X.25 Overview
An Introduction to X.25
X.25 Cloud
LAN
Protocol
S
S
S
S
X.25
LAN
Protocol
X.25
Virtual
Circuit
• IP
• AppleTalk
• Novell IPX
• Banyan VINES
• XNS
• DECnet
• ISO-CLNS
• Apollo
• Compressed TCP
• Bridging
X.25 Protocol Stack
OSI Reference Model
X.25 Protocol
7
Application
•
6
Presentation
•
5
Session
•
4
Transport
•
3
Network
X.25
3
2
Data Link
LAPB
2
1
Physical
Physical
1
X.25 (X.121) Addressing Format
4 decimal digits
Up to 10 or 11 decimal digits
Data Network ID Code
Network Terminal Number
• Addressing set by service provider
X.25 Encapsulation
IP Network
IP Network
X.25
Data-Link
Frame
(LAPB)
X.25
Header
IP Datagram
• Protocol datagrams are reliably
carried inside X.25 frames
X.25 Virtual Circuits
Switched Virtual Circuits
(SVCs)
Permanent Virtual Circuits
(PVCs)
• Numbering for up to 4095 VCs per X.25
interface
X.25 Configuration Example
IP Address: 10.60.8.1
X.121 Address: 311082194567
IP Address: 10.60.8.2
X.121 Address: 311082191234
S1
Token
Ring
S0
X.25
Cisco A
Cisco A
Cisco B
Cisco B
interface serial 1
interface serial 0
encapsulation x25
encapsulation x25
x25 address 311082194567
x25 address 311082191234
ip address 10.60.8.1 255.255.248.0
ip address 10.60.8.2 255.255.248.0
x25 map ip 10.60.8.2 311082191234 broadcast
x25 map ip 10.60.8.1 311082194567 broadcast
X.25 Local and XOT Switching
X.25
PAD
IP Internetwork
Local
Switching
X.25
PAD
Remote
Switching
Router(config)#
x25 route [ # position ] x.121-address [ cud pattern ]
interface type-number
X.25
X.25
PAD
PAD
Frame Relay
Overview
Frame Relay Overview
DCE or
Frame
Relay Switch
DTE or
CPE
router
CSU/DSU
Token
Ring
Frame Relay works here.
• Virtual circuits make connections
• Connection-oriented service
Frame Relay Terminology
LMI
500=Active
400=Active
DLCI=500
Local
Access
Loop=T1
Local Access
Loop=64 kbps
DLCI=400
Local Access
Loop=64 kbps
Frame Relay Terminology
(cont.)
I am congested.
BECN
Bc=32 kbps
CIR=64 kbps
T1
Selecting a Frame Relay
Topology
Full Mesh
Partial Mesh
Star (Hub and Spoke)
Multipoint Subinterfaces
Configuration Example
RTR2
s2.1=10.17.0.2/24
RTR1
s2.2=10.17.0.1/24
RTR3
s2.1=10.17.0.3/24
RTR4
s2.1=10.17.0.4/24
Multipoint Subinterfaces
Configuration Example
<Output Omitted>
!
interface Serial2
no ip address
encapsulation frame-relay
!
interface Serial2.2 multipoint
ip address 10.17.0.1 255.255.255.0
bandwidth 64
frame-relay map ip 10.17.0.2 120 broadcast
frame-relay map ip 10.17.0.3 130 broadcast
frame-relay map ip 10.17.0.4 140 broadcast
!
router rip
network 10.0.0.0
<Output Omitted>
Point-to-Point Subinterfaces
Configuration Example
s2.1
10.17.0.0/24
RTR2
RTR1
s2.2
s2.3
s2.4
10.18.0.0/24
s2.1
10.20.0.0/24
RTR3
s2.1
RTR4
Point-to-Point Subinterfaces
Configuration Example
<Output Omitted>
interface Serial2
no ip address
encapsulation frame-relay
!
interface Serial2.2 point-to-point
ip address 10.17.0.1 255.255.255.0
bandwidth 64
frame-relay interface-dlci 110
!
interface Serial2.3 point-to-point
ip address 10.18.0.1 255.255.255.0
bandwidth 64
frame-relay interface-dlci 120
!
interface Serial2.4 point-to-point
ip address 10.20.0.1 255.255.255.0
bandwidth 64
frame-relay interface-dlci 130
<output omitted>
Common LAN
Technologies
LAN Technology Overview
Ethernet
Token Ring
FDDI
Token
Ring
FDDI
Dual Ring
Ethernet and IEEE 802.3
• Several framing variations exist for this
common LAN technology
Ethernet Frame Variations
Preamble
DA
8
SA
6
Type
6
Data
2
FCS
4
Ethernet Frame
Preamble
8
DA
SA
6
Length
6
802.3 Frame
2
802.2
Header
and Data
FCS
4
Ethernet Frame Variations
SNAP Frame
AA
AA
CTRL
802.2 Frame
DSAP
SSAP
CTRL
Preamble
DA
802.3 Frame
SA
Length
OUI
Ether
Type
Upper Layer Data
IP, AppleTalk
DATA
802.2 Header
and Data
FCS
High-Speed Ethernet Options
• 100BaseTX
• 1000BaseT/SX/LX/LH/ZX
Token Ring and IEEE 802.5
Token
Ring
• IBM’s Token Ring is equivalent to IEEE 802.5
Physical Layer: Token
Ring/802.5
Logical Topology
MSAU
Shielded or
Unshielded
Twisted-Pair
• Logically a ring, but physically a star configuration
to MAU relays
The Token Ring/802.5 Interface
Token
Token
Ring
To0
• Cisco router’s data link to Token Ring/802.5 uses
interface named To plus a number (for example, To0)
Token Ring/802.5 Operation
T
A
T=0
• Token Ring LANs continuously pass a
token
Token Ring/802.5 Operation
T
A
T=0
A
T=1
T Data
• Token Ring LANs continuously pass a
token or a Token Ring frame
Token Ring/802.5 Operation
T
A
A
T=0
T=0
T
A
T=1
T Data
• Token Ring LANs continuously pass a
token or a Token Ring frame
Token Ring/802.5 Media Control
Access Control Field
P P
P
T M R R R
P
Priority bits
T
Token bit
M
Monitor bit
R
Reservation bits
• Fields in a frame determine priority and
reservation for sharing media
FDDI
100 Mbps
FDDI
Dual Ring
• Devices on FDDI maintain connectivity
on dual counterrotating rings
FDDI Dual-Ring Reliability
When a failure
domain
occurs...
FDDI Dual-Ring Reliability
When a failure
domain
occurs...
FDDI Dual-Ring Reliability
When a failure
domain
occurs...
...wrap primary
and secondary
rings...
...wrap primary
and secondary
rings...
...maintaining
network integrity
ATM
ATM Basics Review
•
•
•
•
•
•
A compromise for voice,
data, and video
Hardware-based cell relay
Larger frames are
segmented and
reassembled at ATM
endpoints
Can span both LAN and
WAN
Scales from a few Mbps to
multi-Gbps
QoS defined/negotiated
when initial connection is
made
Voice
Video
Data
ATM cells
48-octet
Payload
ATM 53-octet cells are
switched in hardware
ATM Cell Processing
TCP
IP
TCP Packet
Convergence
Sublayer
LLC
LLC
App Data
TCP Header
App Data
IP Header
IP Datagram
LLC/SNAP
TCP Header
IP Header
IP Header
TCP Header
TCP Header
App Data
App Data
AAL5-PDU
AAL 5
AAL5 SAR-PDU - 48 Bytes
SAR Sublayer
48 Bytes
ATM
5 Bytes
ATM Hdr
AAL5 SAR-PDU - 48 Bytes
53 Bytes ATM Cell
Transmission Convergence (STS3c, STM-1, DS3, 4B/5B, ...)
PHY
Physical Media (MMF, SMF, STP, UTP, Coax, …)
Virtual Path and Virtual Channel
Virtual Channels (VCs)
ATM Physical Link
Virtual Channel Connection (VCC)
Virtual Path (VP)
e.g., OC-3, E3,
OC-12
Virtual Path (VP)
Virtual Channels (VCs)
Virtual Channel Connection
(VCC)
Contains Multiple VPs
Virtual Path
(VP)
Contains Multiple VCs
VC
Logical Path
between ATM End points
Connection Identifier = VPI/VCI
Well-Known VPI/VCIs
Terminating Connections
Type
Signaling
ILMI
PNNI
VPI/VCI
(Logical)
Default
(Physical)
x/5
0/5
x/16
0/16
x/18
0/18
ATM Cell Header—UNI Format
GFC
VPI
4
8
VCI
16
32 Bits
8
7
6
5
PT
CLP
HEC
3
1
8
8 Bits CRC
4
3
2
Bits
Bytes
1
GFC
VPI
1
VPI
VCI
2
3
VCI
PT
User data or maintenance flow
Congestion experienced
End of message (AAL5)
VCI
PT
HEC
CLP
4
5
ATM Cell Header—NNI Format
VPI
VCI
12
16
32 Bits
8
7
6
5
4
3
2
PT
CLP
HEC
3
1
8
8 Bits CRC
Bits
Bytes
1
VPI
1
VPI
VCI
2
3
VCI
VCI
PT
HEC
CLP
4
5
•Larger VP field for trunking
•Distinction not very useful for private network
VP Switching/VP Cross-Connect
VCI 1
VCI 2
VPI 1
VPI 4
VCI 3
VCI 4
VCI 3
VCI 4
VPI 2
VPI 5
VCI 5
VCI 6
VPI 6
VCI 1
VCI 2
VCI 5
VCI 6
VPI 3
VP Switch
•Switching based on VPI only
(central office trunk switching)
VP and VC Switching
VC Switch
VCI
2
VCI
1
VPI
1
VCI
3
VPI
3
VCI
4
VPI
2
VPI 2
VCI 4
VPI 3
VCI 3
VPI 5
VCI 1
VCI 2
VCI 1
VCI 2
VCI 1
VCI 2
VPI 1
VPI 4
VP Switch
ATM Switch Translation Table
Input
Output
Port
VPI/VCI
Port
VPI/VCI
1
2/39
2
4/55
2
4/55
1
2/39
1
6/64
3
2/89
3
2/89
1
6/64
4/55
2/39
6/64
2
1
3
2/39
• ATM switches translate VPI/VCI values
• VPI/VCI value unique only per interface—
e.g., locally significant and may be reused
elsewhere in network
ATM Adaptation Layer
Service Bit Rate Connection Timing
Categories
Mode
Concern
ATM
Adaptation
Layer (AAL)
AAL1
CBR
(Constant)
AAL2
VBR
(Variable)
ATM Layer
AAL5
Physical Layer
ABR
UBR
Yes
ConnectionOriented
Yes
ConnectionOriented
No
• Best effort with
congestion feedback
• Reliable delivery
of bursty traffic if
latency okay
No
• No guarantee
• For SMDS
(CBR)
UBR Connection(Unless
specified)
• Bandwidth and
throughput
guaranteed
• Good for voice
and video
• Best effort bandwidth
and throughput
• Good for live video,
multimedia,
LAN-to-LAN
ConnectionOriented
VBR
AAL3/4
Application
Examples
AAL5
Payload Frame
Data
Frame
Convergence
Sublayer (CS)
Convergence Sublayer PDU
SAR PDU
AAL5
SAR
Sublayer
SAR PDU
SAR PDU
SAR PDU
0x0
ATM Cell
ATM Layer
0x0
ATM Cell
0x0
ATM Cell
•CS trailer
Length
CRC-32
•Cell header
Type: empty, not EOM, EOM
•Payload: 48 bytes
•Primary AAL for data
0x1
ATM Cell
AAL5—Frame and Cell Formats
AAL5 CPCS - PDU Frame = n x 48 bytes
User
P
A
D
Data
0-65535 bytes
0-47
bytes
Control
(CPI and UU)
2byte
Payload
Length
2 bytes
CRC-32
4 bytes
AAL5 CPCS PDU Trailer
SAR-PDU Payload = 48 bytes
AAL5 SAR PDU = 1 Cell Payload
CPI
UU
PAD
Common part indicator, current function: aligning the
trailer to 8 bytes, currently unused and set to 0
User-to-user indication, currently unused and set to 0
0-47 bytes
•53 = 5 (cell header) + 48 (payload)
ATM Signaling,
ATM Address Formats
Switched Virtual Channels (SVCs)
B-ICI
Public ATM Network
NNI
UNI
NNI
Token
Ring
NNI
Private ATM Network
• SVC requires signaling
ATM connection oriented
• ATM Forum UNI 3.x/4.0 signaling specification
Based on ITU-T Q.2931
Signaling Concept
ATM Router A
Connect to B
OK
Connect to B
OK
Connect to B
•Signaling request
Connect to B
•Connection routed—setup path
•Connection accepted/rejected
•Data flow—along same path
•Connection teardown
OK
ATM Router B
ATM SVC Call Setup
Calling Party
Called Party
Set Up
Setup
Call
Proceeding
Call
Proceeding
Connect
Connect
ACK
ES
Connect
Connect
ACK
ATM Network
Connect
ACK
ES
•Call set up in software
•Cell switching in hardware
•Dynamic call establishment (call setup) with
appropriate parameters (e.g., QoS: CLR, CTD, CDV)
ATM SVC Call Release
Calling Party
ATM Network
Called Party
Release
Release
Release
Complete
ES
Release
Complete
ES
•Dynamic call teardown (call release) of virtual connection
Q.2931 Call Control
Q.2931
SSCOP
PD
Call
Ref
Mes Mes
Type Len
Info
Elements
•Q.2931 call control message format at UNI
ATM Addressing Formats
AFI
DCC
DSP
E.164
ESI
HO-DSP
ICD
IDI
Sel
ICD
AFI ICD
AFI = 47
DCC
E.164
AFI
E.164
AFI = 45
High-order Part of DSP
International Code Designator
Initial domain identifier
NSAP Selector
ESI
SEL
ESI
SEL
ESI
SEL
DSP
AFI DCC
AFI = 39
Authority and Format Identifier
Data Country Code
Domain Specific Part
ISDN (telephone) Number
End-system Identifier (IEEE)
DSP
DSP