Transcript X.25 and Frame Relay

X.25 and Frame Relay
Topics to be Discussed
• Datagram versus Virtual Circuit
• X.25
–
–
–
–
History and Overview
Devices
Protocols, Frames and Addressing
Call Setup
• Frame Relay
–
–
–
–
–
History and Overview
Devices
Protocols, Frames and Addressing
Congestion Management
Local Management Interface (LMI)
• Comparison of X.25 and Frame Relay
• X.25 and Frame Relay Today
Datagram Packet Switching
• No call setup
• Each packet can travel across a different route
from sender to receiver
• Delivery and order of packets cannot be
guaranteed
• Most common implementation of datagram
packet switching is Internet Protocol (IP)
Virtual Circuit Packet Switching
• Similar to standard circuit-switched networks
• Call Setup required to define the route between
Sender and Receiver
• Each route is assigned a Virtual Circuit Identifier (VCI)
• All packets using the same VCI will travel the same
route and will arrive in sequence
• Circuit is “virtual” because resources are not
dedicated to a single call
• Most common forms of virtual circuit packet
switching are X.25 and Frame Relay
X.25 History and Overview
• Designed to provide a low cost alternative for data
communication over public networks
– Pay only for bandwidth actually used
• Ideal for “bursty” communication over low quality
circuits
• Standard provides error detection and correction for
reliable data transfer
• X.25 standard approved in 1976 by CCITT (now known as
ITU)
• Can support speeds of 9.6 Kbps to 2 Mbps
• Can provide multiplexing of up to 4095 virtual
circuits over on DTE-DCE link
X.25 Devices
• Data Terminal Equipment (DTE)
– Terminals, personal computers, and network hosts
– Located on premises of subscriber
• Data Circuit-terminating Equipment (DCE)
– Modems and packet switches
– Usually located at carrier facility
• Packet Switching Exchange (PSE)
– Switches that make up the carrier network
Sample X.25 Network
PSE
X.25
WAN
PSE
Modem
DCE
Terminal
DTE
Personal Computer
DTE
Modem
DCE
PSE
PSE
Modem
DCE
Server
DTE
Packet Assembler/Disassembler (PAD)
• Used for DTE devices that are too simple to
implement X.25 (such as character-mode terminals)
• Acts as intermediary device between DTE and
DCE
• Performs three functions
– Buffering to store data until a device is ready to
process it
– Packet Assembly
– Packet Disassembly
PAD in Action
PSE
X.25 Packet
Data
PAD
Terminal
DTE
Modem
DCE
PSE
Data
Assembly/
Disassembly
Buffer
X.25 mapping to OSI Model
Application
Presentation
Other Services
Session
Transport
Network
PLP
Data Link
LAPB
Physical
x.21 bis, EIA/TIA-232, EIA/TIA-449,
EIA-530, G.703
X.25
Protocol
Suite
X.25 Physical Layer
• Several well-known standards are used for X.25
networks
– X.21bis – supports up to 2 Mbps
• 15-pin connector
– RS-232 (EIA/TIA-232) – supports up to 19.2 Kbps
• 25-pin connector
– RS-449 (EIA/TIA-449) – supports up to 64 Kbps
• 37-pin connector
– V.35 – supports up to 2 Mbps
• 34-pin connector
• Uses serial communications in either asynchronous
or synchronous modes
X.25 Data Link Layer
• Link Access Procedure, Balanced (LAPB) is the protocol used
for this layer
• LAPB is a version of HDLC
– HDLC in Asynchronous Balanced Mode (ABM)
– DTE and DCE are peers and can both perform all functions
• LAPB manages communication and packet framing between
DTE and DCE devices
• Makes sure that frames are delivered in sequence and errorfree
– Uses sliding window of 8 or 128 frames
LAPB Frame Types
• Three types of frames
– I-Frames (Information Frames)
• Carry data as well as Next Send (NS) and Next Receive (NR) counts
– S-Frames (Supervisory Frames)
• Controls flow of data with Receiver Ready (RR), Receiver Not
Ready (RNR), and Reject (REJ) frames
– U-Frames (Unnumbered Frames)
• Establish and maintain communications with Set Asynchronous
Balanced Mode (SABM), Unnumbered Acknowledgment (UA),
Disconnect (DISC), Disconnect Mode (DM) and Frame Reject
(FRMR)
LAPB Frame Format
Flag
Address
Control
Data
FCS
Flag
Flag: (8 bits) Indicates start and end of frame (01111110)
Address: (8 bits) DTE address is maintained in higher layer so this field is
used to identify command and responses between DTE and DCE. A value of
0x01 indicates a command from DTE and responses from DCE while a value
of 0x03 indicates commands from DCE and responses from DTE.
Control: (8 bits) Contains sequence numbers, commands and responses for
controlling data flow
Data: (varies is size) Contains upper layer data
FCS: (16 bits) Frame Check Sequence used to determine if an error has
occurred in transmission (variation of CRC)
X.25 Network Layer
• Packet Layer Protocol (PLP) is the X.25
network layer protocol
• PLP manages calls between a pair DTE devices
using a Permanent Virtual Circuit (PVC) or a
Switched Virtual Circuit (SVC)
• PLP handles segmentation, reassembly, bit
padding and error and flow control
• PLP uses X.121 Addressing Scheme to setup a
virtual circuit
PLP Operates in Five Modes
• Call Setup
– Used to setup virtual circuit for SVC
• Data Transfer
– Used for transferring data with both SVC and PVC
• Idle
– Used when SVC call has been established but no data is currently
being transferred
• Call Clearing
– Used to end communication between DTEs for a SVC
• Restarting
– Used to synchronize DTE and DCE for all virtual circuits that exist
between them
PLP Frame Format
GFI
LCI
PTI
User Data
General Format Indicator: (4 bits) Identifies packet parameters, such as
whether the packet carries user data or control information, what kind of
windowing is being used, and whether delivery confirmation is required
Bit 1 – 0=User Data, 1=Data for PAD
Bit 2 – 0=Local Ack, 1=Remote Ack
Bits 3 & 4 – 00=Reserved, 01=Window Size 8, 10=Window Size 128,
11=Extended Format
Logical Channel Identifier: (12 bits) Identifies the virtual circuit (1-4095)
across the local DTE to DCE interface. This field consists of a 4-bit Logical
Channel Group Number (LCGN) and an 8-bit Logical Channel Number (LCN)
Packet Type Identifier: (8 bits) Identifies one of 17 different packet types
User Data: (varies is size but typically 128 bits) Contain encapsulated user
data for data packets or additional control information for other packets
X.121 Addressing
• PLP uses X.121 addressing during the call setup
phase to establish a virtual circuit between DTEs
• Only used for SVC calls
• Address consists of up to 14 digits
– 3 digits for Country Code
– 1 digit for Network Number (only 10 per country)
– Up to 10 digits to define the terminal number on the
network
Country
PSN
National Terminal Number
X.25 Call Request Packet
0
D
X
X
LGCN
LCN
0
0
0
0
Called Address Length
1
0
1
1
Calling Address Length
Called Address
Calling Address
Facilities Length
Facilities*
Call User Data
*Facilities field contains control
information to setup call specific
features such as reverse
charging, simplex mode instead
of full-duplex, max frame size,
and window size.
X.25 Clearing Packet
0
D
X
X
LGCN
LCN
0
0
0
1
0
0
1
1
Clearing Cause
Clearing Diagnostic
Address in Call Request format if used
Facilities in Call Request format if used
Clear User Data
X.25 Data Packet
Q
D
X
X
LGCN
LCN
Pr3 Pr2 Pr1 M Ps3 Ps2 Ps1 0
User Information or Higher Layer Protocol
PrX is a 3-bit Send Sequence Number
PsX is a 3-bit Receive Sequence Number
M indicates whether packets are part of a sequence
X.25 Call Setup
DTE to DCE
Interface
DCE to DTE
Interface
Call Request
Call
Setup
Phase
Data
Transfer
Phase
Call
Clearing
Phase
Incoming Call
Call Accepted
Call Connected
Data Packet
Incoming Data
Data Packet
Incoming Data
Clear Request
Clear Indication
Clear Response
Clear Confirm
Frame Relay History and Overview
• Frame Relay was originally designed for use on Integrated
Services Digital Network (ISDN)
• Usually considered a replacement for X.25 using more
advanced digital and fiber optic connections
• Does not perform error correction at intermediate nodes
making it faster than X.25
– When an error is detected (FCS) the frame is discarded and correction
is left up to higher layer protocols
• Original standard proposed in 1984 but widespread
acceptance did not occur until the late 1980’s
– Service Description Standard (ITU-T I.233)
• Overall service description and specifications, Connection Management
– Core Aspects (ITU-T Q.922)
• Frame Format, Field Functions, Congestion Control
– Signaling (ITU-T Q.933)
• Establishing and Releasing switched connections and status of permanent
connections
Frame Relay Devices
• Data Terminal Equipment (DTE)
– Terminals, Personal Computers, routers, and
bridges typically at the customer location
• Data Circuit-terminating Equipment (DCE)
– Typically packet switches owned by the carrier
that transmit data through the WAN
Sample Frame Relay Network
Packet Switch
DCE
Packet Switch
DCE
Personal Computer
DTE
Terminal
DTE
Frame Relay
WAN
Packet Switch
DCE
Packet Switch
DCE
Network Host
DTE
Frame Relay Assembler/Disassembler
(FRAD)
• To handle frames from other protocols a FRAD
is used to provide conversion to Frame Relay
packets
• A FRAD can either be a separate device or part
of a router/switch
X.25
ATM
PPP
X.25
FRAD
Frame Relay
FRAD
ATM
PPP
Frame Relay mapping to OSI Model
Application
Presentation
Session
Other Services
Transport
Network
Data Link
LAPF
Physical
Any Standard
Frame
Relay
Protocol
Frame Relay Physical Layer
• No specific protocol is defined
• Any protocol recognized by ANSI can be
implemented
Frame Relay Data Link Layer
• Link Access Protocol for Frame Modes Services
(LAPF) is the protocol defined for Frame Relay
Layer 2 services
• LAPF is a version of HDLC
– Does not provide flow or error control
– Uses Address field for DLCI (addressing) as well as
for congestion control
LAPF Frame Format
Flag
Address
DLCI
Information
C/R EA
DLCI
FCS
FECN BECN
Flag
DE EA
DLCI: (10 bits) Data Link Connection Identifier is used to identify the Virtual
Circuit number
C/R: (1 bit) Provided for up layers to determine commands and responses
EA: (1 bit) Determines if this byte is last byte of address (0=more, 1=last)
FECN: (1 bit) Forward Explicit Congestion Notification indicates congestion in
the direction the frame is traveling
BECN: (1 bit) Backward Explicit Congestion Notification indicates congestion
in the opposite direction the frame is traveling
DE: (1 bit) Discard Eligibility indicates that a frame is low priority when set
Extended Addresses
• To increase the number of virtual circuits the
DLCI can be expanded from 10 bits to 16 bits
and 23 bits
• The EA field is set to 0 to indicate that
additional address bytes are present. The last
address byte will have a 1 in the EA field
Three Address Formats
Two-byte Address
(10 bit DLCI)
DLCI
DLCI
C/R 0
FECN BECN
DLCI
Three-byte Address
(16 bit DLCI)
DLCI
FECN BECN
DLCI
DLCI
DE
0
0
1
C/R 0
FECN BECN
DE
DLCI
DLCI
1
C/R 0
DLCI
Four-byte Address
(23 bit DLCI)
DE
0
0
0
1
DLCI Addressing
•
Two Byte Address Format
0
1-15
16-991
992-1001
1002-1022
1023
•
Reserved
Assigned using Frame Relay connection procedures
Layer 2 management of Frame Relay service
Reserved
In channel layer management
Three Byte Address Format
0
1-1023
1024-63,487
63,488-64,511
64,512-65,534
65,535
•
In-channel signaling
In-channel signaling
Reserved
Assigned using Frame Relay connection procedures
Layer 2 management of Frame Relay service
Reserved
In channel layer management
Four Byte Address Format
0
In-channel signaling
1-131,071
Reserved
131,072-8,126,463
Assigned using Frame Relay connection procedures
8,126,464-8,257,535 Layer 2 management of Frame Relay service
8,257,536-8,388,606 Reserved
8,288,607
In channel layer management
Frame Relay Operating States
• Original Frame Relay standard only covered PVC
• SVC support was added but does not have widespread
implementation
• PVC States
– Data Transfer – data is being transmitted between DTE devices
– Idle – connection is still active but no data is being transferred
• SVC required the addition of two additional states
– Call Setup – virtual circuit between DTE devices is established
– Call Termination – virtual circuit between DTE devices is terminated
Congestion Management
• Because of the shared resources of a virtual circuit,
congestion can cause the loss of packets as buffers become
full
• Frame Relay defines a congestion control mechanism using
the FECN and BECN bits in the address field
• When a switch determines that congestion has occurred it will
set the FECN bit on packets traveling in the direction of the
congestion to alert the receiver to slow down requests for
data. The BECN bit will be set for packets going in the
opposite direction of the congestion to let the sender know to
send data more slowly
• The FECN and BECN bits will allow higher layer protocols to
manage flow.
• Discard Eligible bit is used to identify frames that are low
priority and can be discarded in the event of congestion
Local Management Interface (LMI)
• LMI is a set of extensions to Frame Relay developed in 1990
by Cisco Systems, StrataCom, Northern Telecom, and Digital
Equipment Corporation
• LMI provides global addressing which allows additional
management capability such as standard address resolution
and discovery
• LMI allows status messages to be passed between DCE and
DTE devices to provide communication and synchronization
(uses DLCI 1023 on a 2-byte address)
• LMI specifies multicast capability to allow creation of
multicast groups to limit bandwidth use
LMI Frame Format
Flag
LMI DLCI
Unnumbered
Call
Protocol
Information
Discriminator Reference
Indicator
Message
Type
Information
Elements
FCS
Flag
LMI DLCI: Identifies the frame as an LMI frame
Unnumbered Information Indicator: Sets the poll/final bit to zero
Protocol Discriminator: Contains a value indicating that this is an LMI frame
Call Reference: Always contains zeros. This field currently is not used
Message Type: Identifies frame as one of the following message types:
Status-inquiry Message: Allows a user device to inquire about the status of
the network.
Status Message: Responds to status-inquiry messages. Status messages
include keep alive and PVC status messages.
Information Elements: Contains a variable number of individual information
elements (IE) consisting of the following fields:
IE Identifier: Uniquely identifies the IE
IE Length: Indicates the length of the IE
Data: Consists of 1 or more bytes containing encapsulated upper-layer data
Comparison of X.25 and Frame Relay
X.25
Frame Relay
Yes
None
HDLC
HDLC
Layer 3 Support
PLP
None
Error Correction
Node to Node
None
High
Low
Difficult
Easy
Too Slow
Yes
No
Yes
Slow
Yes
Layer 1 Specification
Layer 2 Protocol Family
Propagation Delay
Ease of Implementation
Good for Interactive Applications
Good for Voice
Good for LAN File Transfer
X.25 and Frame Relay Today
• Many X.25 networks have been replaced by
Frame Relay or X.25 over Frame Relay
Networks
• X.25 still in use for low bandwidth applications
such as credit card verification
• It is likely that ATM Networks will ultimately
replace Frame Relay and X.25 Networks
Resources
•
•
•
•
•
•
•
•
•
•
•
•
http://www.rhyshaden.com/x25.htm
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/x25.htm
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm
http://www.protocols.com/pbook/x25.htm#LAPB
http://www.doc.ic.ac.uk/~kpt/Slides/X25/sld001.htm
http://www.raduniversity.com/2004/x25/format.htm
http://www.techfest.com/networking/wan/frrel.htm
http://www.dcbnet.com/notes/framerly.html
http://www.unm.edu/~network/presentations/course/chap5/sld001.htm
Forouzan, Behrouz A., “Data Communications and Networking, Third Edition”;
McGraw-Hill 2004
Kumar, Balaji, “Broadband Communications”; McGraw-Hill 1998
Trivedi, Carol, “Wide Area Networks”; EMCParadigm 2004