Layer 2
Data Framing for
Fun and Profit
Eric L. Michelsen
Inductive Logic
Where In the Stack Are We?
OSI
7. Application
6. Presentation
5. Session
Link Layer
Sublayers
4. Transport
3. Network
2. Link
1. Physical
4/27/2001
1490
PPP Frame AAL2 AAL5
Relay
HDLC
Ether- 802.2
Novell
SNAP
net II (LLC)
IPX
ATM
Bit serial
(V.35, HSSI, DS1, DS3, etc.)
Inductive Logic
Ethernet
2
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
4/27/2001
Inductive Logic
3
Synchronous HDLC (aka SDLC)
1 octet
opening
flag
2 or 4
CRC closing
flag
data
flag
flag
01111110 ... idle ...
01111110
• 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:
flag
flag 0111111011111101111110 flag
4/27/2001
Inductive Logic
4
HDLC Bit Stuffing
bit stuffed 0
01111110 . . .
111110 . . . . 01111110
opening flag
user data:
five 1s
closing flag
• 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
4/27/2001
Inductive Logic
5
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
1 octet
opening
flag
01111110
4/27/2001
2 or 4
data
Inductive Logic
CRC closing
flag
idle...
01111110
6
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
1 octet
2 to 4
opening frame
flag
address
1 to max-length
information
2
CRC closing
flag
F
E
DLCI[9:4]
DLCI[3:0] C
N
7 6 5 4 3 2 1 0 7 6 5 4 3
EA
CR
=0
4/27/2001
1
B
E DE EA
C
=1
N
2 1 0
Inductive Logic
7
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 network
frame address
F
E
DLCI[9:4]
DLCI[3:0] C
N
7 6 5 4 3 2 1 0 7 6 5 4 3
EA
CR
=0
4/27/2001
B
E DE EA
C
=1
N
2 1 0
Inductive Logic
8
RFC 1490
Multiprotocol Encapsulation in FR
• Allows a SNAP-like header in data field to encapsulate
raw LAN frames (“bridged” mode)


1
Bridging header (OUI = 0x00-80-C2 for 802.1) encapsulates
LAN frame
PID specifies with or without LAN FCS
1
1
3
2
6
0 or 4
Ctl pad NLPID
OUI PID dest ...
=3 =0 =0x80
MAC
LAN data
optional
LAN FCS
LAN frame
Frame Relay information field
• Also allows direct IP encapsulation (“routed” mode)
Ctl NLPID
= 3 = 0xCC
IP packet
Frame Relay information field
4/27/2001
Inductive Logic
9
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
frame size 64 to 1518
8 octets
6
6
S
preamble F
D
destination
MAC
source
MAC
48 (min) to 1502 (max)
PID + data
pad (if
needed)
4
FCS
CRC-32
1010 ... 1011
Ethernet Common HW sublayer
4/27/2001
Inductive Logic
10
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
8 octets
preamble
6
S
destination
F
MAC
D
6
source
MAC
Common HW sublayer
4/27/2001
2
46 (min) to 1500 (max)
PID
data
Ethernet II
Inductive Logic
pad (if
needed)
4
FCS
Common HW
sublayer
11
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)
8 octets
preamble
6
6
2
1
1
1
dest source
length DSAP SSAP Ctl
MAC MAC
43 to 1497
data
4
pad (if FCS
needed)
length
4/27/2001
Inductive Logic
12
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)
8 octets
preamble
6
6
2
1
1
1
3
2
dest source
length DSAP SSAP Ctl OUI PID
MAC MAC
38 to 1492
data
4
pad (if
FCS
needed)
length
4/27/2001
Inductive Logic
13
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
8 octets
6
6
2
46 (min) to 1500 (max)
2
IPX
destination source
preamble
length checksum
MAC
MAC
= FFFF
IEEE 802.3
4/27/2001
data
pad (if
needed)
4
FCS
IPX header
Inductive Logic
14
How Can I Tell?
• Check PID/length first, then DSAP/SSAP
dest source PID >=
preamble MAC MAC
data/pad
1536
dest source length DSAP SSAP Ctl
preamble MAC MAC 3-1535
data/pad
dest source length DSAP SSAP Ctl OUI PID
preamble MAC MAC 8-1535 = AA = AA
dest source length
preamble MAC MAC
2-1535
4/27/2001
FFFF
Inductive Logic
FCS Ethernet
II
data/pad
data/pad
FCS 802.2
FCS
802.2
SNAP
FCS
Novell
old IPX
15
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
4/27/2001
Inductive Logic
8 bits 
GFC/
VPI...
VPI...
2 ...VPI
VCI...
1
3
...VCI...
4 ...VCI
5
PT CLP
HEC (8-bit CRC)
6
:
:
Payload
48 octets
53
16
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
packet layer
packets
SAR (AAL5)
cells
ATM layer
cell header
AAL5 packet
cell
PTI =
last cell
4/27/2001
cell
cell
cell
cell
cell
last cell
Inductive Logic
17
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)
N x 48
0 to 65,535 octets
data
0 to 47
pad
1
1
UU ctl
2
4
data
length
CRC-32
data length
4/27/2001
Inductive Logic
18
RFC 1483
Multiprotocol Encapsulation in ATM
• Requires an 802.2 SNAP-like header in data field
• Allows for raw LAN frames (“bridged” mode)


1
Header (OUI = 0x00-80-C2 for 802.1) encapsulates LAN frames
PID specifies with or without LAN FCS
1
1
3
2
2
6
...
DSAP SSAP Ctl OUI PID pad dest ...
=AA =AA =3 00 80 C2 00 xx 00-00 MAC
LAN data
0 or 4
optional
LAN FCS
LAN frame
AAL5 or FUNI data field
• Also allows direct IP encapsulation (“routed” mode)
DSAP SSAP Ctl OUI PID
AA
4/27/2001
AA
03
00 00 00 08 00
IP packet
Inductive Logic
19
Classical IP Over ATM
• RFC 1577 (Classical IP and ARP over ATM)



4/27/2001
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 Logic
20
FUNI
• ATM standard for Frame User-Network Interface
• Q.922 HDLC frames to ATM AAL5 interworking
• Fully supports ILMI, OAM, SVCs: all ATM management
Premise
FUNI Interworking cells
Device
ATM Network
Premise format
HDLC
flag
Q.922
VC payload
CRC
HDLC
flag
variable
VC payload
4/27/2001
Inductive Logic
pad
ATM format
AAL5 trailer
21
Some Common Encapsulations
1483 FUNI
(Ethernet)
HDLC Q.922
flag DLCI 528 AA AA 03 00 80 C2 00 07 00 00
Ethernet frame
CRC
HDLC
flag
1483 FUNI
(IP)
HDLC Q.922
flag DLCI 528 AA AA 03 00 00 00 08 00
IP packet
CRC
HDLC
flag
1490
(Ethernet)
HDLC Q.922
flag DLCI 16 03 00 80 00 80 C2 00 07
Ethernet frame
CRC
HDLC
flag
CRC
HDLC
flag
VC payload
CRC
HDLC
flag
ISO protocol packet
CRC
HDLC
flag
CRC
HDLC
flag
CRC
HDLC
flag
1490
(IP)
HDLC Q.922
flag DLCI 16
03 CC
Q.922
HDLC
flag
Q.922
Q.922
+1490
HDLC
flag
Q.922 03
HDLC
HDLC
flag
HDLC
+ PPP
4/27/2001
NLPID
Optional
1 or 2
HDLC Adrs Ctl
PPP-PID
flag
FF 03
IP packet
HDLC payload
PPP information
Inductive Logic
22
Frame Relay Encapsulations
1483 FUNI
(Ethernet)
HDLC
flag
Q.922
AA AA 03 00 80 C2 00 07 00 00
Ethernet frame
CRC
HDLC
flag
1483 FUNI
(IP)
HDLC
flag
Q.922
AA AA 03 00 00 00 08 00
IP packet
CRC
HDLC
flag
1490
(Ethernet)
HDLC
flag
Q.922
03 00 80 00 80 C2 00 07
Ethernet frame
CRC
HDLC
flag
1490
(IP)
HDLC
flag
Q.922 03 CC
CRC
HDLC
flag
“None”
HDLC
flag
Q.922
CRC
HDLC
flag
CRC
HDLC
flag
PPP- HDLC
Q.922
RFC1973 flag
4/27/2001
IP packet
VC payload
variable
NL03
PPP-PID
PID
PPP information
Inductive Logic
23
ATM Encapsulations
variable
“None”
(AAL5)
pad
VC payload
AAL5 trailer
variable
1483
AA AA 03 00 80 C2 00 07 00 00
(Ethernet)
Ethernet frame
pad
AAL5 trailer
variable
1483 IP
AA AA 03 00 00 00 08 00
(ATM)
IP packet
pad
AAL5 trailer
variable
FRF.5
CiscoPPP
Q.922
VC payload
Optional
1 or 2
Adrs Ctl
PPP-PID
FF 03
PPP information
PPP information
PPP-PID
4/27/2001
FE FE
pad
AAL5 trailer
variable
1 or 2
PPP-2364LLC
AAL5 trailer
variable
1 or 2
PPP-2364NULL
pad
03 CF PPP-PID
pad
AAL5 trailer
variable
PPP information
Inductive Logic
pad
AAL5 trailer
24
DS1 ESF Framing
193-bit frame
M1
DS1-ESF
(unchannelized)
Reference
T1.107
192-bit
Payload
C1
M2
F1=0
M3
C2
M4
F2=0
M5
C3
M6
F3=1
M7
C4
M8
F4=0
M9
C5
M10
F5=1
M11
C6
M12
F6=1
6-bit CRC
4 x 6 = 24 frames
= 1 superframe
Frame Alignment
Signal (FAS)
4 kbps Facility
Data Link (FDL)
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
4/27/2001
Inductive Logic
Note:
SF (aka D4) framing is not
usable for data transport
25
DS3 Framing
680-bit M-subframe
X1
84-bit F1=1
Payload
C1=1
F2=0
C2=1
F3=0
FE
AC
F4=1
X2
F1=1
C1=1
F2=0
C2=1
F3=0
C3=1
F4=1
P1
F1=1
CP
F2=0
CP
F3=0
CP
F4=1
P2
F1=1
FE
BE
F2=0
FE
BE
F3=0
FE
BE
F4=1
M1
=0
F1=1
DL
F2=0
DL
F3=0
DL
F4=1
M2
=1
F1=1
C1=1
F2=0
C2=1
F3=0
C3=1
F4=1
M3
=0
F1=1
C1=1
F2=0
C2=1
F3=0
C3=1
F4=1
M-Frame
Alignment
M-subframe Alignment
Framing is bit-oriented. C-bit-parity shown.
7 M-subframes
= 1 M-frame
Reference T1.107
M23 mode sets C1 in subframe-1 randomly, other C-bits per stuffing
X1 = X2 = not Remote Alarm Indication (RAI)
P1 = P2 = CP = XOR of previous M-frame payload bits
CP are end-to-end parity bits
44.736 Mbps line rate (exact)
44.736 x (84 / 85)  44.210 Mbps payload
rate
Inductive
Logic
4/27/2001
FEAC = Far End Alaram & Control
FEBE = Far End Block Error
DL = Data Link: HDLC EOC
26
STS-1 (component of OCn)
90 Columns
Reference GR-253, chap 3.
A1 A2 J0/Z
0
STS-1 Payload Capacity (84 useable columns)
8000 x 84 x 9 x 8 = 48.384 Mbps
Section
overhead B1 E1 F1
J1
J1
H1 H2 H3
B3
B2 K1 K2
C2
STS-1 Envelope Capacity
Line D4 D5 D6
overhead
ATM cells,
etc.
G1
87 Columns
D7 D8 D9
F2
D10 D11 D12
H4
|
S1/ M0
M1/ E2
Z1 Z2
Z3
Transport
overhead
810 Byte Frame
Fixed Stuff
9 Rows
H1/H2 point to the
Synchronous Payload Envelope
(SPE) at arbitrary byte offset
Fixed Stuff
D1 D2 D3
Z4
Z5
Framing is byte-oriented.
Line rate =
8000 frames/s x 90 x 9 x 8 bits= 51.840 Mbps
4/27/2001
Path
Overhead
(POH)
Inductive Logic
27
STS-Nc SPE
STS-12c SPE: 12 x 87 = 1044 Columns
J1
J1
B3
B3
C2
C2
G1
G1
F2
STS-3c Payload Capacity:
8000 x 260 x 9 x 8 = 149.760 Mbps
F2
H4
260 Columns
H4
Z3
Z3
Z4
Z4
Z5
Z5
Path
Overhead
(POH)
4/27/2001
9 Rows
Fixed Stuff
Fixed Stuff
Fixed Stuff
STS-3c SPE: 3 x 87 = 261 Columns
Path
Overhead
(POH)
Line Rate = N x 51.840 Mbps
Framing chosen to match STM-N
STS-48c (not shown) is exactly 4 times STS-12c
(POH + 15 Stuff)
Inductive Logic
STS-12c Payload Capacity:
8000 x 1040 x 9 x 8
= 599.040 Mbps
1040 Columns
9 Rows
Reference GR-253, chap 3.
28
This slide intentionally left blank
4/27/2001
Inductive Logic
29