Data communications
1 / 62

Data Communications - PowerPoint PPT Presentation

  • Uploaded on

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)

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about ' Data Communications' - jeremy-boyle

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Data communications

Data Communications

Asynchronous Transfer Mode

and Frame Relay

What is atm
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
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
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 architecture1
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 architecture2
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
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 connections1
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

Advantages of virtual paths
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
Call Establishment Using VPs

What are vccs used for
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
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
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

Header 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 format1
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 format2
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
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)

Header error control1
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
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)


  • 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

  • $$$


  • 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)

  • $$


  • 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 adaptation layer
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

Adaptation layer services
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
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 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 real-time)

  • 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
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)

Frame relay
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
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
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
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

Control plane
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
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 Format

Frame fields
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
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
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
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 vc1
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
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
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
Frame Relay Standards

  • Description ANSI ITU

  • Service Description T1.606 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
Frame Relay ForumImplementation Agreements

  • FRF.1.1 User-to-network (UNI) agreement

  • FRF.2.1 Frame relay network to network agree.

  • FRF.4 SVC agreement

  • FRF.5 Frame Relay/ATM PVC Network agree.

  • FRF.7 Frame Relay PVC multicast service and protocol agreement

  • FRF.9 Data compression

  • FRF.11 Voice over frame relay

  • See Frame Relay Forum website for complete, up-to-date list

Where is frame relay used
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
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 service1
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
More Info

  • Frame Relay Forum ( is a good source of information

  • White papers describe basic concepts of frame relay

Review questions
Review Questions

  • 1. What are the main functions of ATM?

  • 2. What are the layers of ATM?

  • 3. What is the relationship between VCC and VPC?

  • 4. What is the layout of a cell?

  • 5. 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 questions1
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?