, 33 a R 2 d b c R 4 B R 3 B sends M 2 to A, CI=22, routed to R 5 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 13

Connection-oriented Routing c, 33, a, 57 A X a R 1 c b d c, 22, b, 79 b d, 79, a, 33 (79, M 2) R 5 c a R 2 d a b c R 4 B R 3 R 5 forwards M 2 from port c CI=22 to port b, changes CI to 79 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 14

Connection-oriented Routing A X c, 22, b, 79 b R 5 c d, 79, a, 33 c, 33, a, 57 a R 2 d ) 2 (33, M b c R 1 a c b d R 4 a B R 3 R 2 forwards M 2 from port d CI=79 to port a, changes CI to 33 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 15

Connection-oriented Routing c, 33, a, 57 (57, M 2) A a R 1 c b d X c, 22, b, 79 b R 5 c d, 79, a, 33 a R 2 d b c R 4 a B R 3 R 1 forwards M 2 from port c CI=33 to port a, changes CI to 57 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 16

Generic Connection-oriented c, 22, b, 79 a, 57, c, 33 b, 79, c, 22 c, 33, a, 57 Network Tables b, 21, c, 35 A X b, 57, c, 21, b, 57 b, 23, d, 12, b, 23 a, 5, b, 17, a, 5 a R 1 c b d a, 12, c, 33, a, 12 a, 33, d, 79, a, 33 a, 21, d, 21, a, 21 a R 2 d b c b a R 4 c Can you trace the connections? R 5 c a c, 35, b, 21 c, 42, a, 14, c, 42 B d R 3 b c b, 33, c, 57, b, 33 c, 33, d, 14, c, 33 CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman Y 17

Generic Connection Teardown b A X a R 1 c b d a R 2 d b c a b a R 4 c b R 5 c a B d R 3 c Y For teardown, either end or intermediate node (router) sends teardown packet CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 18

Generic Connection Teardown A X a, 57, c, 33, a, 57 b, 57, c, 21, b, 57 b, 23, d, 12, b, 23 a, 5, b, 17, a, 5 a R 1 c b d a, 33, d, 79, a, 33 a, 21, d, 21, a, 21 a R 2 d b c b a R 4 c b R 5 c a c, 22, b, 79, c, 22 b, 21, c, 35, b, 21 c, 42, a, 14, c, 42 Teardown ACK B d R 3 c Y B decides to close connection to A, B sends teardown packet to R 5 Teardown Pkt = (Teardown, CI=22) R 5 acknowledges, B frees resources CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 19

Generic Connection Teardown A X a, 57, c, 33, a, 57 b, 57, c, 21, b, 57 b, 23, d, 12, b, 23 a, 5, b, 17, a, 5 a R 1 c b d a, 33, d, 79, a, 33 a, 21, d, 21 b d, 21, a, 21 ACK R 5 c a R 2 d wn a o d r a e b c T a d b R 3 b c a R 4 c b, 21, c, 35, b, 21 c, 42, a, 14, c, 42 B Y R 5 sends teardown packet to R 2 Teardown Pkt = (Teardown, CI=79) R 2 acknowledges, R 5 frees resources CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 20

Generic Connection Teardown A X a, 57, c, 33, a, 57 b, 57, c, 21, b, 57 a, 21, d, 21 b, 23, d, 12 d, 21, a, 21 d, 12, b, 23 a, 5, b, 17 a R 2 d K C b, 17, a, 5 A b c n R 1 w a c Teardo b d b a R 4 c b a b R 5 c a b, 21, c, 35, b, 21 c, 42, a, 14, c, 42 B d R 3 c Y R 2 sends teardown packet to R 1 Teardown Pkt = (Teardown, CI=33) R 1 acknowledges, R 2 frees resources CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 21

Generic Connection Teardown ACK b, 57, c, 21, b, 57 b, 23, d, 12, b, 23 a, 5, b, 17, a, 5 A a R 1 c Teardown b d X a, 21, d, 21, a, 21 a R 2 d b c b a R 4 c b R 5 c a b, 21, c, 35, b, 21 c, 42, a, 14, c, 42 B d R 3 c Y R 1 sends teardown packet to A Teardown Pkt = (Teardown, CI=57) A acknowledges, R 1 and A free resources CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 22

X. 25 • CCITT designed, adopted as ISO 8208 • Specifies DTE-Router interface at three levels: – PHY: connector, voltage, pin semantics, etc. – Link: delimiters, pkt #s, ACKs, flow ctl, etc. – Packet: network layer interface • DTE=end node (Data Termination Equip) • DCE=router (Data Communication Equip) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 23

X. 25 Basics • DTE can have multiple calls over a single link to a DCE • Call first must be established • Then data may flow in both directions • Then the call must be cleared CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 24

X. 25 Basics • Call may be established by DTE in 3 ways: – DTE initiated (SVC – Switched Virtual Circuit) DTE informs DCE of desired destination DTE – DTE received (Incoming Call) DCE informs DTE call request by source DTE – Nailed up (Permanent Virtual Circuit) Administratively set up – DTEs only see data • Call gets locally significant VC number at setup CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 25

X. 25 VC numbers • VC “Call numbers” are 12 bits long: – Logical Channel Group Number: 4 bits – Logical Channel Number: 8 bits • Call number 0 reserved for control pkts for all VCs • Next block of call numbers for PVCs • Then incoming, then either, then outgoing (to avoid CN collisions from DTE and DCE) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 26

X. 25 Call Setup # octets 1 0 0 0 1 group 1 channel 1 type 1 Calling DTE Addr Len Called DTE Addr Len variable Calling DTE Address variable Called DTE Address 1 0 x 0 a for call request units are nybbles Facilities Length variable Facilities variable User Data Call Request Packet Format CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 27

X. 25 Call Overview DTE DCE Call Request Call Connected Data Incoming Data Clear Request Clear Confirm Incoming Call Accepted Incoming Data Clear Indication Clear Response – X. 25 specifies DTE-DCE protocol – But not DCE-DCE protocol CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 28

X. 25 Data Transfer • Call is full duplex (simultaneous bidirectional data flow) • Message and Ack sequence numbers – 3 -bit and 7 -bit formats – Choice at subscription or call setup time • Q bit – end host purposes • D bit – Received by DTE? or DCE only • M bit – more packets in message CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 29

X. 25 Data Packet Formats Q D 0 1 group channel Ack # M Msg # 0 User Data 3 -bit Format Q D 0 1 group channel Msg # 0 Ack # M User Data 7 -bit Format CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 30

X. 25 Data Transfer • Call # (Group+Channel) and msg # allow multiple, simultaneous connections • M bit allow message fragmentation – All but last packet have M=0 b 1 – When fragmented, new sequence #s given – Destination DCE must hold and reassemble – Packet must be full for M bit to be set… CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 31

X. 25 Packet Fragmentation Max pkt size = X DTE DCE Max pkt size = X/3 DTE DCE #1 #2 #1 M #2 M #3 #4 M #5 M #6 – Smaller maximum packet size at destination – Sequence numbers are different CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 32

X. 25 Packet Fragmentation Max pkt size = X/3 DTE DCE Max pkt size = X DCE DTE #1 M #2 M #3 #4 M #5 M #6 #1 #2 – Smaller maximum packet size at source – Destination DCE must hold and recombine fragments – Sequence numbers are different CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 33

X. 25 Flow Control • Sequence numbers + ACKs (piggybacked) – ACK cumulative and inclusive – ACK indicates receiver ready to take w more – Sequence numbers different in each direction • No data for return – Receive Ready packet • Choking flow – No ACKs, or equivalently, Receive Not Ready CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 34

X. 25 Facilities • Window size w – fixed or negotiated per call • Packet size - fixed or negotiated per call • Throughput - fixed or negotiated per call (75 bps – 64 Kbps) • Closed user group – access control – Set of DTEs that can communicate – Closed group numbers of local significance • Allowing only incoming or outgoing calls • Setting lowest outgoing CI number – Boundary for CIs set by DCE and DTE CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 35

Per-call X. 25 Facilities • • • Window size w Maximum packet size Throughput (75 bps – 64 Kbps) Closed user group for call Reverse charging (callee pays) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 36

X. 25 Call Release • Standard method: – One of DTEs issues Clear Request to DCE – DCE informs network, other DCE – Other DCE informs other DTE – Other DTE acknowledges to its DCE – Other DCE confirms to network, first DCE – First DCE confirms to initiating DTE • All packets sent are delivered CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 37

X. 25 Call Release • Network detects problem – DCE may issue Clear Request (hangs up) – DCE may issue Reset (restarts seq#’s at 0) – No guarantees on unacknowledged packets – Network may recover transparently (right) • Potential network problems – DCE or link along route fails – DTE issues packet for a call with unexpected seq# - DCE and DTE have lost sync on call – DCE tears down all calls to a DTE – lost sync CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 38

X. 25 Interrupts • Urgent control packet – DTE may issue one Interrupt packet – Interrupt packet is small (1 to 32 bytes) – Not subject to flow control • One at a time – Other DTE must reply with Interrupt Confirm packet – Another Interrupt packet cannot be sent until the Interrupt Confirm packet is received CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 39

X. 25 Implementation • Circuit Method – – VC set up through the network CAC due to resource reservation Performance guaranteed Router failure leads to dropped calls (routers only hold packet until ACKed) • Reliable Connections over DG Method – Routers with end node neighbors implement fullservice transport layer – DCEs set up connection within network – DCE holds packet until other DCE acknowledges it CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 40

X. 25 Implementation Comparison • Circuit Method – – Service guarantees May refuse a call Faster routing (based on small CIs) Route failures not transparent • Reliable Connections over DG Method – No router buffer or BW reservations – More utilization for bursty traffic – Transparent route modifications CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 41

ATM Basics • Cells – Fixed size packets (48 bytes data, 5 header) – 48 bytes an unhappy compromise • Virtual Circuits, virtual paths • Service categories • Adaptation layers CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 42

ATM VCs and VPs • Virtual circuits created with CIs • CI has two subfields: VPI and VCI – Virtual Path ID is 12 bits – core routing – Virtual Circuit ID is 16 bits – calls • End node to switch – 4 bits of Generic Flow Control – These are “borrowed” from the VCI field • Switch to switch - Full 16 -bit VCI CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 43

ATM VCs and VPs • VPI – Virtual Path ID is 12 bits – Used internally for routing – Changes on each link in ATM network • VCI – Not used in ATM network – Used with VPI by edge routers to route calls • VPI/VCI – used as CI outside network CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 44

ATM Link Negotiations • Customer and Provider ATM networks – Customer NWs route on VPI/VCI – Provider NWs route on VPI only – VPIs in provider NWs typically permanent • Link parameters – Maximum number of active bits in VCI, VPI – Maximum number of VCs, VPs – Maximum allowable VPI number – Min and max VCI number for switched VCIs CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 45

ATM Service Categories • CBR – Constant Bit Rate – Data accepted, delivered at constant rate – Network reserves resources accordingly • UBR – Unspecified Bit Rate – Best effort • ABR – Available Bit Rate – Like UBR with congestion feedback • VBR – Variable Bit Rate – Reserve resources for less than peak rate – Specify burst characteristics, mark cells – RT (real time) and NRT (non-real time) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 46

ATM Cell Header Format GFC or VPI VPI VCI VCI PT CLP Header CRC GFC = Generic Flow Control (end node-switch only) PT = Payload Type CLP = Cell Loss Priority (CLP=1 means vulnerable) end node may set CLP, or switch may set CLP if rate is excessive CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 47

ATM Payload Type Field Value 000 001 010 011 100 101 110 111 Payload Type data, no congestion yet, not last cell data, no congestion yet, last cell data, congestion, not last cell data, congestion, last cell of packet control info btw/neighbor switches control info btw/end switches on path ABR rate control info reserved for future use CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 48

ATM Call Setup/Release • • Similar to X. 25, called signaling VCI=5, VPI=0 for signaling cells Use AAL 5 since longer than 48 bytes Call Setup – Source & destination addresses – Traffic characteristics – Transit network (like LD carrier) • Call Proceeding – Sent as link ACK for Setup message CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 49

ATM Call Setup/Release (cont. ) • Connect – Sent by destination to source – Accepts call • Connect ACK – Sent by source to destination • Release – Sent by either end to other end to end call • Release ACK – Sent to acknowledge Release to initiator CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 50

ATM Qo. S Parameters Parameter TLA Meaning PCR Maximum rate cell will be sent Peak cell rate SCR Long term average cell rate Sustained cell rate Minimum cell rate MCR Minimum acceptable cell rate Cell delay variation tolerance CDVT Maximum acceptable cell jitter Cell loss ratio CLR Fraction of cells lost or too late Cell transfer delay CTD Mean and max latency Cell delay variation CDV Variance in latency Cell error rate CER Fraction of cells delivered w/o error Severely-errored cell block r. SECBR Fraction of blocks garbled Cell misinsertion rate CMR Fract. of cells delivered to wrong dest. CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 51

ATM Services Class Timing Bit rate Mode A B C RT None Constant Variable Connection oriented D RT None Constant Variable Connectionless CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 52

ATM Adaptation Layer Structure AAL Convergence Sublayer (service specific part) Convergence Sublayer (common part) Segmentation and Reassembly Sublayer (SAR) ATM Layer PHY Layer Convergence layers – interface to application (messages) SAR – break into chunks and add header/trailer (cells) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 53

ATM Adaptation Layers • Interface above ATM to allow packet sizes other than 48 bytes • AAL is specified in call setup • AAL 0 = direct ATM • AAL 1 is for CBR • AAL 2 – for VBR, watch this space…. • AAL 3/4 – for data, but clunky • AAL 5 – for data by computer guys CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 54

ATM AAL 1 • Accepts CBR stream from source • Delivers CBR to destination • Does this over network with variable delays, losses, etc. – Buffers data to smooth delays – Numbers cells so losses/duplicates detected – First byte of cell for control info, rest is payload – Ctl= C bit (signaling), Seq (3 bits), SNP (4 bits for sequence # protection – CRC-3 + parity) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 55

ATM AAL 2 • Intended for simple, RT streams • 1 byte header, 2 byte trailer – Seq# (SN) and Info Type (IT) in header – Length Indicator (LI) and CRC in trailer • Field sizes not defined! • Hopelessly broken CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 56

ATM AAL 3/4 • • First cut at providing for larger data packets Complex and inefficient Essentially superseded by AAL 5 AAL 3/4 steps: Take datagram M and – Frame M into F with header, trailer, padding – Chop F into cell-sized chunks – Put header, trailer, and padding on each chunk – On receipt, check each cell, reassemble – If error, all of F must be resent CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 57

ATM AAL 3/4 Framing • Take datagram M and make F – Add 4 byte header • First byte always 0 • 1 byte tag for reassembly • 2 bytes of length (including header, trailer, padding) – Pad data to a 32 -bit boundary – Add 4 byte trailer • 1 constant byte • 2 bytes of length (actual data length) • 1 byte tag (must match header’s tag) CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 58

ATM AAL 3/4 Framing • Take F and break into 44 -byte chunks – Each chunk is put into cell – Add 2 byte header • 2 bits to indicate beginning, end of F • 4 bits sequence number • 10 bit field for multiplexing cells on single circuit – Pad data to 44 bytes, if necessary – Add 2 byte trailer • 6 bits length (data length in cell, 44 unless end) • 10 bit CRC covering cell CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 59

ATM AAL 3/4 Framing Message (1 to 65 K) 1 1 2 0 -3 1 CPI Btag BA size Payload (1 to 65 K) Padding 44 -byte chunk 40 2 ATM header ST 44 -byte chunk 4 10 SN MID … 1 2 bytes Etag Length 44 -byte chunk Payload (44 bytes) 6 10 LI CRC CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman bits 60

ATM AAL 5 • Same purpose as AAL 3/4 – datagrams • Take message M and form frame F by – Adding an 8 -byte trailer – Padding M so M, trailer, and padding are a multiple of 48 bytes (fit neatly into cells) – Trailer has 2 unused bytes, plus • 2 -byte length field (data only, not padding or trailer) • 4 -byte CRC – Use 1 bit from cell header to indicate “end” – Use end bit to find trailer, reassemble, check – If error, resend all of F CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 61

ATM AAL 5 Framing UU field for use by higher layer Message (1 to 65 K) Length 1 Payload (1 to 65 K) Pad UU 1 2 4 Length CRC bytes … 48 -byte chunk 5 ATM header 48 -byte chunk … 48 Payload 48 -byte chunk CRC checks entire message, including trailer (with CRC set to 0 x 0000) PTI field bit set in last cell of message CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 62

AAL Summary/Comparisons Item AAL 1 AAL 2 AAL 3/4 AAL 5 Service Class A B C/D Multiplexing No No Yes No Message Delimiting None Btag/Etag PTI bit User bytes 0 0 0 1 CS Padding 0 0 32 -bit word 0 -47 bytes CS Protocol OH 0 0 8 8 CS Checksum None CRC-32 SAR payload 46 -47 45 44 48 SAR proto OH 1 -2 3 4 0 SAR checksum None 10 bits None CEN 5501 C - Computer Networks - Spring 2007 - UF/CISE - Newman 63