Transcript Week Nine

Data Communications
Asynchronous Transfer Mode
and Frame Relay
What is ATM?
A packet switched, connection-oriented service
Local area, metro area, and wide area service
Can support real-time traffic and non-real-time
traffic (data arrives in order, low delay)
Can support various levels of service (continuous,
variable, available, and unspecified)
Very fast (up to 622 Mbps)
A complex technology and typically expensive
More What is ATM?
Similarities between ATM and packet switching
Transfer of data in discrete chunks
Multiple logical connections over single physical
interface
In ATM flow on each logical connection is in fixed
sized packets called cells
Minimal error and flow control
Reduced overhead
Data rates - 25.6Mbps to 622.08Mbps (155.5
Mbps necessary for full-motion video)
Overview of ATM Network
ATM is similar to IP – a mesh network of “routers”
(ATM switches)
Two types of links in ATM
NNI (network – network interface) connects two ATM
switches; UNI (user – network interface) connects
switch to user device
ATM is connection-oriented
User must create a virtual circuit thru the ATM network
(using virtual circuit ID); signals create circuit, maintain
circuit, dissolve circuit
Protocol Architecture
Protocol Architecture
User plane
Provides for user information transfer along with flow
control and error control
Control plane
Performs call and connection control functions
Management plane
Plane management
Management functions related to system as a whole; make
sure the various planes coordinate their activities properly
Layer management
Provides operations, administration, and maintenance (OAM)
services thru info packets that switches exchange to keep
system running effectively
Protocol Architecture
Physical plane
Designed to run over SONET but can also run over
FDDI, T-1, and T-3
ATM Layer
Defines the cell format and how to respond to info found
in the header. Also responsible for setting up and
releasing connections, and performs congestion control
ATM Adaptation Layer (AAL)
Provides the interface between applications and the ATM
layer
ATM Logical Connections
Virtual channel connections (VCC)
Analogous to virtual circuit in X.25
Basic unit of switching between two end users
Full duplex
VCCs used for data, user-network exchange
(control), and network-network exchange
(network management and routing)
ATM Logical Connections
Two types of virtual circuits
Permanent virtual circuit – analogous to a leased
telephone line
Switched virtual circuit – created using a connection
protocol based on ITU-T Q.2931
Virtual path connection (VPC)
Bundle of VCCs with the same end points
ATM Connection Relationships
Advantages of Virtual Paths
Simplified network architecture
Network transport functions can be applied to a channel
or a path of channels
Increased network performance and reliability
Network deals with fewer entities
Reduced processing and short connection setup
time
Much work is setting up path, reserving capacity for
future channels
Enhanced network services
Path is used internally but also visible to user
Call
Establishment
Using VPs
What Are VCCs Used For?
Between end users
Used to carry end to end user data, control signals
VPC provides overall capacity, VCC organization done
by users
Between end user and network
Used to carry control signaling between user and
network (typos top of page 353 – VPC should be
VCC)
Between network entities
Used to carry network traffic management and routing
information
Control Signaling – VCC
Done on separate connection ; Four methods for
establishing a VCC:
Semi-permanent VCC – no control signaling necessary
Meta-signaling channel - used as permanent control signal
channel – this channel is used to set up other VCC
signaling channels between user and network
User-to-network signaling virtual channel – Used for
control signaling - Used to set up VCCs to carry user
data
User-to-user signaling virtual channel
Within pre-established VPC
Used by two end users without network intervention to establish
and release user-to-user VCC
ATM Cells
Fixed size
5 octet header (cell tax)
48 octet information field
Why so small?
Small cells reduce queuing delay for high priority cells
Small cells can be switched more efficiently
Easier to implement switching of small cells in hardware
Fixed-size makes programming more easy
ATM Cell Format
Header Format
Generic flow control
Used at user to network interface
Controls flow of data from user device into the ATM
network only
Essentially two classes of connections – controlled and
uncontrolled
Controlled – network provides info to user regarding
how many cells it can send – like a credit mechanism
for flow control
Uncontrolled – network simply enables or disables
sending of cells – like X-ON/X-OFF flow control
Header Format
Virtual path identifier
An 8-bit (UNI) or 12-bit (NNI) path ID
Virtual channel identifier
A 16-bit channel ID. Together, VPI and VCI identify a
logical connection
Payload type
Various types of user info or network management info
For example: leftmost bit identifies payload as user
data or OAM info; second bit indicates whether cell
has passed thru any congested switches; third bit
might be used to indicate last cell in a sequence of
cells
Header Format
Cell loss priority
CLP bit indicates a cell’s priority level
If congestion occurs, ATM has option of deleting cells to
relieve congestion. Cells with CLP = 1 go first.
Header error control
See the following slides
Header Error Control
Provides for error checking on the header only
Payload is unprotected. Is this a good idea?
Fiber optic used – so low error rates
Some other layer can error detect the payload
Does it really make sense to error detect real-time
traffic?
ATM needs the speed!
Uses x8 + x2 + x + 1 checksum
Allows some error correction (single-bit errors, which
AT&T says happens 99.5% of time)
HEC Operation at Receiver
Header Error Control
HEC can also be used for providing
synchronization
Apply error-checking method using 40 consecutive bits.
If it does not generate a result consistent with the last 8
bits, shift one bit and try again.
Repeat above step until a consistent result is found.
Could it be a coincidence? Try it three more times. All
four succeed? You are in sync.
ATM Service Categories
An ATM network can support many types of
traffic:
Real time
Constant bit rate (CBR)
Real time variable bit rate (rt-VBR)
Non-real time
Non-real time variable bit rate (nrt-VBR)
Available bit rate (ABR)
Unspecified bit rate (UBR)
CBR
Fixed data rate continuously available
Tight upper bound on delay
Can support uncompressed audio and video
Video conferencing
Interactive audio
A/V distribution and retrieval
Tightly controlled by Peak Cell Rate (PCR), Cell
Transfer Delay (CTD), and Cell Delay Variation
(CDV)
$$$$
rt-VBR
Time sensitive application
Tightly constrained delay and delay variation
rt-VBR applications transmit at a rate that varies
with time
Examples include bursty voice and video
Can statistically multiplex connections
Parameters include Peak Cell Rate, Sustainable
Cell Rate, and Maximum Burst Size
$$$
nrt-VBR
Non-real time VBR
Intended for bursty traffic with no tight constraints
on delay and delay variation
Examples include airline reservations, banking
transactions
Parameters include Peak Cell Rate, Sustainable
Cell Rate, Maximum Burst Size, Cell Loss Ratio,
Cell Transfer Delay
$$$
ABR
Application specifies Peak Cell Rate (PCR) and
Minimum Cell Rate (MCR)
Resources allocated to give at least MCR
Spare capacity shared among all ABR sources
Examples include LAN interconnection and basic
critical data transfer systems such as banking,
defense information
(flying standby)
$$
UBR
For application that can tolerate some cell loss or
variable delays (non-critical apps)
Cells forwarded on FIFO basis
Do not specify traffic related service guarantees
Examples include text/data/image transfer,
messaging, remote terminals
Best effort service (wear your parachute)
$
ATM Bit Rate Services
ATM Adaptation Layer
Essentially the “translation layer” between ATM
layer and other layers, such as PCM and IP:
PCM (voice)
Assemble bits into cells
Re-assemble into constant flow
IP
Map IP packets onto ATM cells
Fragment IP packets
Use LAPF over ATM to retain all IP infrastructure
AAL Protocols
Adaptation Layer Services
Handle transmission errors
Segmentation and re-assembly
To enable larger blocks of data to be carried in the
information field of ATM cells
Handle lost and misinserted cells (cells routed the
wrong way)
Perform flow control and timing control
Supported Application types
Four AAL protocols defined:
AAL 1: CBR traffic, e.g. circuit emulation (T-1 over ATM),
voice over ATM, real-time video
AAL 2: rt-VBR traffic, e.g. MPEG voice and video
AAL 3/4: nrt-VBR traffic, e.g. general data service (not
really used by anyone)
AAL 5 (successor to AAL 3/4): e.g. nrt-VBR: voice on
demand; nrt-VBR: frame relay, ATM; UBR: IP over ATM
AAL 1
AAL 1 is the interface between a real-time
uncompressed byte stream and ATM
Got to be fast!
No convergence sublayer, only SAR sublayer
AAL 1 takes 46 or 47 bytes of data and puts a one
or two byte header on front
AAL 1 continued
AAL 1 header consists of following:
One bit pointer – tells whether this is a one byte header
or a two byte header. If second byte is included, this
byte tells where the data starts within the payload (in
case the payload does not contain a full 46 bytes of
data)
Three-bit sequence number – used to tell if a cell is lost
or mis-inserted (which may be too late anyway for realtime)
Four bits of error checking on preceding 3-bit sequence
number (yikes!)
AAL 2
AAL 2 format is used for compressed data, which
needs to indicate where each frame of
compressed data ends and begins
Similar to AAL 1 – no convergence sublayer, only
the SAR sublayer
Unlike AAL 1, AAL 2 adds a header and a trailer
AAL 2 continued
The AAL 2 format has the following fields:
Sequence number – same as AAL 1
Type field – helps identify message boundaries by
indicating when a cell corresponds to the first, last, or
intermediate cell of a message
Length field – specifies the number of bytes in the
payload
Checksum – applied to the entire cell, including the
data!
AAL 5
AAL 5 packets can be very large – up to 65,535
byte payload
AAL 5 not designed for real-time traffic
SAR sublayer takes the potentially large
convergence sublayer packets and breaks them
into 48 byte chunks, ready for the ATM layer
SAR sublayer also adds a 32-bit CRC at the end of
the packet, which is applied to the entire packet
(see next slide for example)
Example AAL 5 Transmission
Frame Relay
What is it?
A high-speed communications technology that is
used in hundreds of networks throughout the
world to connect LAN, SNA, Internet, and even
voice traffic.
Designed to be more efficient than X.25
Developed before ATM
Larger installed base than ATM
ATM now of more interest on high speed networks
Recall X.25
Call control packets, inband signaling
Multiplexing of virtual circuits at layer 3
Layer 2 and 3 include flow and error control
Considerable overhead!
Not appropriate for modern digital systems with
high reliability
Frame Relay - Differences
Call control carried in separate logical connection
Multiplexing and switching at layer 2
Eliminates one layer of processing
No hop-by-hop error or flow control
End-to-end flow and error control (if used) are
done by higher layer
Single user data frame sent from source to
destination and ACK (from higher layer) sent
back
Advantages and Disadvantages
Lost hop-by-hop error and flow control
Increased reliability makes this less of a problem
Streamlined communications process
Lower delay
Higher throughput
Tulsa, OK to NYC and back:
X.25: 1 sec delay round trip
Frame relay: 70 msec delay round trip
Protocol Architecture
Control Plane
Between subscriber and network
Separate logical channel used
Similar to common channel signaling for circuit
switching services
Data link layer
LAPD (Q.921)
Reliable data link control
Error and flow control between user (TE) and network
(NT)
Used for exchange of Q.933 control signal messages
User Plane
End to end functionality
Transfer of info between ends
LAPF (Link Access Procedure for Frame Mode
Bearer Services) Q.922
Frame delimiting, alignment and transparency
Frame mux and demux using addressing field
Ensure frame is integral number of octets (zero bit
insertion/extraction)
Ensure frame is neither too long nor short
Detection of transmission errors
Congestion control functions
Frame
Format
Frame Fields
DLCI – Denotes the port to which the destination
LAN (or device) is attached
The routing tables at each intervening frame relay
switch use the DLCI to route the frames to the
proper destination
FECN and BECN – Congestion control techniques
DE – Discard Eligibility bit – Have you exceeded
your data rate + burst rate for more than two
seconds?
Frame Relay Operation
Each frame relay switch performs following:
1. Check integrity of frame (FCS)
2. Look up DLCI in a table
3. Relay frame out appropriate port or trunk
Any Problems?
Just discard the frame!
Frame check error? Discard frame
Congestion? Discard frame
Invalid DLCI? Discard frame
Who informs the sender that a frame was
discarded?
Not frame relay! (Let TCP do it)
Virtual Circuits (VC)
VCs are full duplex, software-defined data paths
between two ports
You can have permanent virtual circuits (PVC) and
switched virtual circuits (SVC)
PVCs are set up by network provider and not
dynamic
To establish a PVC you and your provider agree
upon data transfer rate, burst rate, latency,
network availability, and delivery rate
(throughput)
Virtual Circuits (VC)
SVCs are available on call-by-call basis
Establishing a call by using the SVC signaling
protocol (Q.933) is similar to POTS call
Much more work needed by the network to set up
an SVC
Thus, you pay more for an SVC (make sure you
really need an SVC and not a PVC)
Congestion Notification
An important part of frame relay
If traffic increases to a point where delays are
unacceptable, congestion occurs
Serious congestion causes frames to be dropped
Network must inform users to reduce their offered
load
Status of Connections
Along with congestion signaling, there is optional
status of connection signaling
Special management frames with DLCI = 0 may
be passed between network and access device
These frames provide the following info:
Keep alive signal (still active?)
Valid DLCIs for this interface
Status of each VC, e.g. congested or not
Frame Relay Standards
Description
Service Description
ANSI
T1.606
ITU
I.233
Core Aspects
T1.618
Q.922 A
Access Signaling
T1.617
Q.933
Note: ANSI and ITU standards essentially equivalent
Frame Relay Forum
Implementation Agreements
FRF.1.1
FRF.2.1
FRF.4
FRF.5
FRF.7
protocol
FRF.9
FRF.11
User-to-network (UNI) agreement
Frame relay network to network agree.
SVC agreement
Frame Relay/ATM PVC Network agree.
Frame Relay PVC multicast service and
agreement
Data compression
Voice over frame relay
See Frame Relay Forum website for complete, up-to-date list
Where is Frame Relay Used?
Connect multiple LANs over larger distance (e.g.
interconnection a company’s sites)
SNA over frame relay (e.g. data center connecting
to multiple branch banks)
Voice over frame relay (VoFR)
Frame relay to ATM internetworking (frame relay
to ATM to frame relay, or frame relay to ATM)
Creating a Frame Relay Service
Let’s say you want to interconnect three sites
Each site connects to the frame relay cloud via a
port
Each site also needs some form of
telecommunication service to connect to this
port
Each connection through the frame relay cloud
requires a PVC
Creating a Frame Relay Service
Port price depends upon capacity
PVC price depends upon capacity, delivery rate,
and latency (service level agreement)
Telecommunications line depends upon type of
service
When specifying PVC one defines the capacity
(committed information rate CIR) and burst rate
If user exceeds CIR plus burst rate for more than
2 seconds, frames get Discard Eligible bit set
More Info
Frame Relay Forum (www.frforum.com) is a good
source of information
White papers describe basic concepts of frame
relay
Review Questions
1.
2.
3.
4.
5.
What are the main functions of ATM?
What are the layers of ATM?
What is the relationship between VCC and VPC?
What is the layout of a cell?
What are the different service categories and
when might each be used?
6. What is the function of the AAL?
7. What are the sublayers of AAL?
8. What are the main functions of frame relay?
Review Questions
9. What are the differences between frame relay
and X.25?
10. What is the frame format of frame relay?
11. Why is frame relay so much faster than X.25?
12. What is the relationship between PVC, port,
and connecting medium?