CS294-3: Distributed Service
Architectures in Converged
Networks
Randy H. Katz
Computer Science Division
Electrical Engineering and Computer Science Department
University of California
Berkeley, CA 94720-1776
1
Outline
•
•
•
•
•
Historical Background
Structure of the PSTN
Intelligent Network/Services
SS7 Structure
Common Themes and Observations
2
The Telegraph Learns to Speak
Alexander Graham Bell
– 1876: Demonstrates the telephone at US Centenary
Exhibition in Philadelphia
– Bell offers to sell patents to Western Union for
$100,000--they refuse. Bell Telephone Company founded
9 July 1877
– 1878: Western Union enters into competition with rival
system designed by Thomas Edison and Elisha Gray. Bell
sues and wins.
3
Bell’s Early Telephones
4
Building the Network
Almon Brown Strowger (1839 - 1902)
– 1889: Invents the “girl-less, cuss-less” telephone system,
also known as the mechanical switching system
5
“Ma Bell”
• Bell’s patents expire in 1890s; over 6000
independent operators spring up
– 1910: Bell System controls 50% of local telephone market
– 1913: AT&T & U. S. government reach Kingsbury
Agreement: AT&T becomes regulated monopoly while
promising "universal" telephone service; Controls “toll”
services in U.S.
– Long distance interconnection withheld as a competitive
weapon
– 1950: Bell System controls 84% of the local telephone
access market
6
Bell Telephone
Equipment
7
Public Switched Telephone
Network (PSTN)
• Time Division Multiplexed Network
• Single application: uncompressed voice (i.e.,
POTS—Plain Old Telephone Service)
• 99.9994% uptime
• Low latency and jitter
• Highly scalable call routing through SS7
infrastructure
• Value-added voice services: e.g., voice
messaging, caller ID
8
Structure of the PSTN
• Separation of data (switching) and control (signaling)
planes
• Hierarchical switches
– Local Loop/Central Office/End Office
– Intermediate/Regional Switches
– Trunks (Long Distance)
• Control signaling evolution
– “Old” PSTN (1960s): analog signaling, encoded in voice band
» Dual-tone multifrequency (DTMF) between CPE and EO;
I.e., touch tone dialing
» Single frequency (SF) between COs
– “New” PSTN (1980s): digital signaling/separate control network (SS7)
» SF still exists though made obsolete by SS7
9
Old vs. New Signaling
Call
“I’m busy!”
Voice
Trunk
Call Set-up
“I’m busy!” message
Control
Voice
Trunk
Free
10
Kinds of Signaling Messages
• “I’m forwarding to you a call placed from 212555-1234 to 718-555-5678. Look for it on
trunk 067.”
• “Someone just dialed 800-555-1212. Where do
I route the call?”
• “The called subscriber for the call on trunk 11
is busy. Release the call and play a busy tone.”
• The route to XXX is congested. Please don’t
send any messages to XXX unless they are of
priority 2 or higher.
• I’m taking trunk 143 out of service for
maintenance.
11
Advantages of Out-of-Band
Signaling for PSTN
• It allows transport of more data at higher
speeds, due to digital encoding rather than
audio/analog encoding
• Signaling takes place any time during call, not
only at beginning
• Allows signaling to network elements to which
there are no direct trunk connections
12
PSTN Hierarchy
• Before Deregulation
– Class 5: End Office/Local Office
» Interconnection among EOs only within a geographic
service area
– Class 4: Tandem Office
» Also known as a toll office
» Higher bandwidth trunking among service areas
– Class 3: Primary Center
» Part of toll (long distance) network
– Class 2: Sectional Center
– Class 1: Regional Center
– Not a strict hierarchy in that Class 5 switch can be connected
to switch other than a Class 5 or 4 switch
13
PSTN Hierarchy
Local Area: 510
LATA
8778
Analog “local loop”
Twisted Pair
Digital
CO
CO
642
CO
CO
CO
CO
LEC
Trunk
IXC
64 Kbps links, time division multiplex/demultiplex
Voice band: 3.4 KHz between 350 Hz and 3750 Hz
8000 samples/second x 8 bits (mLaw)
14
PSTN Hierarchy
• Post AT&T Breakup (1980s)
IXC: Interexchange Carrier
LATA: Local Access Transport Area
POP
Tandem Office
Class 4 Switch
Intra-LATA connections
Inter-LATA connections
TO
EO
EO
Allocate a facility
(circuit) per switch
per call
AT
TO
EO
Local Loop
CPE: Customer
Premises Equipment
EO
EO
EO
End Office
Central Office
Class 5 Switch
ILEC: incumbent
CLEC: competitive
“Reciprocal Termination Revenue”
15
PSTN Hierarchy
• Deregulation Complexities
– RBOCs traditional service areas divided into LATAs
(originally 146, now 300+)
– Local operating company must use long distance provider
to interconnect LATAs
– Long distance carriers gain access to local networks
through Point-of-Presence (POP) center
– Every LATA must have a POP
– LATA further divided into local market and toll market
– Intra-LATA toll calls also open for competition
16
PSTN Hierarchy
• Synchronous Network
– Maintaining clock synchronization
PRS
PRS
Stratum 1
Primary Rate Source
PRS
Stratum 2
Toll Office
Stratum 3
End Office
Stratum 4
PBX
Channel Banks
Sync Region
Sync Region
Sync Region
Sync Region
17
PSTN Hierarchy
Digital Signal
Destination
DS0
DS1
DS1C
DS2
DS3
DS3
Bandwidth
64 kbps
1.544 mbps
3.152 mbps
6.312 mbps
44.736 mbps
274.176 mbps
Channels
(DS0s)
1 channel
24 channels
48 channels
96 channels
672 channels
4032 channels
Carrier
Designation
None
T-1
T-1c
T-2
T-3
T-4
• Digital Signaling Hierarchy
– SS-7 uses DS0s for signaling links
– Channel banks: divide DS1 into 24 DS0s
– Copper facilities replaced by fiber:
» SONET, speeds to 9.6 gbps and beyond
» OC-1: 51.84 mbps, OC-3: 155.52 mbps, OC-12: 622.08 mbps
» OC-48: 2488.32 mbps, OC-96: 4976.64 mbps, OC-192: 9953.28 mbps
18
Intelligent Network
• Motivation
–
–
–
–
–
More rapid evolution of the network
Easier to configure
Tailored “services” for subscribers
Services on demand: voice + communication + audio + video
But the most common services are “redirection”: e.g., 800
numbers, cellular network support, call forwarding, etc.,
but also services like call waiting, caller id, voice mail, etc.
– Implemented by GUI to switching networking: Service
Creation Environment (SCE)
• SS-7 is IN’s backbone
– Network that links together the Service Switching Points
– Old PSTN: command signaling embedded in audio channel
– New PSTN: interoffice signaling between switches via
orthogonal network
19
Intelligent Network
• Basic Call Sequencing
– Features/services invoked on state transitions
Disconnect
Put down phone
Quiescent
Talking
Pick up phone
Off hook
Remote user
Picks up phone
Ringing
Dialing
Setup complete
Setup
Waiting for
Setup Complete
Send dial tone
Dial
Collect dial string
Busy detection
Three way calling
20
Intelligent Network
• Call Sequencing
– Call set-up and tear-down handled by conventional SS-7 protocols
» Dialed digits determine how to connect call
– Request for call handling instructions sent to SCP
– DB provides handling instructions
» E.g., to map 800 number to local number under rules
– Routing tables determine which trunks to use to connect call
» SS7 message sent to adjacent exchange requesting circuit
connection on specific trunk
» Target exchange grants permission to connect to trunk by
sending ACK to originating exchange
– Uses Transaction Capabilities Applications Part (TCAP) Protocol
21
Kinds of Signals
• Message Orientation
–
–
–
–
–
Bell ringing
Off hook
Voice transport/full duplex set-up
Bill recording
On-hook/call termination
22
Example Services
•
•
•
•
•
•
•
•
•
Find Me Service
Follow Me Service
Computer Security Service
Call Pickup Service
Store Locator Service
Call Routing Service
Multilocation Extension Dialing
Name Delivery
Outgoing Call Restriction
23
Example Services
• Find Me Service
– Call forwarding to another location
– Call screening capabilities
• Follow Me Service
– Call forwarding based on time schedule
• Computer Security Service
– In-coming call blocking (e.g., to modem line)
– Based on access code or originating phone number
– Essentially Caller ID checking against SCP DB
• Call Pickup Service
– Unanswered call generates page on pager
– Dial special code from any phone to connect to caller
24
Example Services
• Store Locator Service
– Advertise one number, map to local number based on caller
phone number
– Caller number prefix matched against SCP DB of store
locations
• Call Routing Service
– Reroute calls during congestion times or outside of
business hours
– Routing instructions in SCP DB
• Multilocation Extension Dialing
– Abbreviated extension numbers to reach personnel
regardless of location and without need for a PBX
25
Example Services
• Name Delivery
– Display caller name based on in-coming caller number
– Based on line subscriber database
– More sophisticated version of Caller ID (Automatic
Number Identification-ANI)
• Outgoing Call Restriction
– Block calls to specific numbers and area codes, including
900/976 services
26
Intelligent Network
SSP
SSP
SSP
• SSP: Service
Switching Point
SSP
– Computers where
service logic resides
SCP
STP
SCP
STP
• STP: Signal Transfer
Point
– Packet routers
carrying the messagebased signaling
protocol
SMS • SCP: Service Control
SMS
STP
SSP
SSP
Point
STP
SSP
– Service databases
• SMS: Service
Management System
SSP
– User interface to
service DBs &
descriptions
27
SS-7 Signaling Points
• Service Switching Points (SSP)
– Computers
• Signal Transfer Points (STP)
– Packet Routers
– Usually deployed in mated pairs and interconnected
• Service Control Points (SCP)
– Databases, usually deployed in pairs (not direct connected)
• Redundancy and Diversity
– Signaling points deployed in pairs
– 56 kbps facilities (links) between signaling points deployed
in pairs
28
Service Switching Points
• Communicates signals to and from the switches of
the voice network
– Command primitives and packets/signal units
– Translates voice switch signals into SS7 signal units
• Database access
– Queries through SS7 network to centrally located computers
providing the function of SCPs
– E.g., 800 number lookup
• Majority of traffic is circuit-related messages for
call set-up and tear-down
29
Signal Transfer Points (STP)
• Provide the routing paths for SS7 signaling
messages
– Usually embedded in voice switches as integrated functionality
or as an attached computer; rarely found as a standalone box
• Three kinds of STPs
– National Signal Transfer Point
» Router for a national network (ANSI);
Does not support international standards (ITU-TS)
– International Signal Transfer Point
» Part of international network;
Interconnects different national networks
– Gateway Signal Transfer Point
» Interworks between national and international protocols,
between two different national protocols, or between
PSTN protocols and the cellular network
» Implements screening mechanisms/packet filtering
30
Signal Transfer Points (STP)
• Two basic operations provided by SSP
– Call Connections
» Use dial string to route connection message to
terminating end office
– Database Queries
» Message is routed to an SCP
» SSP provides lookup request to STP keyed on dialed
digits (aka global title digits)
» Global Title Translation: STP uses internal translation
tables to map this to database address
» Address consists of a Subsystem Number and a Point
Code of the interface to the DB
31
Signal Transfer Points (STP)
• Network Measurements
– Counts on types of messages entering and leaving the network
– Network event logging (e.g., link outage duration, local
processor outage duration)
• “Peg Counts”
– Count of originating and terminating messages
– Aggregated and sent to Regional Accounting Office (RAO)
– Usage invoice generated and sent to customers
» Owners: Regional Bell Operating Companies through
Bellcore (is this still true?)
» Customers: Long Distance Telcos and Independent Telcos
32
Service Control Points (SCP)
• Interface to Telco databases
–
–
–
–
Subscriber service definition
Routing for special service numbers (800, 900)
Credit card validation/fraud protection
Subscriber service creation
• SCP is really a computer intermediary between
SS7 and the database
– Identified by subsystem number, basis for SSP routing
• Standard databases:
–
–
–
–
–
Call Management Services
Line Information
Business Services
Home Location Register
Visitor Location Register
33
SCP Databases
• Call Management Services DB (CMSDB)
– Call processing
» Routing instructions for special service numbers (800,
877, 976, 900, …)
» Billing information: billing address, 3rd party billing
– Network management
» Instructions for rerouting around congestion
– Call sampling/traffic studies
» Report generation for kinds of calls being made
» Used in provisioning studies
34
SCP Databases
• Line Information DB (LIDB)
– Subscriber information: calling card service, 3rd party
billing instructions, originating line number screening
– Calling card validation, e.g., PIN storage
– Originating line number screening: call forwarding, speed
dialing, etc.
– Subscriber features are network-specific: not all features
implemented by all operators, or implemented in the exact
same fashion
• Business Services Database (BSDB)
–
–
–
–
Call processing instructions
Network management procedures
Specific aspects of private network
Time of day internal rerouting of numbers
35
SCP Databases
• Home Location Register (HLR)
– Billing information, allowed services
– Current location of the cell phone
– Uniquely identified by Mobile ID number (MIN)
» Used just like POTS number
• Visitor Location Register (VLR)
– Track roaming subscribers
– Verify validity of MIN via HLR
• Operations Support Systems (OSS)
– Remote maintenance centers monitor networks
– Allows remote reconfiguration/repair
– SMS: standard interfaces for commands for DB
administration and monitoring/measurement
36
Routing in SS7
• Link sets
– All links with same adjacent STP are called a link set
– Switching equipment does round robin scheduling to insure
equal usage
• Routes
–
–
–
–
Sequence of links to reach a destination switching point
Set of routes called a route set
Members of route set allow alternative routing
Router tables indicate next hop link set to follow to reach
destination
37
Routing Redundancy
Combined
Link
STP
SSP
SSP
Alternative “Extended”
Connect Link
STP
STP
STP
STP
SSP
STP
Same destination
Alternative signaling points
SSP
Alternative Direct
Connect Link
SSP
38
Link Types
• Access Links (A-links)
– Direct link between SSP--STP or SCP—STP
– At least two A links
SSP
SSP
SSP
STP
STP
STP
STP
SSP
39
Link Types
• Bridge Links (B-links)
– Connect mated STP pairs to other mated STP pairs
SSP
SSP
SSP
STP
STP
STP
STP
SSP
40
Link Types
• Cross Links (C-links)
– STP to mated STP
– Used primarily to deal with congestion situations, enhance
reliability
SSP
SSP
SSP
STP
STP
STP
STP
SSP
41
STP
Link Types
• Diagonal Links (D-links)
– Connect mated STP pairs at
different hierarchical levels
STP
SSP
SSP
SSP
STP
STP
STP
STP
SSP
42
Link Types
STP
• Extended Links
(E-links)
– Connect SSP to
remote STP pairs
to achieve
diversity in
routes
SSP
– Backup paths
for greater
reliability
SSP
STP
SSP
STP
STP
STP
STP
SSP
43
Link Types
STP
• Fully Associative
Links (F-links)
– Direct
interconnection
among SSP in the
presence of large
amounts of
traffics
SSP
SSP
STP
SSP
STP
STP
STP
STP
SSP
44
Cellular Network and SS-7
HLR
BSS
BTS
BSC
BTS
BSC
BTS
BSC
MSC
VLR
EIR
AC
SS7
STP
PSTN
MSC
BTS
BTS: Base Transceiver Station
BSC: Base Station Controller
BSS: Base Station Subsystem
MSC: Mobile Switching Center
BSC
HLR: Home Location Register
VLR: Visitor Location Register
EIR: Equipment Register
AC: Authentication Center
45
Basic Call Setup
X
W
Subscriber
line
A
Voice Trunk
Signaling
links
B
Subscriber
line
• Switch A analyzes the dialed digits and determines
that it needs to send call to switch B
• A selects idle trunk, formulates initial address
message (UAM) addressed to B; identifies initiating
switch, destination switch, trunk selected, calling
and called numbers, misc other information
• A picks A-link AW to xmit message to B
46
Basic Call Setup
X
W
Subscriber
line
A
Voice Trunk
Signaling
links
B
Subscriber
line
• W receives msg, determines dest, sends it to B along
BW link
• B receives msg, B determines it serves called number
which is idle
• B forms address complete message (ACM), containing
recipient switch (A), sending switch (B), selected trunk
• B sends ACM msg to X, sends ring tone to A over trunk,
rings called subscriber’s phone
47
Basic Call Setup
X
W
Subscriber
line
A
Voice Trunk
Signaling
links
B
Subscriber
line
• X receives msg from B, forwards it to A
• A connects calling line to indicated trunk, caller
hears ringing tone
• Called subscriber picks up phone, B formulates
answer message (ANM), indicating intended
recipient switch (A), sending switch (B), selected
trunk
48
Basic Call Setup
X
W
Subscriber
line
A
Voice Trunk
Signaling
links
B
Subscriber
line
• B same link as ACM to send ANM message (BX).
Trunk now connects calling/called lines
• X forwards ANM to A via AX
• A insures calling subscribed is connected to correct
trunk in both directions
• If calling subscriber hangs up first, A generates
Release (REL) msg, addressed to B, via AW
49
Basic Call Setup
X
W
Subscriber
line
A
Voice Trunk
Signaling
links
B
Subscriber
line
• W forwards REL to B via WB
• B receives REL, disconnects trunk from called
subscriber, return trunk idle, generate release
complete (RLC) msg, send it back via BX
• X receives RLC, forwards to A via AX
• A receives the RLC, idles the indicated trunk
50
Database Query Example
W
L
A
X
M
• Subscriber served by switch A wants to
reserve rental car, calls 800 number, should be
mapped to closest location
• When finished dialing, A recognizes 800
number, needs assistance to handle
• X receives query from A, selects database M
to respond to query
51
Database Query Example
W
L
A
X
M
• X sends query to M over MX link; M receives query,
extracts passed info, selects real tel # or net to
route call
• M forms response msg to process call, send to A via
MW link
• W receives msg, routes to A over AW link
• A receives msg, determine call routing, picks trunk,
generates IAM, does call set-up as before
52
SS7 Protocol Stack
OMAP: Operations, Maintenance, Admin Part
MAP: Mobile Application Part
Transaction
Capabilities
Application Part
TCAP
ASP
Signaling
Connection
Control Part
NSP
T
U
P
SCCP
I
S
U
P
Telephone
User Part
B
I
S
U
P
ISDN
User Part
Network
Data Link
Physical
MTP
Message
Transport
Protocol
53
Message Transfer Part
• Routing
–
–
–
–
–
Incoming message not for local node
Physical addresses are called point codes
Called/dialing party address determines next hop physical address
Routing tables as service provider maintained/network dependent
Destination point code not necessary the final destination, but
the penultimate address: allows rerouting at the last step
• Message Distribution
– Incoming message for local node
– Routes message to internal user based on user part of message
(telephone vs. ISDN higher level protocols)
• Network Management
– Link Management
– Route Management
– Traffic Management
54
SS7 Protocols
• Built on MTP
• Communications to Telco Databases (SCPs)
– Signaling Connection Control Part
» Allows addressing of specific applications within a
signaling point, i.e., a specific database on an SCP
» Apps identified by subsystem number, e.g., 800 call
processing, calling-card processing, AIN processing,
custom local-area signaling service (CLASS) services (e.g.,
repeat dialing, call return, etc.)
» Incremental routing via Global Title Translation (GTT)—
basically query is sent to an intermediary STP with a GTT
request, it will then determine how the message should be
routed and perform the routing
» This feature used to break down single database into
multiple regional or network operator databases
55
SS7 Protocols
• Communications to Telco Databases (SCPs)
– Application Service Part
» Not widely used: Connection-oriented services
– Transaction Capabilities Applications Part
» Uses SCCP for transport
» Query, retrieval, results transmitted as messages
» Remote control of entities (switches) via TCAP messages
• Addressing
–
–
–
–
Point codes: uniquely identify a signaling point
Network/Cluster/Member Number (8 bits x 8 bits x 8 bits)
0 not used, 255 reserved “for future use”
Networks: RBOCs, major independent telcos, IXCs
56
Keshav’s Common Design
Techniques
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Multiplexing: Trading time and space
Pipelining and parallelism: trading computation for time
Batching: trading response time for throughput
Exploiting locality: trading space for time
Optimizing the common case
Hierarchy
Binding and indirection
Virtualization
Randomization
Soft State
Exchanging state explicitly
Hysteresis
Separating data and control
Extensibility
57
Common Themes
• Pervasive use of redirection/number mapping
• Call blocking & filtering
• Exploitation of geographic proximity based on
phone number prefixes
• Enabled by DB lookups indexed by phone numbers
58
Observations
• Complicated! Teasing out principles is difficult
• Separation of data from control
• Separation of control network into service
logic, routing, and databases
• Addresses/locality very strongly tied to
telephone number semantics
• Ownership of the signaling system not so clear!
59
Drivers for Convergence
• PSTN is expensive, limits innovation from start-ups; complex
and expensive system maintenance; requires large operations
staffs to manage
• Proprietary, limits integration of external apps like billing,
customer service
• No support for IP-based products, multimedia, content delivery
• No model as a contributing element of a “system of systems”;
trying to evolve to be the be-all network
• On-line billing/reporting via Web desirable but difficult to
integrate with legacy systems
• Sprint study [ref?]: data equipment 70% less expensive, data
access lines 60-80% cheaper than voice, maintenance of packet
networks 50%, provisioning costs 72% less
• CLECs looking for a lower cost way to enter local market
• Dial-up circuit overload
60