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Inductive Logic4/27/2001 2
Where In the Stack Are We?
1. Physical
3. Network
5. Session
4. Transport
6. Presentation
7. Application
2. Link
OSI
Bit serial (V.35, HSSI, DS1, DS3, etc.)
HDLC ATM
AAL2Frame Relay
AAL5PPP
Ethernet
Ether-net II
802.2 (LLC)
SNAPNovell
IPX
1490
Link Layer Sublayers
Inductive Logic4/27/2001 3
Layers Upon Layers• Synchronous HDLC (SDLC)• RFC 1662 (Asynchronous HDLC)• Frame Relay• RFC 1490 (Multiprotocol in Frame Relay)• Four Flavors of Ethernet• ATM (cells, AAL5 packets)• RFC 1483 (Multiprotocol in AAL5)• FUNI• Reference formats: DSL, Frame Relay, ATM,
DS1, DS3, SONET
Inductive Logic4/27/2001 4
Synchronous HDLC (aka SDLC)
• Sends frames (packets) over a serial (synchronous) bit stream
• Frames are delimited by flags (8 bits: 01111110)• Shared flag closes one frame and opens the next• Idle is usually flags, but may be 1s (mark idle)• Shared zero flag is deprecated, and often banned:
01111110 ... idle ...
0111111011111101111110flag
flag
flag
2 or 4
opening flag
1 octet
01111110
closing flag flag flagCRCdata
Inductive Logic4/27/2001 5
HDLC Bit Stuffing
• Transparency is by “bit stuffing” everything between the flags: any sequence of five 1s has a 0 bit stuffed after it
• Stuffed frame length need not be a multiple of 8 bits• Stuffing makes the “effective data rate” pattern sensitive• 3 kinds of CRC: CRC-SDLC (aka CRC-CCITT), CRC-16,
and CRC-32 (used by CCITT).• CRC-16 detects errors better, but wasn’t known when
CRC-SDLC was chosen. Rarely used.• Hardware sends/detects flags, bit stuffing, CRC
01111110 . . . 111110 . . . . 01111110opening flag closing flaguser data:
five 1s
bit stuffed 0
Inductive Logic4/27/2001 6
RFC 1662 (Asynchronous HDLC)• Sends frames over a synchronous or asynchronous octet
stream (e.g., modem, X.25, SONET)• Based on ISO/IEC 3309:1991 (HDLC framing)• Frames delimited by flag octets: 01111110 (0x7E)• Byte stuffing: escape = 0x7D,
Following byte = (user data XOR 0x20)• Flag and Escape bytes in user data MUST be escaped• PPP Async Control Character Map (ACCM) specifies
other values from 0 - 0x1F to escape (Tx), and ignore (Rx)• Byte stuffing is often done in software
idle...opening flag
2 or 41 octet
01111110
closing flagCRC
01111110
data
Inductive Logic4/27/2001 7
Frame Relay• Uses synchronous HDLC framing sublayer• Data Link Control Identifier (DLCI) multiplexes virtual
circuits. Usually only local significance. • Uses Q.922 header: Usually 2-octet address (1024 DLCIs),
extensions for 3 or 4 (EA=1 on last address octet)• Max information length is configurable, 4096 max
21 octet
frame address
1 to max-length2 to 4
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
DLCI[9:4] DLCI[3:0]CREA=0
FECN
BECN
DEEA=1
1
opening flag
closing flagCRCinformation
Inductive Logic4/27/2001 8
Frame Relay (cont.)• Link Management Interface (LMI) on fixed DLCI• Forward Explicit Congestion Notification (FECN) set by
Frame Relay network if frame experienced congestion• Backward ECN set if reverse direction is congested• FECN & BECN set by network only
Q.922 is not clear if user equipment can set them or not
• Discard Eligible, set by network for overcommitted, or user for low priority, frames
• C/R bit carried transparently by Frame Relay networkframe address
7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0
DLCI[9:4] DLCI[3:0]CREA=0
FECN
BECN
DEEA=1
Inductive Logic4/27/2001 9
RFC 1490Multiprotocol Encapsulation in FR
• Allows a SNAP-like header in data field to encapsulate raw LAN frames (“bridged” mode)
Bridging header (OUI = 0x00-80-C2 for 802.1) encapsulates LAN frame
PID specifies with or without LAN FCS1 3 2 0 or 4
LAN frame
6
Frame Relay information field
1 1
• Also allows direct IP encapsulation (“routed” mode)Ctl= 3
Frame Relay information field
NLPID= 0xCC
destMAC
LAN dataoptional
LAN FCSCtl=3
OUI PID ...pad=0
NLPID=0x80
IP packet
Inductive Logic4/27/2001 10
Four Flavors of Ethernet• Ethernet II• IEEE 802.2• IEEE 802.2 SNAP• Novell IPX (“raw 802.3”)• All use a common hardware sublayer:
This is the basis of the “64-byte” minimum frame size Finally recognized by IEEE 802.3-1998 as the “MAC” layer
preamble sourceMAC
PID + data FCS
1010 ... 1011
8 octets 6 6 48 (min) to 1502 (max) 4
CRC-32
frame size 64 to 1518
pad (ifneeded)
SFD
Ethernet Common HW sublayer
destinationMAC
Inductive Logic4/27/2001 11
Ethernet II
• First widespread Ethernet standard• PIDs administered by IEEE• All IP on Ethernet is required to support this (RFC
1122), and almost all use it.• All PIDs are >= 1536 (decimal)• Exact packet length is not available from frame
data alone, because of padding
preamble data
8 octets 6 6 42
pad (ifneeded)
SFD
Ethernet IICommon HW sublayer
46 (min) to 1500 (max)
Common HW sublayer
sourceMAC
destinationMAC
PID FCS
Inductive Logic4/27/2001 12
Ethernet IEEE 802.2• Destination and Source Service Access Points (DSAP and
SSAP) are essentially protocol IDs • Used by IPX since Netware 3.12 (SAP=0xE0)• Not used for IP• Exact length available because ‘length’ specifies valid
data, not including padding• Don’t confuse Service Access Point (SAP) with Service
Advertising Protocol (SAP)
data
2 43 to 14971 1 1
length
pad (ifneeded)
preamble
8 octets 4
FCSdest
MACsourceMAC
6 6
length DSAP SSAP Ctl
Inductive Logic4/27/2001 13
IEEE 802.2 SNAP
• SubNetwork Access Protocol (SNAP)• OUI + PID allows vendors to define globally
unique protocol identifiers OUI = Organizationally Unique Identifier
• OUI = 0 implies PID is an Ethernet II PID• Optional support in IP (RFC 1042)
data
6 6
length
pad (ifneeded)
38 to 1492
preamble
8 octets 4
FCSdest
MACsourceMAC
2
length DSAP SSAP
111
Ctl OUI PID
3 2
Inductive Logic4/27/2001 14
Novell IPX (“raw 802.3”)
• Defined by Novell before 802.3 was complete• Phased out by Novell in Netware 3.12• “Checksum” is forced to 0xFFFF (=> none), takes
place of DSAP/SSAP• Does not allow a real IPX checksum
preambledestination
MACdata
8 octets 6 6 4
IPX header
pad (ifneeded)
46 (min) to 1500 (max)2
IPXchecksum = FFFF
IEEE 802.3
FCS
2
sourceMAC
length
Inductive Logic4/27/2001 15
preamble
preamble
preamble
sourceMAC
destMAC
sourceMAC
destMAC
sourceMAC
destMAC
How Can I Tell?
• Check PID/length first, then DSAP/SSAP
data/pad FCS
data/padSSAP Ctl
data/pad
data/pad
preamblesourceMAC
length3-1535
EthernetII
802.2
802.2SNAP
Novellold IPX
length2-1535
SSAP = AA
length8-1535
destMAC
DSAP
FFFF
DSAP = AA
PID >= 1536
FCS
FCS
FCS
FCS
Ctl OUI PID
Inductive Logic4/27/2001 16
ATM Cells• Synchronous bit stream• Bit stream may be framed (DS3, OC3)• Cells are 53 bytes:
5 header + 48 payload• Idle with null or unassigned cells• Cell delineation usually by Header
Error Check (HEC)• ATM switches operate primarily on
cells, ignoring payload type (PT)• VPI/VCI is virtual circuit identifier• VPI is 8 bits (UNI) or 12 bits (NNI)• VCI is 16 bits• CLP is like Frame Relay DE
GFC/VPI...
VCI...
...VCI...
...VCI
VPI...
...VPI
PT CLP
HEC (8-bit CRC)
Payload48 octets
1
2
3
4
5
6
:
:
53
8 bits
Inductive Logic4/27/2001 17
ATM AAL5 (Packets)
• Packet framing above cell layer
• ATM Adaptation Layer 5 (AAL5) includes Segmentation and Reassembly (SAR) of packets
• User-user bit in cell header payload-type (PTI) indicates last cell in packet
SAR (AAL5)
cells
ATM layer
packet layer
packets
cell cell cell cell cell cell
AAL5 packet
last cellPTI =
last cell
cell header
Inductive Logic4/27/2001 18
ATM AAL5 (Packets), cont.• Control, length, and CRC-32 at end of last cell
• Control field currently unused
• ‘data length’ provides precise length
• Length in trailer allows starting to send packet before length is known
(cut through switches, e.g. frame to ATM)
CRC-32
42
ctl
1
pad
0 to 65,535 octets 0 to 47
N x 48
UU
1
data length
datadata
length
Inductive Logic4/27/2001 19
destMAC
LAN dataDSAP=AA
SSAP=AA
Ctl=3
pad00-00
1 1 1 3 2 2
LAN frame
6 ...
AAL5 or FUNI data field
...
RFC 1483 Multiprotocol Encapsulation in ATM
optionalLAN FCS
0 or 4
• Requires an 802.2 SNAP-like header in data field• Allows for raw LAN frames (“bridged” mode)
Header (OUI = 0x00-80-C2 for 802.1) encapsulates LAN frames PID specifies with or without LAN FCS
• Also allows direct IP encapsulation (“routed” mode)
DSAPAA
SSAPAA
Ctl03
OUI PID00 00 00 08 00
OUI PID00 80 C2 00 xx
IP packet
Inductive Logic4/27/2001 20
Classical IP Over ATM
• RFC 1577 (Classical IP and ARP over ATM) Specifies RFC 1483 Routed IP format (802.2 SNAP) Defines Inverse-ARP for identifying your IP peer Defines a bunch of LAN Emulation (LANE) SVC stuff
that no one uses
Inductive Logic4/27/2001 21
FUNI• ATM standard for Frame User-Network Interface• Q.922 HDLC frames to ATM AAL5 interworking• Fully supports ILMI, OAM, SVCs: all ATM management
Interworking Device
cellsPremise
VC payload pad AAL5 trailer
variable
VC payloadHDLC flag CRC HDLC
flagQ.922
Premise format
ATM format
ATM Network
FUNI
Inductive Logic4/27/2001 22
VC payloadHDLC flag CRCQ.922
Q.922 +1490 ISO protocol packetHDLC
flag CRC
1490 (IP)
1483 FUNI (IP)
IP packetHDLC flag CRC03 CC
IP packetHDLC flag CRC
HDLC flag
HDLC flag
HDLC flag
HDLC flag
1483 FUNI (Ethernet) Ethernet frameHDLC
flagQ.922
DLCI 528 CRCAA AA 03 00 80 C2 00 07HDLC
flag
Q.922 DLCI 528 AA AA 03 00 00 00 08 00
00 00
1490 (Ethernet) Ethernet frameQ.922
DLCI 16 03 00 80 00 80 C2 00 07 CRC HDLC flag
Some Common Encapsulations
HDLC flag
Q.922 DLCI 16
Q.922
Q.922 03 NL-PID
HDLC payloadHDLC flag CRCHDLC HDLC
flag
HDLC + PPP PPP informationHDLC
flagAdrs CtlFF 03 CRC HDLC
flag
Optional
PPP-PID
1 or 2
Inductive Logic4/27/2001 23
VC payloadHDLC flag CRC“None”
HDLC flag
PPP informationHDLC flag CRC
PPP-RFC1973
HDLC flag 03 PPP-PID
v a r i a b l e
NL-PID
Frame Relay Encapsulations
1490 (IP)
1483 FUNI (IP)
IP packetHDLC flag CRC03 CC
IP packetHDLC flag CRC
HDLC flag
HDLC flag
1483 FUNI (Ethernet) Ethernet frameHDLC
flag CRCAA AA 03 00 80 C2 00 07HDLC
flag
AA AA 03 00 00 00 08 00
00 00
1490 (Ethernet) Ethernet frame03 00 80 00 80 C2 00 07 CRC HDLC
flagHDLC
flag
Q.922Q.922
Q.922
Q.922
Q.922
Q.922
Q.922
Inductive Logic4/27/2001 24
VC payload“None”(AAL5)
pad AAL5 trailer
1483 IP(ATM) IP packet pad AAL5 trailer
variable
variable
1483 (Ethernet) Ethernet frameAA AA 03 00 80 C2 00 07 00 00 pad AAL5 trailer
variable
ATM Encapsulations
AA AA 03 00 00 00 08 00
Cisco-PPP PPP informationAdrs Ctl
FF 03
Optional
pad AAL5 trailer
variable
PPP-PID
1 or 2
PPP-2364-NULL
PPP information pad AAL5 trailer
variable
PPP-PID
1 or 2
FE FE 03 CFPPP-2364-LLC
PPP information pad AAL5 trailer
variable
PPP-PID
1 or 2
VC payloadQ.922 pad AAL5 trailer
variable
FRF.5
Inductive Logic4/27/2001 25
M11 M12 F6=1C6
DS1 ESF Framing
DS1-ESF (unchannelized)
M1 C1 M2 F1=0
M3 C2 M4 F2=0
M5 C3 M6 F3=1
M7 C4 M8 F4=0
M9 C5 M10 F5=1
192-bit Payload
Frame Alignment Signal (FAS)
6-bit CRC
4 kbps Facility Data Link (FDL)
193-bit frame
4 x 6 = 24 frames = 1 superframe
Framing is bit-oriented (rather than octet-oriented)8k frames/s x 193 = 1.544 Mbps line rate (exactly)8k frames/s x 192 = 1.536 Mbps payload rate (exactly)8k frames/s 24 333 superframes/s
Reference T1.107
Note:SF (aka D4) framing is not usable for data transport
Inductive Logic4/27/2001 26
C1=1 C2=1 C3=1
C1=1 C2=1 C3=1
FEBE
FEBE
C1=1F1=1 C2=1F2=0 FEACF3=0 F4=1
C1=1F1=1 C2=1F2=0 C3=1F3=0 F4=1
CPF1=1 CPF2=0 CPF3=0 F4=1
FEBEF1=1 F2=0 F3=0 F4=1
DLF1=1 DLF2=0 DLF3=0 F4=1
F1=1 F2=0 F3=0 F4=1
F1=1 F2=0 F3=0 F4=1
M2=1
DS3 Framing
X1
X2
P1
P2
M1=0
84-bit Payload
M-subframe Alignment
680-bit M-subframe
7 M-subframes = 1 M-frame
Framing is bit-oriented. C-bit-parity shown. M23 mode sets C1 in subframe-1 randomly, other C-bits per stuffingX1 = X2 = not Remote Alarm Indication (RAI)P1 = P2 = CP = XOR of previous M-frame payload bits
CP are end-to-end parity bits44.736 Mbps line rate (exact)44.736 x (84 / 85) 44.210 Mbps payload rate
M3=0
M-Frame Alignment
Reference T1.107
FEAC = Far End Alaram & ControlFEBE = Far End Block ErrorDL = Data Link: HDLC EOC
Inductive Logic4/27/2001 27
STS-1 (component of OCn)90 Columns
810 Byte Frame
Framing is byte-oriented.Line rate = 8000 frames/s x 90 x 9 x 8 bits= 51.840 Mbps
Reference GR-253, chap 3.
A1 A2 J0/Z0
B1 E1 F1
D1 D2 D3
H1 H2 H3
B2 K1 K2
D4 D5 D6
D7 D8 D9
D10 D11 D12
S1/Z1
M0 | M1/Z2
E2
9 Rows
STS-1 Envelope Capacity87 Columns
Section overhead
Line overhead
Path Overhead
(POH)
STS-1 Payload Capacity (84 useable columns) 8000 x 84 x 9 x 8 = 48.384 Mbps
Fix
ed S
tuff
Fix
ed S
tuff
ATM cells, etc.
H1/H2 point to the Synchronous Payload Envelope
(SPE) at arbitrary byte offset
J1
Transport overhead
J1
B3
C2
G1
F2
H4
Z3
Z4
Z5
Inductive Logic4/27/2001 28
STS-Nc SPE
STS-3c Payload Capacity:8000 x 260 x 9 x 8 = 149.760 Mbps
STS-3c SPE: 3 x 87 = 261 Columns
Reference GR-253, chap 3.
9 Rows
260 Columns
J1
B3
C2
G1
F2
H4
Z3
Z4
Z5
STS-12c Payload Capacity:8000 x 1040 x 9 x 8 = 599.040 Mbps
STS-12c SPE: 12 x 87 = 1044 Columns
9 Rows
1040 Columns
Path Overhead
(POH)
J1
B3
C2
G1
F2
H4
Z3
Z4
Z5
Fix
ed S
tuff
Fix
ed S
tuff
Fix
ed S
tuff
Path Overhead
(POH)Line Rate = N x 51.840 MbpsFraming chosen to match STM-NSTS-48c (not shown) is exactly 4 times STS-12c (POH + 15 Stuff)