ATM is both a layer-2 protocol and a protocol stack, similar to the way
in which IP is a layer-3 protocol and a protocol stack. The
ATM Reference Model table illustrates the
protocol stack for ATM.
ATM Reference Model
ATM Adaptation Layer (AAL)
Convergence Sublayer (CS)
Segmentation and Reassembly (SAR) Sublayer
Generic flow control (GFC)
Cell header creation and verification
Cell virtual path identifier (VPI) and virtual channel
identifier (VCI) translation
Cell multiplex and demultiplex
Transmission Convergence (TC) Sublayer
Header error control (HEC) generation and verification
Physical Medium Dependent (PMD) Sublayer
Bit timing (time recover)
Line coding for physical medium
The physical layer consists of two sublayers. The upper half of the
physical layer is the TC sublayer, which implements such functions as cell
scrambling and unscrambling, cell delineation, and HEC generation and
The purpose of this document is to review the benefits of scrambling
and the different commands used to enable scrambling on ATM interfaces,
depending on the physical layer interface module (PLIM).
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
For more information on document conventions, refer to the
Cisco Technical Tips
Scrambling is designed to randomize the pattern of 1s and 0s carried in
ATM cells or in the physical layer frame. Randomizing the digital bits can
prevent continuous, non-variable bit patterns; in other words, long strings of
all 1s or all 0s. Several physical layer protocols rely on transitions between
1s and 0s to maintain clocking.
One problem symptom that can be a good candidate for scrambling is link
flaps, which occur when particular files cross an ATM link. Such files may be
producing such a long string of all 1s or all 0s.
If you choose to enable cell-payload scrambling, ensure that both ends
of a virtual channel (VC) are configured with the same scrambling setting. Note
that most ATM interfaces do not include a default scrambling statement in the
configuration. For example, with the PA-A3-T3 port adapter, only a non-default
setting of cell-payload scrambling enabled will
appear in the configuration. In contrast, a scrambling statement always appears
in the configuration for an NM-4T1-IMA network module.
Cisco IOS® Software supports three commands to enable scrambling on
router ATM interfaces:
atm scrambling cell-payload—All other ATM
interface hardware (except the PA-A1).
atm ds3-scramble—Digital signal level 3
(DS-3) ATM interfaces only.
Note: DS-3 interfaces now use the atm scrambling
cell-payload command for equivalent functionality.
payload-scrambling—2600 and 3600 inverse
multiplexing over ATM (IMA) network modules
The following sections discuss each of these commands in further
Most ATM interfaces on Cisco routers support the atm
scrambling cell-payload command. Use the show atm
interface atm command to confirm the scrambling settings.
router(config-if)# atm scrambling ?
cell-payload SONET in cell payload scrambling mode
sts-stream SONET in sts-stream scrambling mode
7200-1# show atm interface atm 3/0
AAL enabled: AAL5 , Maximum VCs: 4096, Current VCCs: 1
Maximum Transmit Channels: 0
Max. Datagram Size: 4528
PLIM Type: SONET - 155000Kbps, TX clocking: LINE
Cell-payload scrambling: ON
sts-stream scrambling: ON
0 input, 0 output, 0 IN fast, 0 OUT fast, 0 out drop
Avail bw = 155000
Config. is ACTIVE
That output shows that the Synchronous Optical Network (SONET)
interfaces support two levels of scrambling. The first level, sts-stream
scrambling mode, is required by the GR-253 standard of the International
Telecommunication Union Telecommunication Standardization Sector (ITU-T). It
uses a 1 + x6 + x7
algorithm and scrambles all but the first row of the section overhead of the
SONET frame. Consider this definition of the use of sts-stream scrambling in
section 5.1.3 of GR-253:
“SONET optical interface signals use binary line coding and therefore
must be scrambled to assure an adequate number of transitions (0s to 1s and 1s
to 0s) for such purposes as line rate clock recovery at the receiver. SONET
electrical interface signals use line codes that assure adequate transitions;
however, they are also scrambled for consistency between the electrical and
optical interfaces. The scrambler shall be reset to '11111111' on the
most-significant bit of the byte following the Z0 byte in the Nth STS-1 (in
other words, the byte following the last Z0 byte). The scrambler shall run
continuously from that bit on throughout the remainder of the STS-N frame. Note
that the framing bytes (A1 and A2), the Section Trace byte (J0), and the
Section Growth (Z0) bytes are not scrambled.”
The second level of scrambling, cell-payload scrambling, is optional
and is defined in ITU-T
, section 4.5.3. It uses a polynomial of 1 +
x43. Cell-payload scrambling randomizes the bits in
only the payload portion of an ATM cell and leaves the 5-byte header
unscrambled. Cell-payload scrambling is designed to ensure successful ATM cell
delineation, which is the process of recognizing the start of each new
In summary, it is important to understand that SONET-level or
sts-stream scrambling must be enabled on every SONET device. Cell-payload
scrambling can be enabled or disabled with a configuration command.
Note that the Cisco IOS Software command line presents an option to
disable sts-stream scrambling. Although this command is accepted, it does not
actually disable this level of scrambling.
will remove this command in a future release.
Line-coding protocols on DS-3 and E3 interfaces can benefit from
scrambling. Specifically, scrambling helps to ensure accurate clock recovery on
the receiving ATM interface.
Originally, Cisco IOS Software used the
ds3-scramble and atm
ds3-scramble commands on DS-3 interfaces and the atm
e3-scramble command on E3 interfaces. In Cisco IOS Software
Release 12.2, these commands are hidden and—when configured—will appear as
atm scrambling cell-payload in the
Router# show atm interface atm 2/0/0
ATM interface ATM2/0/0:
AAL enabled: AAL5, Maximum VCs: 4096, Current VCCs: 12
Max. Datagram Size:4528, MIDs/VC: 1024
PLIM Type:DS3 - 45Mbps, Framing is C-bit ADM,
DS3 lbo: short, TX clocking: LINE
227585 input, 227585 output, 0 IN fast, 0 OUT fast
Config. is ACTIVE
The IMA network module for the 2600 and 3600 router series supports the
command. Cisco IOS Software Release 12.0(5)T and 12.0(5)XK introduced support
for the IMA module and for this command.
By default, payload scrambling is off for T1 links and on for E1 links.
The default binary 8-zero substitution (B8ZS) line coding for T1 links normally
is sufficient for proper cell delineation. The scrambling setting must match
the far end.
Use the show atm interface atm or
show controller atm commands to view the status of
scrambling on your IMA interfaces.
router# show controller atm 0/2
Interface ATM0/2 is administratively down
Hardware is ATM T1
!--- Output suppressed.
SAR Scheduling channels: -1 -1 -1 -1 -1 -1 -1 -1
Part of IMA group 3
Link 2 IMA Info:
group index is 1
Tx link id is 2, Tx link state is unusableNoGivenReason
Rx link id is 99, Rx link state is unusableFault
Rx link failure status is fault,
0 tx failures, 3 rx failures
Link 2 Framer Info:
framing is ESF, line code is B8ZS, fdl is ANSI
cable-length is long, Rcv gain is 26db and Tx gain is 0db,
clock src is line, payload-scrambling is disabled, no loopback
ATM network processor modules for the 4x00 router series use the
atm scrambling cell-payload command to configure
resolves a condition in which the router removes the payload scrambling command
and enters a no scrambling statement in the
running configuration upon reload.
Note: The default scrambling state on these modules is no
The LS1010 and Catalyst 8500 series of ATM switches support both modes
of SONET scrambling. Both modes are enabled by default on SONET
ls1010# show controllers atm 12/0/3
IF Name: ATM12/0/3 Chip Base Address: A8E0E000
Port type: OC3 Port rate: 155 Mbps Port medium: SM Fiber
Port status:Good Signal Loopback:None Flags:8308
TX Led: Traffic Pattern RX Led: Traffic Pattern
TX clock source: network-derived
Framing mode: sts-3c
Cell payload scrambling on
Sts-stream scrambling on
Cell-payload scrambling is disabled by default on DS-3 interfaces and
is enabled by default for E3 interfaces.
Use the show controllers atm command to
confirm any configuration changes to these default settings.
Packet over SONET (POS) interfaces support ATM-style payload scrambling
over the synchronous payload envelope (SPE) portion of a SONET frame to ensure
sufficient bit-transition density. Such scrambling is off by default and is
enabled with the pos scramble-atm command.
Router(config)# interface pos 3/0
Router(config-if)# pos scramble-atm
Note: Scrambling changes the value of the C2 byte in the path overhead. The
two values are 16 for scrambling enabled and
CF for scrambling disabled. Scrambling does not change the C2
byte when used with ATM over SONET links.
Cell scrambling does not provide security. Use it to randomize the data
pattern carried over a virtual connection. For secure ATM connections, consider
implementing security at a higher layer or using an encryptor.