1. Frame Relay Network Components
Module 1

2. Module Overview
After successfully completing this module, you should be able to:
Describe the relationship between local area networks and Frame Relay wide area network services
Describe the basic components of a Frame Relay network
Describe the role of Virtual Circuits in a Frame Relay environment
Describe how DLCIs are used to identify Virtual Circuits
List the functions of Link Management Protocols
Describe the basic differences between a User-to-Network Interface (UNI) and a Network-to-Network Interface (NNI)
Define the need for multiprotocol encapsulation in a Frame Relay network
In this module we discuss several issues related to the Frame Relay Protocol. These topics will help you understand the basics of Frame Relay and its advantages when used for wide are networking. Upon completion of this module you will be able to identify the following:
The relationship between Frame Relay switching over a Wide Area Network and Local Area Networking.
The characteristics of Frame Relay.
Virtual Circuit Service and the differences between a Permanent Virtual Circuit and a Switched Virtual Circuit .
Addressing for a Permanent Virtual Circuit.
The purpose and functions of Link Management for circuit management.
The formatting of each frame passing through the Frame Relay network.
Network management capabilities of Frame Relay.
Multiprotocol encapsulation in a Frame Relay network.

3. Frame Relay Technology
Designed to be simpler than existing packet-switched networks
Designed to provide a high-throughput, low-delay network interface
Provides limited error-recovery capabilities
Relies on the end-user’s system to recover from problems
Developed as a replacement solution to existing frame-based wide area networks
X.25
Point-to-point HDLC encapsulation
Multiple circuits can be multiplexed through one physical interface
Frame relay is a packet switching technology that uses frames to encapsulate user data for transmission across the network. The operation of frame relay is simple, compared to other WAN protocols, by design. Unlike other WAN protocols (such as X.25 or TCP/IP), frame relay provides error detection capabilities but is not designed to correct the errors it identifies. It relies on the end systems to make any corrections to data that has been identified as corrupted. The lack of error correction capabilities, and other factors to be discussed, account for the speed frame relay is able to provide across the WAN. The designers of frame relay examined the existing WAN technologies, and what they learned from them was used in the design of frame relay services.
Frame relay was designed to be simple, fast and efficient. It is specifically designed to address the problem of variable burst sizes and unpredictable traffic patterns from bandwidth intensive applications. It makes efficient use of the available bandwidth in a wide area network environment.

4. Frame Relay and the OSI Model
Frame Relay protocol operates in OSI Model layers 1 and 2
Layer 7 6 5 4 3 2 1
Application
Presentation
Session
Transport
The frame relay protocol operates at layers 1 and 2 of the OSI model; in fact it operates in the physical and MAC layers. Layers 1 and 2 deal with the physical connection requirements and the data link layer control functions as described below. Each layer in the OSI model interfaces with the layer below and the layer above it.
OSI Model “Stack” Overview
Layer Number Operational Characteristics
7 - Application The layer at which the user interfaces with the application This is where the user interfaces with the Mail, Telnet, File Transfer, (etc.), application programs.
6 - Presentation This layer deals with format and code conversions; blinking characters, underscores, etc.
5 - Session Establishes, synchronizes and manages the dialog between application programs at the endpoints. Administrative functions, etc., are handled at this layer.
4 - Transport Error recovery, data flow regulation, and segmentation of data for the network layer.
3 - Network Network layer addressing and routing multiple segments of data across the network.
2 - Data Link Controls access to the physical layer and communicates with the upper layer protocols.
1 - Physical Mechanical and electrical specifications for the media used.
Network
LLC
Data Link
MAC
Frame Relay
Physical

5. Encapsulation of User Data
Host A
Host B
LAN A
LAN B
Router
Router
IP Data
Ethernet
Header
Trailer
IP Data
Ascend
FR Header FR
Trailer
IP Data
Ethernet
Header
Trailer
IP Data FR
Header
Trailer
IP Data FR
Header
Trailer
IP datagram encapsulated at entry point to Frame Relay cloud
Within the cloud:
IP portion of packet never examined by frame switches, resulting in lower processing time (latency)
Error checking done, but no error correction
Frame Relay header/trailer removed on exiting cloud
As shown in the diagram, if an IP datagram is sent from terminal or host A, the packet will be forwarded to the Frame Cloud via the router attached to LAN A. LAN A’s router will convert the IP data into Frame Relay format by appending a Frame Relay header and trailer to the IP data. The IP data will traverse the Frame Relay cloud encapsulated in the Frame Relay frame. Upon exiting the Frame Relay cloud, the router at LAN B will strip the Frame Relay header and trailer and operate on the contents of the IP datagram, forwarding the information to LAN B, if necessary.

6. Frame Relay Packet Format
HDLC Packet Format
Flag
(7E)
Addr
(1byte)
Cntrl
(03)
Information Field
FCS
Flag
(7E)
Frame Relay Packet Format
Flag
(7E)
FCS
Information Field
Addr
(2 bytes)
Cntrl
(03)
Frame Relay packet is a variation of standard HDLC packet
Q.922 header makes address field larger
SDLC and HDLC Frames
Bit-synchronous protocols like X.25 or SNA use variations of the HDLC frame as the basis of their transmission structure.
Frame Relay also uses a derivative of the HDLC packet format. ITU recommendation Q.922 defines a 2-byte, 3-byte, and 4-byte addressing structure in the Frame Relay header. However, most implementations use only the 2-byte format.
Possible error conditions that may occur in a frame relay transmission include:
Missing or improper delimiters at the start and end of a frame
A packet which is smaller than the minimum packet size defined for frame relay
Packet which do not contain the proper number of octets as defined by the frame standard
Packets with FCS errors
Packets with improper frame relay addressing information
Packets which use addresses that are reserved and not available to the user
Packets which exceed the maximum size agreed to by the network

7. Frame Relay Header Details
(Flag)
Octet 1
DLCI (MSB)
C/R EA
DLCI
FECN
BECN DE EA
User Data
Frame Relay Header
FCS
The Frame Relay packet format includes the information necessary to carry user data through a Frame Relay network. The individual octets are divided into a header, data and trailer section. The fields within the individual octets include:
Flags: Octet 1 and Octet “n” (the last octet in the frame) will always be equal to a 7E (hex). These two octets identify the start and end of a frame relay packet. The last octet is labeled Octet “n” because the user field is variable in length
DLCI, Data Link Connection Identifier: 10 bit address which identifies a PVC
C/R, Command/Response Indication: Identifies control frames as commands or responses.
DE, Discard Eligibility: Identifies frames that are eligible for discard for congestion avoidance.
EA: Extended addressing bits. Address field may be greater than 2 octets for ANSI and CCITT specifications.
FECN, Forward Explicit Congestion Notification: Used by the network for congestion notification.
BECN, Backward Explicit Congestion Notification: Used by the network for congestion notification.
User Data: The network does not interpret or modify the contents of user data except for the virtual circuit dedicated to the LMI.
FCS
(Flag)
Octet n

8. Virtual Circuit Service
PVCs
Access Line
There are two types of Virtual Circuits
Permanent Virtual Circuits (PVCs)
Switched Virtual Circuits (SVCs)
Permanent Virtual Circuits are most prevalent today
A PVC is a fixed logical connection between two end points
End points are typically multiprotocol routers
Routers need an access line to the Frame Relay network to take advantage of PVC service

9. Data Link Connection Identifier (DLCI)
PVCs
DLCI
DLCI
DLCI
DLCI
DLCI
DLCI
A PVC is defined by its end points
The end point of a PVC is called a Data Link Connection Identifier (DLCI)
A Frame Relay header contains 10 bits for DLCIs
0: ANSI link management
1 - 15: Reserved for future use
: Available range for virtual circuits
: Layer 2 management for network
: Reserved for future use
1023: LMI

10. DLCI Local Significance
Chicago
Orlando
New York
DLCI 50
DLCI 25
DLCI 20
Address is significant only to local link
Local CPE never knows remote DLCI number
Remote DLCI number can be same as local DLCI number
Multiple PVCs terminating at one end point must have unique DLCI numbers
“Global addressing” is a DLCI numbering scheme used to identify destination of PVC

11. Role of Trunks Trunks interconnect Frame Relay switches
A PVC is provisioned across a trunk, reserving bandwidth on the trunk
Trunks can be “oversubscribed,” allowing PVCs to reserve more bandwidth than is physically available
On Ascend switches, OSPF routing protocol runs across trunks
OSPF routing table used
To find PVC path
Route SNMP management packets from Network Management Station (NMS)

12. Frame Relay Interface Definitions
UNI
NNI
Access Segment
Transit Segment
There are two basic types of Frame Relay interfaces
User-to-Network Interface (UNI) - defined in FRF.1
Network-to-Network Interface (NNI) - defined in FRF.2
There are two types of UNIs
UNI DCE
UNI DTE
Differences between UNIs and NNI ports are best learned by understanding their role in the Local Management Interface (LMI) protocol…the next subject
The interface for the Frame Relay access device into the Frame Relay network is called the User-to-Network Interface (UNI).
Frame Relay services may traverse across multiple Frame Relay networks. The interface between adjacent Frame Relay networks is called the Network-to-Network Interface (NNI).
Frame Relay Service Providers may offer Access Services and Transit Services. Access Service provides a direct interface to the end user. Transit Services provide connections to other Access Services. A carrier can provide Access or Transit Services only for a given connection.

13. Local Management Interface (LMI) Protocol
UNI
NNI
Access Segment
Transit Segment
LMI
LMI
Local Management Interface (LMI) protocol runs across UNI and NNI connections
Uses same physical line as user data
Uses reserved DLCI numbers, allowing switch and CPE to differentiate LMI data from normal user data
A form of “out-of-band” signaling
The interface for the Frame Relay access device into the Frame Relay network is called the User-to-Network Interface (UNI).
Frame Relay services may traverse across multiple Frame Relay networks. The interface between adjacent Frame Relay networks is called the Network-to-Network Interface (NNI).
Frame Relay Service Providers may offer Access Services and Transit Services. Access Service provides a direct interface to the end user. Transit Services provide connections to other Access Services. A carrier can provide Access or Transit Services only for a given connection.

14. How Does LMI Function?
LMI is a polling mechanism, with the DTE as the master and the DCE as the slave
LMI provides:
Verification of the integrity of the link
PVC status information
DCE
DTE
LMI
The network management entity provides dynamic notification to the user of the addition and deletion of PVCs. It also monitors each connection to the network through a periodic heartbeat polling process.
The heartbeat polling process is an exchange of sequence numbers between the network and the user device to ensure that both are operational. This polling process provides information to the user about its physical connection to the network through a Link Integrity Verification Report as well as a report on the status of all PVCs through a Full Status Report .
Status Enquiry (DTE)
Status Response (DCE)
Full Status Enquiry (DTE)
Full Status Report (DCE)

15. The Three LMI Standards
UNI
NNI
Access Segment
Transit Segment
LMI Rev 1
The original Frame Relay LMI standard created for UNIs
Developed through a consortium of Frame Relay supporters
DLCI 1023 used for management
ANSI Annex D
“Standard” for link management on UNIs
DLCI 0 used for management
CCITT Q.933 Annex A
“Standard” used on all NNI links
Add text from page 1-27 of the present student guide.

16. LMI Timers and Counters
T391 DTE Poll Timer - how often DTE polls DCE
T392 DCE Poll Verification Timer - how often DCE expects to be polled by DTE
LMI counters
N391 Full Status Poll Count - determines frequency of full status enquiry (default = every 6th poll)
N392 Allowable Errors Count - threshold of errors
N393 Events Count - tracks number of enquiries/responses that have occurred
By default, after 3 errors out of 4 events, the link is considered to have failed
LMI operates on both UNI and NNI interfaces. Various counters are implemented on the link to manage the activity and for error determination. The counters include:
N391 counter - is used to initiate a full status polling sequence on the local link. The values for this counter range from 1 to 255 with the default of 6. This counter determines, using the default of 6, that on every 6th poll, a full status poll will be initiated.
N392 counter - is used to determine a threshold of allowable errors on the link. The range is from 1 to 10 with a default of 3. Thus is three errors are found in the count determined by the N393 counter, the link is determined to be inactive.
N393 events counter - is used to monitor a count of events, polls, which take place on the link. If N392 errors occur in N393 counts, the link is considered to have gone inactive.
T391 counter (DTE poll timer) - starts at the beginning of a status enquiry message transmission and is used to measure if a response to the status enquiry message has been received in the interval defined by the counter. The range is from 5 to 30 with a default of 10. No response indicates a “miss”. N392 misses in N393 tries identifies a link inactive condition.
T392 counter (DCE polling verification counter) - determines the allowable time for the DCE to respond to a status enquiry. The range is from 5 to 30 with a default of 15; T392 must always be larger than T391.
In frame relay bi-directional mode, the network can also initiate status enquiry messages. It is possible for the DTE and DCE sides to have different T392 counters in bidirectional mode.

17. Putting It All Together: LMI on a UNI
DTE
DCE
LMI
Status Enquiry
Status Response
Full Status Enquiry
Full Status Response
On UNI connections
LMI procedures are unidirectional
DTE always generates the status enquiry
DCE only responds to enquiries
LMI should be configured for LMI Rev 1 or Annex D
A UNI connection will use unidirectional LMI procedures for status enquiry, link integrity or full status updates. The first three octets of the information element contain information on the type of report and the length of the information element.
Three report types are available:
Report type information element octet:
Full status =
Link integrity verification =
Single PVC asynchronous status =
Cascade Implementation
LMI maintains the operation of a UNI (or a Network-to-Network Interface (NNI)). It also reports status of virtual circuits over the local link asynchronously or via full status poll/response. Functions performed are dependent on the type of links:
UNI/DCE: Responds to link integrity and status polls
UNI/DTE: Polls the DCE end of the link, handles asynchronous reports
NNI: Bi-directional polling and asynchronous reports
For links where only PVCs are enabled, both the "gang-of-four" LMI and ANSI Annex D are supported. For links where both PVCs and SVCs are enabled, the ANSI Annex B is supported.

18. Putting It All Together: LMI on an NNI
Bidirectional
Procedures
Status
Enquiry
Status Response
Bidirectional LMI procedure
Both ends generate status enquiry and response messages
Both ends operate with independent periodic poll timers
Both ends use unique number sequences
LMI should be configured for Annex A

19. Summary: Components of a Frame Relay Network
Trunk
NNI
DLCI
DLCI
UNI
UNI
DLCI
DLCI
(LMI)
(LMI)
PVCs