Pag. 1
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 1 Copyright Gruppo Reti – Politecnico di Torino
Data link layer (layer 2)
Gruppo Reti TLC
http://www.telematica.polito.it/
COMPUTER NETWORKS – Data link layer protocols - 2 Copyright Gruppo Reti – Politecnico di Torino
Copyright
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NoDerivs-NonCommercial. Per vedere una copia di questa
licenza, consultare
http://creativecommons.org/licenses/nd-nc/1.0/
oppure inviare una lettera a: Creative Commons, 559
Nathan Abbott Way, Stanford, California 94305, USA.
• This work is licensed under the Creative Commons
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or send a letter to Creative Commons, 559 Nathan Abbott
Way, Stanford, California 94305, USA.
COMPUTER NETWORKS – Data link layer protocols - 3 Copyright Gruppo Reti – Politecnico di Torino
Data link layer functions
• Frame delineation
– Explicit delimiters (flag) before and after packet transmission
– Lenght indicator
– Fixed length
– Silence between packets
• Multiplexing (of higher layer protocols)
• Addressing
• Error detecion
• Window protocol
– Flow control
– Sequence and error control through retransmissione
• Multiple access protocols for shared media
COMPUTER NETWORKS – Data link layer protocols - 4 Copyright Gruppo Reti – Politecnico di Torino
Data link layer protocols
• Most derived from the SDLC (Syncronous Data
Link Control) defined in the IBM netowrks
architecture
• Ansi standardized SDLC as ADCCP (Advanced
Data Communication Control Procedure) and ISO
as HDLC (High-level Data Link Control)
• CCITT (later ITU-T) derived LAP (Link Access
Procedure) and LAPB (Link Access Procedure
Balanced)
• In Local Area Networks (LANs) a MAC sublayer
solves the multiple access problem
COMPUTER NETWORKS – Data link layer protocols - 5 Copyright Gruppo Reti – Politecnico di Torino
Data link layer protocols
• Several protocols belong to the HDLC family
– LAP-D (Link Access Procedure D-Channel)
– LAP-F (Link Access Procedure to Frame Mode
Bearer Service)
– LLC 802.2 (Logical Link Control)
– PPP (Point-to-Point Protocol)
– LAPDm (LAP for the mobile D channel)
COMPUTER NETWORKS – Data link layer protocols - 6 Copyright Gruppo Reti – Politecnico di Torino
Data link layer protocols
• Starting from HDLC…
• ….. we move to
– LAP-B, X.25 and ISDN B channel
– LLC, for LANs
– PPP, for point to point connections • Used at home
– LAP-F, used in Frame Relay
– ATM, not derived from HDLC, used in B-ISDN
Pag. 2
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 7 Copyright Gruppo Reti – Politecnico di Torino
Example of public network
• DTE: Data Terminal Equipment
• DCE: Data Circuit-terminating Equipment
NETWORK
DCE
DCE
DCE
DCE
DTE
DTE
DTE
DTE
Datsa link layer
protocols
COMPUTER NETWORKS – Data link layer protocols - 8 Copyright Gruppo Reti – Politecnico di Torino
• PDU format:
• Bit oriented protocol, with bit-stuffing to
ensure data transparency (flag 01111110
must not appear in other fields)
• Address field is used in multi-point
configuration (master/slave)
• Control field differentiates the PDUs
01111110 address control data CRC 01111110
8 8 8/16 >=0 16 8
Data format (many layer 2 protocols)
COMPUTER NETWORKS – Data link layer protocols - 9 Copyright Gruppo Reti – Politecnico di Torino
HDLC: Master/Slave Configuration
• Protocol used to manage the communication among master and slaves
• PDUs sent from the master are named command, PDUs send from slaves are named response
master
slave
COMPUTER NETWORKS – Data link layer protocols - 10 Copyright Gruppo Reti – Politecnico di Torino
HDLC: operational modes
• Normal Response Mode (NRM)
– Suited for point-to-point of unbalanced multi-
point.
– One primary station (Master) and several slaves
– The Master sets the P/F bit to 1 to enable slave
transmission
– The slave sets the P/F bit to 1 in the last PDU
COMPUTER NETWORKS – Data link layer protocols - 11 Copyright Gruppo Reti – Politecnico di Torino
HDLC: operational modes
• Asynchronous Response Mode (ARM)
– Unbalanced configuration. The slave can send
data without waiting to be polled by the master.
• Asynchronous Balanced Mode (ABM)
– Balanced point-to-point configurations. The P/F
bit requires immediate response.
– Most widely used in data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 12 Copyright Gruppo Reti – Politecnico di Torino
HDLC: three types of PDUs
• Control field
– Differentiates three types of PDUs
• P/F = poll/final bit
0 N(S) P/F N(R)
1 0 S S P/F N(R)
1 1 M M P/F M M M
Information
Supervision
Unnumbered
Pag. 3
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 13 Copyright Gruppo Reti – Politecnico di Torino
HDLC: three types of PDUs
• Information
– Data sent after connection opening
• Supervision
– ACKs (positive and negative)
• Unnumbered
– Link management
– Data sent in connectionless mode
COMPUTER NETWORKS – Data link layer protocols - 14 Copyright Gruppo Reti – Politecnico di Torino
HDLC: two numbering schemes
• Normal numbering (modulo 8) and extended
numbering (modulo 128)
• Control field equal to either 1 o 2 byte
0 P/F
1 0 S S x x x x P/F N(R)
1 1 M M P/F M M M
Information
Supervision
Unnumbered
N(R) N(S)
COMPUTER NETWORKS – Data link layer protocols - 15 Copyright Gruppo Reti – Politecnico di Torino
HDLC vs LAP-B
• LAP-B (ISDN B channel) uses only a subset
of PDUs defined by HDLC
• We briefly describe the PDUs used by LAP-B
COMPUTER NETWORKS – Data link layer protocols - 16 Copyright Gruppo Reti – Politecnico di Torino
LAP-B: information PDU
• Data transfer
• N(S) and N(R) fields needed by the window
protocol to provide error and sequence
control
– N(S) = transmitted PDU sequence number
– N(R) = acknowledge number, refers to the
expected PDU at the receiver
0 N(S) P/F N(R)
COMPUTER NETWORKS – Data link layer protocols - 17 Copyright Gruppo Reti – Politecnico di Torino
LAP-B: supervision PDUs (S)
• ACK transfer
• RR (Receiver Ready - C/R)
– Positive ACK
• RNR (Receiver Not Ready - C/R)
– Positive ACK and flow control signal sent from the
receiver which is unavailable (ON-OFF flow control)
• REJ (Reject - C/R)
– Request for retransmission of all PDU starting from N(R)
1 0 S S P/F N(R)
COMPUTER NETWORKS – Data link layer protocols - 18 Copyright Gruppo Reti – Politecnico di Torino
LAP-B: unnumbered PDUs (U)
• Mainly used to control connection
management
• 5 M bits. A limited number of PDUs is used
by LAP-B among the 32 available PDUs
• Command PDUs:
– SABM(E) (Set Asynchronous Balanced Mode),
used to (re)open the connection
• E = Extended numbering scheme
– DISC (Disconnect): the connection is aborted
1 1 M M P/F M M M
Pag. 4
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 19 Copyright Gruppo Reti – Politecnico di Torino
LAP-B: unnumbered PDUs (U)
• Response PDUs
– UA (Unnumbered Acknowledgment):
• ACK for PDUs opening the connection or to answer to
DISC commands
– DM (Disconnect Mode)
• Connection was not set up correctly
– FRMR (FRaMe Reject)
• Answer to the reception of a correct but unknown
PDU
• 24 additional bits to explain the reason why the PDU
was rejected
COMPUTER NETWORKS – Data link layer protocols - 20 Copyright Gruppo Reti – Politecnico di Torino
format command response code in control field
1 2 3 4 5 6 7 8
Information I (Information) 0 N(S) P N(R)
Supervision RR (Receiver Ready) RR (Receiver Ready) 1 0 0 0 P/F N(R)
RNR (Rec. Not Ready) RNR (Rec. Not Ready) 1 0 1 0 P/F N(R)
REJ (Reject) REJ (Reject) 1 0 0 1 P/F N(R)
Unnumbered SABM (Set Asynchr. Balanced Mode) 1 1 1 1 P 1 0 0
DISC (Disconnect) 1 1 0 0 P 0 1 0
DM (Disconnect Mode) 1 1 1 1 F 0 0 0
UA (Unnumbered Acknowledgement) 1 1 0 0 F 1 1 0
FRMR (Frame Reject) 1 1 1 0 F 0 0 1
LAPB: command and response PDUs
COMPUTER NETWORKS – Data link layer protocols - 21 Copyright Gruppo Reti – Politecnico di Torino
LAPB: Poll/Final Bit
• In command PDUs
– the P/F bit is used to poll stations (i.e. to require an answer) when
set to 1
• In response PDUs
– the P/F bit is used to answer (final) to command PDUs with a P/F bit
set to 1
• Poll bit set to 1 by DTE (or DCE) is a request to answer
immediately for DCE (or DTE).
• Final bit set to 1 specifies the answer to the poll request
• DTE and DCE cannot send a command PDU with the P bit
set to 1 unless a response PDU with the F bit set to 1 was
received
COMPUTER NETWORKS – Data link layer protocols - 22 Copyright Gruppo Reti – Politecnico di Torino
LAPB addresses
• DTE has address 00000011 (3)
• DCE has address 00000001 (1)
• 3 is the value of the address field in
– command PDUs DCE DTE and in
– response PDUs DTE DCE
• 1 is the value of the address field in
– command PDUs DTE DCE and in
– response PDUs DCE DTE
• The address field permits to distinguish command
PDUs from response PDUs and to understand
whether the P/F bit is a poll or a final
COMPUTER NETWORKS – Data link layer protocols - 23 Copyright Gruppo Reti – Politecnico di Torino
Layered architecture:
X.25 and ISDN B channel
L3
L2
L1
LAP-B
Physical
LAP-B
Physical
Switching node User terminal
Error control
User
terminal
X.25
packet layer
Physical
LAP-B
X.25
packet layer
Physical
LAP-B
X.25 packet layer
COMPUTER NETWORKS – Data link layer protocols - 24 Copyright Gruppo Reti – Politecnico di Torino
Q V Q V
V,UA,F
Q,I00
V,I10
V,I00,P
V,I21 Q,I10,P
V,RR2,F
V,I32
V,I42
V,I52
V,I62
V,I72,P
V,RR3
V,RR4
Q,I25
Q,I36
V,I03
V,I14
V,I24 Q,I47
V,RR2,F
V,RR5,P Q,I52,P
V,RR3,F Q,RNR6,F
V,RNR6,P
V,RR3,F
V,DISC,P
V,UA,F
V,SABM,P
modo non
connesso
modo non
connesso
Q,RR2,F
LAPB: example of data transfer
Pag. 5
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 25 Copyright Gruppo Reti – Politecnico di Torino
Q V
V,I10
V,I00
V,I20
V,I30
V,I40
V,I50
V,I60 V,RR7
trasferim.
dati
tim
eout
V,RR0,P V,RR7,F
V,I70
LAPB: error recovery via timeout
COMPUTER NETWORKS – Data link layer protocols - 26 Copyright Gruppo Reti – Politecnico di Torino
Q V
Q,I00
V,I10
V,I00
V,I21
Q,I10
V,I32
V,I23
Q,I21
V,REJ2 Q,I32
Q,I42
trasferim.
dati
V,I34
V,I45 Q,I53
LAPB: error recovery via REJ
COMPUTER NETWORKS – Data link layer protocols - 27 Copyright Gruppo Reti – Politecnico di Torino
Q V
Q,I10
V,I11
V,I00,P
V,I22
V,I21,F
V,I33,P
V,I45
Q,I31
Q,I41
Q,I51
trasferim.
dati
V,I15,P
V,I26 V,RR2,F
Q,I00
V,RR1,F
LAPB: error recovery via P/F bit
COMPUTER NETWORKS – Data link layer protocols - 28 Copyright Gruppo Reti – Politecnico di Torino
Q
V,RR7
tim
eout
V,REJ7
V
V,I10
V,I00
V,I20
V,I30
V,I40
V,I50
V,I60
trasferim.
dati
V,I00
V,I70
V,I10
LAPB: error recovery via
time-out and REJ
COMPUTER NETWORKS – Data link layer protocols - 29 Copyright Gruppo Reti – Politecnico di Torino
Data link layer in LANs
• IEEE 802.3 data link layer in LANs has two
sub layers
– LLC: Logical Link Control
– MAC: Medium Access Control
LLC
MAC
physical
network 3
2
1
COMPUTER NETWORKS – Data link layer protocols - 30 Copyright Gruppo Reti – Politecnico di Torino
IEEE 802.2 Logical Link Control
• Derived from HDLC
• Standard ISO 8802/2.
• Main characteristics
– Byte oriented protocol
– Does not use flag for packet delineation (done
by the MAC)
– No erroro control (done by the MAC)
– PDU have source and destination addresses
used for higher layer protocol multiplexing
– Variable packet lenghth
Pag. 6
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 31 Copyright Gruppo Reti – Politecnico di Torino
LLC PDU
• One unused bit in the source address is
exploited to distinguish between command
and response.
• Control field
– 8 bit in unnumbered PDUs
– 16 bit otherwise
• Maximun packet size depends on the MAC
DSAP SSAP control information
address address 8 bit 8 bit 8 o 16 bit 8 x M bit
COMPUTER NETWORKS – Data link layer protocols - 32 Copyright Gruppo Reti – Politecnico di Torino
LLC PDU
• LLC addresses identify the higher layer
protocols transported bt the LLC protocol
• Address field has two bit, unused for
addressing, reserved for control purposes
• All 0s address identifies the LLC itself
• Up to 63 addresses available
COMPUTER NETWORKS – Data link layer protocols - 33 Copyright Gruppo Reti – Politecnico di Torino
SNAP PDU
• Extended addressing scheme, named SNAP
(SubNetwork Access Protocol)
• Used for connectionless transfer
• Identifies IP protocol among others
0AAH 0AAH 03H 000000H 0800H (3)-PDU
DSAP SSAP controllo informazione
OUI protocol type (3) (2)
COMPUTER NETWORKS – Data link layer protocols - 34 Copyright Gruppo Reti – Politecnico di Torino
Point to Point Protocol (PPP)
RFC 1661 • PPP (Point to Point Protocol) is mainly used
to connect customers to Internet ISP over
telephone or ADSL lines
• Also available over ISDN connections and
even over SONET/SDH
• Point-to-point connection is easier to manage
with respect to braodcast or radio channel
• PPP is simple (likely the simplest data link
layer protocol)
COMPUTER NETWORKS – Data link layer protocols - 35 Copyright Gruppo Reti – Politecnico di Torino
PPP functionalities
• PDU delineation
• Content transparency
• Error detection (but no correction)
• Higher layer protocols multiplexing
• Monitoring of link activity
• Negotiation of layer three (IP) addresses
– Useful for ISPs
COMPUTER NETWORKS – Data link layer protocols - 36 Copyright Gruppo Reti – Politecnico di Torino
Functions not available in PPP
• Error correction
• Flow control
• Sequence control
– Point to point link
• Multipoint link management
Pag. 7
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 37 Copyright Gruppo Reti – Politecnico di Torino
PPP PDU
• Flag for “framing” or PDU delineation
• Address
– Not used and kept for compatibility with HDLC
• Control
– Not used and kept for compatibility with HDLC
• Protocol
– Higher layer protocol multiplexing
COMPUTER NETWORKS – Data link layer protocols - 38 Copyright Gruppo Reti – Politecnico di Torino
Byte Stuffing
• Data transparency is obtained by defining a mechanism to
send, in the data portion of the packet , the byte with value
01111110
• The sender inserts an escape byte 01111101 before any
01111110 byte or 01111101 byte
• The first byte equal to 01111101 is discarded by the
receiver
COMPUTER NETWORKS – Data link layer protocols - 39 Copyright Gruppo Reti – Politecnico di Torino
PPP Link Control Protocol
• PPP-LCP protocol opens (and close) the PPP
connection
– Options are negotiated
– Max frame length; authentication protocol; skip address
and control fields
• DEAD is the initiaI state
• Once the PPP connection has been established
and the authentication process was successful, for
the IP protocol
– the IP Control Protocol (RFC1332) is used to set up IP
parameters (addresses, datagram compression, etc)
COMPUTER NETWORKS – Data link layer protocols - 40 Copyright Gruppo Reti – Politecnico di Torino
PPP Data Control Protocol
COMPUTER NETWORKS – Data link layer protocols - 41 Copyright Gruppo Reti – Politecnico di Torino
Frame Relay
• Standard to create packet networks based on virtual circuits
(normally permanent VCs) on a wide area
• The standard was originally proposed within the ISDN
framework
• Today used
– to create VPNs (Virtual Private Networks) for companies
– to interconnect LANs
– to build logical topologies to interconnect Internet routers for ISP
• Bit rate ranging from 64 kb/s to 2 Mb/s
• Variable size packets (well suited to data traffic)
– Maximum size 4096 byte
• http://www.frforum.com
COMPUTER NETWORKS – Data link layer protocols - 42 Copyright Gruppo Reti – Politecnico di Torino
Frame Relay
• Operates on Permanent Virtual Circuit
(although signaling protocols to deal with
SVC are defined)
Frame Relay
network
DTE
DTE DTE
DTE
DCE
DCE
DCE
DCE
Virtual
Circuits
Pag. 8
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 43 Copyright Gruppo Reti – Politecnico di Torino
LAPF
• Frame Relay defines the LAPF protocol (Link Access Procedure to Frame mode bearer services)
• LAPF is divided in two parts:
– DL-CORE (reccomendation I.233) • Used in all network nodes
– DL-CONTROL • Optionally used only by end users (today, mainly IP
routers)
• In most applications, it is not used
COMPUTER NETWORKS – Data link layer protocols - 44 Copyright Gruppo Reti – Politecnico di Torino
Core and edge approach
L>=
3
L2
L1
DL-CORE
Physical
DL-CORE
Physical
Frame Relay
switching node
User terminal
Error control
User terminal
Higher layer
protocols
DL-CONTROL
Physical
DL-CORE
Higher layer
protocols
DL-CONTROL
Physical
DL-CORE
COMPUTER NETWORKS – Data link layer protocols - 45 Copyright Gruppo Reti – Politecnico di Torino
LAPF packet
• Packet delineation through flag and
bitstuffing to guarantee data transparency
Flag Address Control CRC Flag Information
DL-CORE
DL-CONTROL (like HDLC with extended numbering)
COMPUTER NETWORKS – Data link layer protocols - 46 Copyright Gruppo Reti – Politecnico di Torino
LAPF packet
• ADDRESS field
contains
– the DLCI (Data Link
Connection
Identifier),the virtual
circuit identifier
– some additional bits for
congestion control and
traffic policing
DL-CORE
DL-CONTROL
ADDR (LSB)
FLAG
FLAG
ADDR (MSB)
information
CRC (LSB)
CRC (MSB)
control
COMPUTER NETWORKS – Data link layer protocols - 47 Copyright Gruppo Reti – Politecnico di Torino
ADDRESS field • DLCI: Data Link
Connection
Identifier
• FECN/BECN:
forward/backward
explicit congestion
notification
• DE: discard
eligibility
• C/R:
command/response
• D/C: DLCI or DL-
CORE
• EA: extension bit
default format (2 byte)
format 3 byte
format 4 byte
EA
0
EA
1
EA
0
EA
0
EA
1
EA
0
EA
0
EA
0
EA
1
lower DLCI or DL-CORE control
C/R
C/R
C/R
DE
DE
DE
BECN FECN
FECN
FECN
D/C
D/C
upper DLCI
upper DLCI
upper DLCI
DLCI
DLCI
DLCI
lower DLCI
lower DLCI or DL-CORE control
BECN
BECN
COMPUTER NETWORKS – Data link layer protocols - 48 Copyright Gruppo Reti – Politecnico di Torino
B-ISDN
• Private and public networks
• Integrated network
– Support all type of services, with different transmission
speeds and quality of service requirements over the
same network infrastructure
• Standardized by ITU-T and ATM Forum
• ISDN (re)evolution
• Exploit ATM as a transport, multiplexing and
switching technique
Pag. 9
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 49 Copyright Gruppo Reti – Politecnico di Torino
Asynchronous Transfer Mode:
ATM • Defined as a data link layer protocol
• Wide area network exploiting packet
switching networks with virtual circuit service
• Characteristics
– High bit rate (starting from 622 Mb/s)
– Low latencies for voice services
– Fixed size data units (cells)
• 53 byte (48 byte of data)
• http://www.atmforum.com
COMPUTER NETWORKS – Data link layer protocols - 50 Copyright Gruppo Reti – Politecnico di Torino
B-ISDN: reference model
Management
plane
Control plane User plane
Higher layers Higher layer
AAL (ATM Adaptation Layer)
ATM layer
Physical layer
Pla
ne
ma
na
ge
me
nt
La
ye
r ma
na
ge
me
nt
COMPUTER NETWORKS – Data link layer protocols - 51 Copyright Gruppo Reti – Politecnico di Torino
Core and edge approach
in the user plane
L>=3
L2
L1
ATM
Physical
ATM
Physical
ATM switching node User
terminal
Error detection only on-demand
for some AAL
User
terminal
Higher layers
protocols
AAL
Physical
ATM
Higher layers
protocols
AAL
Physical
ATM
COMPUTER NETWORKS – Data link layer protocols - 52 Copyright Gruppo Reti – Politecnico di Torino
ATM: which OSI layer?
• Original idea
– End-to-end transport
– ATM from desktop to
desktop
• Reality:
– IP router interconnection
– IP over ATM
– ATM is yet another data
link technology
COMPUTER NETWORKS – Data link layer protocols - 53 Copyright Gruppo Reti – Politecnico di Torino
ATM cell format
• Header (5 bytes) + payload (48 bytes)
• Fixed size cell
– To ease the switching task at high speed (synchronous
switching)
• Small cell size
– Reduced latency (can be obtained by increasing
transmission speed)
– Small packetization delay for interactive voice services
– Segmentation overhead
• Slightly different format at network edge and core
COMPUTER NETWORKS – Data link layer protocols - 54 Copyright Gruppo Reti – Politecnico di Torino
GFC
VCI
VPI
VCI CLP
HEC
PT
DATA
VPI VCI
VCI
VCI CLP
HEC
PT
DATA
VPI VCI
VPI
UNI CELL NNI CELL
ATM cell format
5 BYTE
48 BYTE
Pag. 10
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 55 Copyright Gruppo Reti – Politecnico di Torino
ATM cell format
• ATM cell header (5 bytes = 40bit)
– GFC (4 bit): Generic Flow Control
– VPI (8-12 bit): Virtual Path Identifier
– VCI (16 bit): Virtual Circuit Identifier
– PT (3 bit): Payload Type
– CLP (1 bit): Cell Loss Priority
– HEC (8 bit): Header Error Code
COMPUTER NETWORKS – Data link layer protocols - 56 Copyright Gruppo Reti – Politecnico di Torino
ATM cell format
• GFC - Generic Flow Control
– Only at the UNI interface.
– The network issues information to user on the
number of cells that can be sent
– Two control algorithms:
• ON-OFF
• Credit based
COMPUTER NETWORKS – Data link layer protocols - 57 Copyright Gruppo Reti – Politecnico di Torino
ATM cell fromat
• VPI - Virtual Path Identifier
– Variable length:
• 8 bit at the UNI (256 VP’s)
• 12 bit at the NNI (4096 VP’s)
– Some VPIs are reserved to network
management functions and to signalling
COMPUTER NETWORKS – Data link layer protocols - 58 Copyright Gruppo Reti – Politecnico di Torino
ATM cell format
• VCI: Virtual Circuit Identifier
– Identifies a single virtual circuit within a given
VPI.
– 65536 VC’s are available for each VP.
– Example: link at 2,4 Gb/s, 1 VP, all VCs with the
same capacity 36Kb/s for each VC.
COMPUTER NETWORKS – Data link layer protocols - 59 Copyright Gruppo Reti – Politecnico di Torino
ATM cell format
• PT - Payload Type
– Classifies the payload information type.
– It contains an identifier named Payload Type
Identifier (PTI).
– Among the eight possible codes,
• four are reserved to network functions
• four to user function
COMPUTER NETWORKS – Data link layer protocols - 60 Copyright Gruppo Reti – Politecnico di Torino
PT MEANING
0 0 0
User cell EFCI No congestion
AAL 5 indication=1 1 0 0
User cell EFCI Congestion
AAL 5 indication=0 0 1 0
User cell EFCI Congestion
AAL 5 indication=1 1 1 0
PT field (Payload Type)
User cell EFCI No congestion
AAL 5 indication=0
Pag. 11
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 61 Copyright Gruppo Reti – Politecnico di Torino
PT SIGNIFICATO
0 0 1
OAM cell
(Operation and Maintenance) 1 0 1
RM cell
(Resource Management) 0 1 1
Not used
Reserved for future use 1 1 1
Campo PT (Payload Type)
OAM cell
(Operation and Maintenance)
COMPUTER NETWORKS – Data link layer protocols - 62 Copyright Gruppo Reti – Politecnico di Torino
ATM cell format
• CLP - Cell Loss Priority
– Two priority levels at the ATM layer (within each
VC)
– In ATM switches, it permits to selectively discard
cells in case of buffer congestion
– CLP=0 indicates a high priority cell
COMPUTER NETWORKS – Data link layer protocols - 63 Copyright Gruppo Reti – Politecnico di Torino
ATM cell format
• HEC - Header Error Code
– It permits to check the correctness of the ATM
cell header only
– No error detection on paylod!
– Single errors are corrected
– Two errors are detected
• SEC/DED: Single errore correction/ Double Error
Detection
COMPUTER NETWORKS – Data link layer protocols - 64 Copyright Gruppo Reti – Politecnico di Torino
B-ISDN: reference model
Management
plane
Control plane User plane
Higher layers Higher layer
AAL (ATM Adaptation Layer)
ATM layer
Physical layer
Pla
ne
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La
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COMPUTER NETWORKS – Data link layer protocols - 65 Copyright Gruppo Reti – Politecnico di Torino
AAL: ATM Adaptation Layer
• Integrates ATM transport to offer service to users
• Servide dependent layer
• Examples of AAL functions:
– Transmission errors detection and managment
– Segmentation and reassembly
– Cell loss management
– Flow control
– Synchronization
COMPUTER NETWORKS – Data link layer protocols - 66 Copyright Gruppo Reti – Politecnico di Torino
AAL: ATM Adaptation Layer
• It defines four classes of service (service
classes)
– Through three main parametrs:
• Source transmission speed
• Type of connection (connection
oriented/connectionless)
• Temporal relation between end user
Pag. 12
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 67 Copyright Gruppo Reti – Politecnico di Torino
AAL: 4 service classes
• A: CBR traffic, constant but rate, connection
oriented, synchronism required AAL 1
• B: VBR traffic, connection oriented,
synchronism required AAL 2
• C: VBR traffic, connection oriented,
synchronism not required AAL 3/4
• D: VBR traffic, connectionless, synchronism
not required AAL 5
COMPUTER NETWORKS – Data link layer protocols - 68 Copyright Gruppo Reti – Politecnico di Torino
Class A Class B Class C Class D
Synchronism required between source and dest
Speed
Connection
type
AAL type
Possible
applications
required not required
costant
(CBR)
variable
(VBR)
Connection oriented connectionl
ess
AAL 1 AAL 2 AAL 3/4 - 5
voice 64kbit/s
video CBR
video/audio
VBR data data
AAL service classes
COMPUTER NETWORKS – Data link layer protocols - 69 Copyright Gruppo Reti – Politecnico di Torino
AAL layer: architecture
• The AAL layer is subdivided into two sub-
layers
– convergence sublayer (CS):
• Service and ATM traffic convergence
• Multiplexing
• Error detection
• Synchronism recovery
– segmentation and reassembly (SAR):
• Segmentation in transmission, reassembly in
reception of CS PDUs
COMPUTER NETWORKS – Data link layer protocols - 70 Copyright Gruppo Reti – Politecnico di Torino
AAL
SSCS
CPCS CS
SAR
AAL architecture
• CS convergence sublayer
• SAR segmentation and reassembly
• SSCS service specific CS
• CPCS common part CS
• Some sub-layers can be empty
COMPUTER NETWORKS – Data link layer protocols - 71 Copyright Gruppo Reti – Politecnico di Torino
AAL format
AAL 1
AAL 2
AAL 3
AAL 4
AAL 5
ATM Cell Header SN IT SAR - SDU LI CRC
44 byte
ATM Cell Header ST SN RES SAR - SDU LI CRC
44 byte
ATM Cell Header ST SN MID SAR - SDU LI CRC
44 byte
ATM Cell Header SN SNP SAR - SDU
47 byte
ATM Cell Header
48 byte COMPUTER NETWORKS – Data link layer protocols - 72 Copyright Gruppo Reti – Politecnico di Torino
AAL 3/4
CPI B Tag
BA size
AAL payload pad AL Lenght
SAR header
SAR trailer
SAR header
SAR trailer
SAR header
SAR trailer
2 byte 44 byte 2 byte SAR - PDU
1B 1B 2B 0-3B 1B 2B 2B
ST SN MID LI CRC
2 4 10 bit 6 bit 10 bit
ST=EOM
ST=COM
ST=BOM
E Tag CPCS PDU
SAR PDU
Pag. 13
COMPUTER NETWORKS – Data link layer protocols
COMPUTER NETWORKS – Data link layer protocols - 73 Copyright Gruppo Reti – Politecnico di Torino
AAL 5
SAR
Layer
PDU
CS
Layer
PDU
End of segment = 1
CS Layer Payload
Le
ng
ht
CR
C -
32
48 bytes
SAR
payload
48 bytes
SAR
payload
48 bytes
SAR
payload
1- 65535B 0-47B 2B 2B 4B
COMPUTER NETWORKS – Data link layer protocols - 74 Copyright Gruppo Reti – Politecnico di Torino
AAL 5
• No CS layer
• SAR layer exploit all 48 byte payload
• Last cell created by the segmentation
process has the third bit in the PT field of the
ATM header set to 1
– Layer separation principle violated!
• Error control over the full CS-PDU
COMPUTER NETWORKS – Data link layer protocols - 75 Copyright Gruppo Reti – Politecnico di Torino
AAL 5
• Advantages
– simplicity
– efficiency
– Improved reliability (CRC - 32)
• Disadvantages
– Use the third bit of the PT field in the ATM
header
• Breaks layer separation principle
– Loss of the cell with the PT bit set =1 implies that
two full CS-PDUs are lost
COMPUTER NETWORKS – Data link layer protocols - 76 Copyright Gruppo Reti – Politecnico di Torino
Layer 2 protocols: a comparison
Protocol Packet delineation Layer 3 protocol
multiplexing
Error detection Error correction
(window
protocol)
LAPB Delimiter Higher layer YES YES
LAPF core +
LAPF control
Delimiter Via virtual
circuits
YES in
LAPF core
Optional in
LAPF control
(edge)
ATM (core)+
AAL (edge)
Done at the
physical layer
Via virtual
circuits
YES in
AAL (edge)
NO
PPP Delimiter YES YES NO
LLC Done in MAC
IEEE 802.3
YES Optional Optional
Ethernet MAC Silence YES YES NO
In gray info either not presented in this class or later descriebd
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