Transcript Current Telecommunications Infrastructure

309
Packet Relay
Relaying: Switching packets asynchronously
 Types of packet relay:

1. Cell relay:


Fixed-size packets
Used in ATM and SMDS
2. Frame relay:

University of Arizona
Variable-length packets
ECE 478/578
310
Advantages of Fixed-Size Packets

Simple switch-hardware design


Hardware store-and-forward is easier
Dynamic storage allocation is easier (no memory
fragmentation)
More deterministic scheduling (for performance
guarantees)
 High degree of parallelism in large switches



Synchronized multiprocessors
Multiple levels of buffering can be easily clocked
University of Arizona
ECE 478/578
311
Disadvantages of Fixed-Size Packets
Segmentation and reassembly (SAR)
 Overhead in the case of small-size cells

University of Arizona
ECE 478/578
312
Integrated Services Digital Network (ISDN)
Evolutionary technology from digital telephony
 Intended as a digital interface for voice and data
 More popular in Europe
 ISDN terminology:



Functional grouping: A set of capabilities in an ISDN
user interface (similar to layer functions)
Reference points: Logical interfaces between functional
groupings (similar to SAPs)
University of Arizona
ECE 478/578
313
ISDN Specification

Types of functional groupings:


Terminal Type 1 and 2 (TE1 and TE2)
Network Termination 1 and 2 (NT1 and NT2)
 Four
University of Arizona
reference points: R, S, T, and U
ECE 478/578
314
Typical ISDN Topology
T
S
TE 1
NT 2
U
NT 1
LT/ET
Network
R
TE 2
University of Arizona
T
S
TA
NT 2
U
NT 1
LT/ET
ECE 478/578
315
Typical ISDN Topology (Cont.)
TE1 = end-user ISDN terminal
 TE2 = non-ISDN terminal
 TA = terminal adaptor
 NT1 = device that connects 4-wire subscriber
wiring to 2-wire local loop. Responsible for
physical layer functions
 NT2 = more complex than NT1. Contains layer 2
and 3 functions. Performs concentration

University of Arizona
ECE 478/578
316
ISDN Access Rates

Basic Rate Interface (BRI)





Two 64-kbps B channels for data
One 16-kbps D channel for control (out of band)
Designated as 2B+D
Up to eight TE1s can be multiplexed onto a BRI
Primary Rate Interface (PRI)




23 64-kbps B channels for data (total of 1.544 Mbps)
One 64-kbps D channel for control
Designated as 23B+D
In Europe the PRI consists of 31B+D (E1 line)
University of Arizona
ECE 478/578
317
Broadband ISDN (B-ISDN)
Set of protocols that is standardized by ITU-T
 Started as an extension of ISDN. However, ISDN
and ISDN interfaces are NOT compatible
 ATM is the transport protocol for B-ISDN

University of Arizona
ECE 478/578
318
Driving Forces Behind B-ISDN
Emergence of bandwidth-intensive applications
 Desire to integrate data, voice, and video over a
single channel (why?)
 Need to provide performance guarantees for realtime applications

University of Arizona
ECE 478/578
319
Synchronous Transfer Mode (STM)



Based on time-division multiplexing (TDM)
Each connection is reserved a time slot
Bandwidth is wasted if user is idle
Stream #1
frame
Stream #1
frame
frame
frame
Time
Division
Multiplexer
Stream #1
wasted bandwidth
University of Arizona
ECE 478/578
320
Asynchronous Transfer Mode (ATM)
Based on statistical (i.e., asynchronous) multiplexing
 Bandwidth is allocated on demand
 Each packet (cell) carries its connection ID

Stream #1
1 1 1 1
2
Stream #1
Stream #1
University of Arizona
3
2
Statistical
Multiplexer
1 2 3 1 1 2 1 3
3
ECE 478/578
321
So What is ATM?
Transport technology for B-ISDN
 Based on fixed-length packets (cells)
 A cell consists of 53 bytes:



User payload: 48 bytes
Cell header: 5 bytes
Hardwired store-and-forward architecture
 Connection-oriented fast packet switching!

University of Arizona
ECE 478/578
322
What Does ATM Provide?
Efficient bandwidth utilization (via statistical
multiplexing)
 Quality of service (QoS)




Maximum cell transfer delay
Cell delay variation (jitter)
Cell loss rate
 Cell
sequencing (important for real-time apps)
 Unified transport solution for diverse traffic types
 Scalability
University of Arizona
ECE 478/578
323
Cell Size Considerations

Transmission efficiency
E  PL /( PL  HD)



Impact of cell loss on voice quality



PL  no. of payload bytes
HD  no. of header bytes
Loss of 32-byte cell  4 ms interruption
> 32 ms interruption is quite disruptive
Echo cancellation
University of Arizona
ECE 478/578
324
B-ISDN Protocol Stack

Three “planes”:



User plane
Control plane
Management plane


Plane management
Layer management
Plane Management
Layer Management
Control Plane
User Plane
Higher Layers
ATM Adaptation Layer
ATM Layer
Physical Layer
University of Arizona
ECE 478/578
325
B-ISDN Physical Layer

Consists of two sublayers:



Transmission Convergence (TC) sublayer
Physical Medium (PM) sublayer
Functions of the TC sublayer:





Generation/recovery of transmission frames
Transmission frame adaptation
Cell delineation
Cell header processing (HEC generation)
Cell rate decoupling
University of Arizona
ECE 478/578
326
B-ISDN Physical Layer (Cont.)

Functions of the PM sublayer:



Bit timing
Line encoding
Other medium-dependent functions
University of Arizona
ECE 478/578
327
Common Physical Layer Interfaces

Multimode Fiber:


155 Mbps SONET STS-3c (SDH)
100 Mbps 4B/5B coding
 Single-Mode
Fiber at 100 Mbps 4B/5B coding
 Coax cable at 45 Mbps DS3 rate
 Subrates (for ATM over unshielded twisted pair)



51.84 Mbps
25.92 Mbps
12.96 Mbps
University of Arizona
ECE 478/578
328
SONET Hierarchy
Rate (Mbps)
Optical Level
Electrical Specs
SDH
51.84
OC-1
STS-1
---
155.52
OC-3
STS-3
STM-1
622.08
OC-12
STS-12
STM-4
1244.16
OC-24
STS-24
STM-8
2488.32
OC-48
STS-48
STM-16
University of Arizona
ECE 478/578
329
SONET STC-3c Physical Layer
Transmission
Convergence
Sublayer
Physical
Media
Dependent
Sublayer
University of Arizona
- HEC generation/verification
- Cell scrambling/descrambling
- Cell delineation
- Path signal identification
- Frequency justification/Pointer processing
- Multiplexing
- Scrambling/descrambling
- Transmission frame generation/recovery
B-ISDN
Specific
Functions
B-ISDN
Independent
Functions
- Bit timing, Line coding
- Physical medium
ECE 478/578
330
ATM Layer Functions

Cell multiplexing and demultiplexing
Switch
1 2 3
1 1 2 1 3
1 2
4 5 5 4
5
4
4
4 1 1 2 1
3 5 5
4
5 3
VPI/VCI translation
 Traffic management (e.g., shaping, policing)
 Cell header processing (except for the HEC field)
 Cell rate decoupling (for SONET and DS3)
 OAM functions
University of Arizona
ECE 478/578

331
ATM Cell Format
BIT
8
7
6
5
4
3
GFC
VPI
1
VCI
2
PT
Cell Payload
(48 octets)
CLP 4
OCTET
3
HEC
University of Arizona
1
VPI
VCI
VCI
2
5
6
.
.
53
ECE 478/578
332
Remarks

The previous cell format is for the User-toNetwork Interface (UNI)



Between an end-system and an ATM switch
An end system could be, an IP router with an ATM
interface, a PC/workstation, or a LAN switch
In the Network-to-Network Interface (NNI), the
GFC field is used as part of the VPI field


NNI is typically between two ATM switches
Two flavors of NNI are used (Private and Public)
University of Arizona
ECE 478/578
333
ATM Switching
User A
User B
AAL
AAL
ATM
ATM
ATM
Network
Physical
UNI
PNNI
Physical
PNNI
UNI
UNI = User Network Interface
University of ArizonaPNNI
= Private Network Node Interface
ECE 478/578
334
Generic Flow Control (GFC)
Four bits in the cell header
 Only in cells at UNI (intermediate switches
overwrite it)
 Intended for link-by-link flow control
 Typically, GFC is not used

VPI
GFC
VCI
VPI
VCI
VCI
PT
CLP
HEC
Cell Payload
(48 octets)
University of Arizona
ECE 478/578
335
Connection Identifiers

ATM uses a 2-level connection hierarchy:


Virtual channel connection (VCC or VC)
Virtual path connection (VPC or VP)
 A VP is
a bundle of VCs
 Each connection has a VP identifier (VPI) and a
VC identifier (VCI)
 Cell switching is performed based on:
GFC
VPI
VPI
VCI
VCI


VPI alone (VP switching), or
Both VCI and VPI (VP/VC switching)
University of Arizona
VCI
PT
CLP
HEC
Cell Payload
(48 octets)
ECE 478/578
336
Connection Identifiers (Cont.)

Some VPI and VCI values are reserved for
signaling and control functions:





Connection requests: VPI=0, VCI=5
PNNI topology state packets: VPI=0, VCI=18
Resource Management (RM) cells: VCI=6
VCI values < 32 are reserved for control functions
VCIs and VPIs have local scope
University of Arizona
ECE 478/578
337
VP and VP/VC Switching
VPI 3
VPI 3
VCI 1
VCI 2
VCI 5
VCI 3
VCI 1
VCI 2
VPI 4
VPI 1
VCI 1
VCI 2
VPI 1
VP/VC Switching
University of Arizona
VP Switching
ECE 478/578
338
Payload Type (PT) Field
VPI
GFC
VCI
VPI
VCI
VCI
PT
CLP
HEC
Cell Payload
(48 octets)
Payload Type
000
001
010
011
100
101
110
111
University of Arizona
Meaning
user cell, no congestion, cell type 0
user cell, no congestion, cell type 1
user cell, congestion indication, cell type 0
user cell, congestion indication, cell type 1
OAM cell (link-by-link)
OAM cell (end-to-end)
RM cell (used in ABR service)
reserved for future use
ECE 478/578
339
Cell Loss Priority (CLP)
VPI
GFC
VCI
VPI
CLP = 1 for low priority
 CLP = 0 for high priority
 CLP is used in selective cell discarding to:

VCI



VCI
PT
CLP
HEC
Cell Payload
(48 octets)
penalize greedy users (traffic policing)
request differential QoS (e.g., coded video)
CLP is a key parameter in traffic management
University of Arizona
ECE 478/578
340
Header Error Control (HEC)
VPI
GFC
VCI
VPI
Checksum over cell header
 Performed by the physical layer
 Corrects all single-bit errors
 Detects about 84% of multiple-bit errors


VCI
VCI
PT
CLP
HEC
Cell Payload
(48 octets)
Cells with multiple errors are discarded
University of Arizona
ECE 478/578
341
ATM Layer in the OSI Model

Different opinions:




Network layer (since it performs routing)
Data-link layer (in IP over ATM and in MPOA)
Physical layer (in LAN emulation)
Conclusion:

There is no 1-to-1 correspondence between
B-ISDN and OSI layered models
University of Arizona
ECE 478/578
342
ATM Adaptation Layer (AAL)
Purpose: Adapt upper “applications” to ATM layer
 Different applications have different needs
 Four AALs are used
 AALs were originally classified according to:




Real-time versus non-real-time
Connection oriented versus connectionless
Constant bit rate (CBR) versus variable bit rate (VBR)
University of Arizona
ECE 478/578
343
AAL Functions
Segmentation and reassembly of upper-layer PDUs
 Delay variation recovery
 Cell losses recovery
 Circuit emulation (e.g., voice over ATM)
 Connectionless service over ATM
 Clock synchronization
 And others ...

University of Arizona
ECE 478/578
344
AAL Structure
Convergence Sublayer (service specific part)
Convergence Sublayer (common part)
Segmentation & Reassembly Sublayer
Note: In some AALs, the convergence sublayer consists of one
part only
University of Arizona
ECE 478/578
345
Types of AAL
1. AAL1



Intended for TDM-like circuit emulation
Supports clock synchronization and timing recovery
Provides sequence numbers
2. AAL2



Optimized for the transport of VBR video traffic
Provides timing information and sequence numbers
Not quite popular
University of Arizona
ECE 478/578
346
Types of AAL (Cont.)
3. AAL3/4



Provides both connectionless and connection-oriented
services over ATM
Supports the multiplexing of messages from multiple
users over the same VC
Not popular either
4. AAL5



Intended for data applications (e.g., TCP over AAL5)
Provides minimal functionality
Most popular AAL
University of Arizona
ECE 478/578
347
Barriers to the Deployment of ATM
Lack of “killer applications”
 Cost of new infrastructure
 Other competitive technologies for LANs
 Uncertainty about the new technology
 Incomplete standards

ATM is mainly being deployed in the Internet backbone and
within specialized networks
University of Arizona
ECE 478/578