lecture 2 advanced networking cse 8344 southern methodist university fall 2003 mark e. allen
TRANSCRIPT
Lecture 2
Advanced Networking CSE 8344
Southern Methodist University
Fall 2003
Mark E. Allen
Introduction
• Miscellaneous notes:– Exam dates: (posted on web also)
SONET
• SONET byte oriented frame format– Path, line, and section– Multiplexing format
• Virtual Containers• Synchronous payload envelope (SPE)• Pointers
– Timing issues– This is what makes SONET synchronous -- the payload can float in
the SONET frame.
• Overhead– Line, Section, and Path
• Performance monitoring
SONET HierarchySignal Bit rate Capacity
STS-1, OC-1 51.840 Mbps 28 DS1s, 1 DS3
STS-3, OC-3 155.520 Mbps 84 DS1s, 3 DS3s
STS-12, OC-12 622.080 Mbps 336 DS1s, 12 DS3s
STS-48, OC-48 2488.320 Mbps 1344 DS1s, 48 DS3s
STS-192, OC-192 9953.280 Mbps 5376 DS1s, 192 DS3s
SONET Network
ADMor
DCS(LTE)
ADMor
DCS(LTE)
REGREG REGREG
Router(PTE)
Router(PTE)
Router(PTE)
Router(PTE)
Terminal(LTE)
Terminal(LTE)
Terminal(LTE)
Terminal(LTE)REGREG
LINE
SECTION
PATH
SONET Networking
3 columns of transport overhead:
Section overhead
Line overhead
Path overhead
OH PAYLOAD OH PAYLOADOH PAYLOAD
9 rows
90 columns (87 columns of payload)
STS-1Synchronous
PayloadEnvelope
810 bytes x 8000 frame/sec x 8 bits = 51,840,000 bps
90 Columns
9 R
ows
87 Columns of Payload
STS-1 SynchronousPayload Envelope
(STS-1 SPE)
A1 A2 JO/Z0
B1 E1 F1
D1 D2 D3
H1 H2 H3
B2 K1 K2
D4 D5 D6
D7 D8 D9
D10 D11 D12
S/Z1 M0/M1Z2
E2
J1
B3
C2
G1
F2
H4
Z3
Z4
Z5
Section Trace/Growth
BIP-8/BI: Parity Checking
BIP-8/B2: Error Monitoring
STS-1 Frame
SONET Overhead
• Path overhead – Overhead for entire end-to-end circuit. Path terminating equipment (PTE) terminates this overhead.
• Line overhead – Overhead for connection between terminals, cross-connects (DCS), or add-drop mulitplexer (ADM). Line terminating equipment (LTE) terminates this overhead.
• Section overhead – Overhead for connection between regenerators. Section terminating equipment (STE) terminates this overhead.
Path overhead• J1 – Used for tracing the circuit• B3 – Bit interleaved parity byte to check for
errors• C2 – Path signal label byte to indicate the
contents of the SPE• H4 – Virtual tributary (VT) multiframe
indicator byte to describe multiframe VT payloads (pointer)
• G1 – Path status byte so PTE can detect problems on the path
• F2 – Path user channel byte for communications between path elements
• Z3,Z4,Z5 – User bytes reserved for future
J1
B3
C2
G1
F2
H4
Z3
Z4
Z5
Line overhead• H1,H2 – pointer to beginning of SPE• H3 – pointer action byte used to hold
data when pointer adjustment is made• B2 – byte interleaved parity for line• K1, K2 – used to manage Auto-
Protection Switching (APS)• D4 to D12 – Data communication
channel (DCC) bytes are 576 kbps• Z1, Z2 – Not defined (Z2 used for FEBE
in STS-3)• E2 – express 64 kbps channel between
LTE (for STS-1 only)
H1 H2 H3
B2 K1 K2
D4 D5 D6
D7 D8 D9
D10 D11 D12
S1/Z1 M0 or M1/Z2
E2
Section overhead• A1,A2 – Framing bytes• C1 – STS ID set for each
STS1• B1 – Byte interleaved parity
for monitoring for errors• E1 – 64 kbps orderwire• F1 – Used by section
equipment• D1 to D3 – Data
communication channel
A1 A2 C1
B1 E1 F1
D1 D2 D3
SONET multiplexing
• To create OC-N signals, SONET streams are BYTE INTERLEAVED.
• No bit-stuffing is used because network is synchronous
• Pointers are used to account for phase differences in SPE of tributary signals (tribs).
• Overhead from all tribs is aligned.
Typical point to point SONET link
OC192
W
P
OC48OC48OC48OC48
OC192
40 mile amplifier spacing
Dallas Atlanta
Traditional physical layer switching
Purpose of switches
• Voice networks– Connect dialing party to called party. Required making
connections at CO, Tandem, IXC switching nodes
• Layer 1 data network– Connection at SONET or optical layer to connect a
DS3 or OC3 through the network
• Layer 2 data network– Connections via ATM or Ethernet switch to establish a
flow, or PVC through which two PCs or routers are connected
Switching systems
• Switches vs. Routers– Switches are typically connection oriented, routers are
based on datagram routing• Routers use routing lookup tables to send out the packet.• Switches are based on a connection, flow, or circuit that usually
traverses several switches from source to destination• Packet switches have queues, circuit switches do not.
• Focus on circuit switching for now– Packet switching and routing are an extension of circuit
switching.– Optical switching is simpler (conceptually not
technically) subset, to be discussed later.
Basic crosspoint switch (Bellamy)
Space switch
• Most basic switch, sometimes called crosspoint switch.– Rectangular fabric: any input can connect to any output
– Number of crosspoints is N X M
• Graded switches – Each input has access to a select group of outputs
– Used when crosspoints are expensive or switch would be too big.
Graded matrix (Bellamy)
Switches (cont)
• Square vs. triangular– Square fabrics have two possible ways of making
connection– Triangular get rid of extra cross points but require
compare
• Why multistage switches– For square fabric, N(N-1) switches required– For triangular array, N(N-1)/2 required– This results in too many pieces for a practical sized
switch, 5 Billion crosspoints for 100,000 port switch.– Multistage switching is the answer
Three-stage (Bellamy)
3 stage switches
• Number of cross points in 3 stage switch is:• Nx=2Nk + k (N/n)^2
– Where N is number of inputs– k is number of center stages– n is size of inlet / outlet group
• Consider what happens with blocking– There is no center stage that can make a connection to
output stage that can switch to the desired output.
• Clos showed that if– k = (n-1) + (n-1) + 1 then switch is non-blocking
3 stage switch (cont)
• Using this k for number of crosspoints yields (equ 5.2, Bellamy)
•
• Solving for the minimum number of crosspoints yields (equ 5.3)
3-stage (cont, Bellamy)
• Note the reduction in required cross points in (Table 5.1) by using a 3 stage Clos switch.
Crosspoint reduction (Bellamy)
Blocking switches
• In reality, Clos switches are “rearrangeably” non blocking. Not strictly non-blocking.
• In real-world, connections are continuously being made and torn down. – So we can’t pick the perfect path for each
connection beforehand.– Clos switch still requires a fair number of
crosspoints.
Switches with blocking
• It’s often practical to make a switch that is “blocking”– There is some small probability the switch can’t
connect an input to an output– Recall it depends on what other connections
have been made (i.e. how “busy” is the switch?)– Many switches aren’t very busy– Considerable cost savings can be enjoyed by
reduction in cross points
Analysis of blocking switches
• These equations provide probability blocking through a switch fabric– Lee graphs
– Jacobaeus
• Discrete event simulation software packages are often used in practice when designing switches – Modeling input behavior is a challenge
• Call times, relationship between inputs and outputs, etc.
– Examples: OPNET
Time Division switching
• Time Division Switching allows multiple connections to share cross-points– Results in even fewer cross points than
multistage switches
• Goes well with Time division multiplexing– Many times, the individual circuits have been
TDM’d prior to being connected to the switch
Time slot interchange (TSI)
• This is an important function of digital switches. • Memory is used to rearrange data in the time slots• Allows information to arrive at the Space switches
at the right time.• Normally used with Space switching to create
TST, TSST, etc. matrices that combine both. • The Lucent 4ESS switch uses TSSSST.
– 4 internal Switches wrapped in two TSI switches– Can handle from 100,000 to 200,000 calls.
Keshav, Chapter 8
Keshav, Chapter 8
Bellamy Ch 5
Cross-connects
• Digital Cross-connect is a specialized switch fabric– Combines muxing and switching– Used to aggregate (fill) and groom
• Typically appear as 3/3, 3/1, 3/1/0, etc.– 3/3 cross connects DS3s– 3/1 cross connects DS1 within DS3s or entire DS3.– 3/1/0 groom to the DS0 level.– Lower granularity of grooming costs more (more
crosspoints)– Hierarchy is often used (see figure 5.34)
Big switch example
• Example:– SONET cross-connects– 256 OC48 external interfaces– What is total switch capacity?
• 256 X 2.5 Gbps = 640 Gbps
– IF DS0s were to be groomed, how many possible connections?
• 256 X 48 X 28 X 24 = 8.25 Million input channels• Using N(N-1)/2, would be huge!, even Clos is too big
– What about STS-1 granularity?• Switches exist to do this (barely!)
Voice Network Signaling
Voice network signaling
• Signaling function– Supervisory
• On hook, Off hook, dial tone, ringing, on-hook, busy signal
– Information bearing• Dialed digits, toll charges, etc.
– In-channel signaling• In band
– Single Frequency (SF), dual tone multifrequency (DTMF), multifrequency (MF) which all operate in voice band
• Out of band– DC levels on the loop portion or out of band using FDM– Pulses on phones for dialed digits are out of band
Signaling (cont)
• Common channel signaling (CCS)– Here, the signaling information is contained in a separate
signaling channel– Channel is carries signaling for several lines– Good for fraud prevention– Simpler to manage signaling between switches– Disadvantages:
• Signaling may not propagate through the network to free resources
• No automatic testing• Trunks may not all terminate at the same switch (signaling must
be forwarded)
Analog interfaces
• Subscriber loop interfaces1. Battery: 48 volts is supplied to operate the phone
2. Overvoltage protection: protection from lightning, etc.
3. Ringing: 20Hz 86 volt rms signal to ring the phone.
4. Supervision: Detection of on/off hook
5. Test: Access to testing the loop
BORSCHT (Battery, Overvoltage, ….)
(Hybrid and Coding are also required at switch end.)
Analog interfaces (cont)
• Loop start trunks– Simple connection between switches
• Central Office to Private Branch Exchange (PBX)
• Problem of “glare” exists
• Ground start trunk solve this problem– More elaborate communication between PBX and CO
• Direct Inward Dial (DID) trunks– Allow incoming calls to PBX to connect directly to
called party (no attendant necessary)
Analog interfaces (cont)
• E&M (ear and mouth) trunks– 5 types of E&M interfaces are defined– Type II E&M is a 8 wire interface– TX pair, RX pair, E pair, and M pair– Supervisory signaling happens over the E&M
leads– Typically used to connect PBX to CO, PBX to
PBX
Digital Networking
• Analog loops will exist for some time– Businesses will move to digital phones more quickly
• For switching and transport, analog has serious drawbacks– Noise, Ease of multiplexing, switching,
• Current approach is to convert to digital at the ingress of the network– Digitization schemes will be discussed later– Time Division Multiplexing is done in digital domain– Digital signals are better to regenerate– Performance monitoring– Ease of encryption (digital can be scrambled easier)
Advantages of Digital
• DSP chips have enabled the transition to digital networking– Echo cans now use DSP algorithms (LMS)– Modems– Vocoders / Decoders
• Cell phones, Secure phones, Voice over packet, etc.
• There are a few drawbacks to digital– Bandwidth management – Network synchronization– Analog interfaces– Multi-access is complicated (drop and insert)
Switched voice architecture
From: Digital Telephony Bellamy, chapter 1
Keshav, chapter2
The voice network
Central office
Telephone Telephone Telephone
Tandem switch
Central office
Public Switch
Public Switch Public Switch
Interexchange tollswitch network
Bandwidth is allocated through the network using a parallel SS7 network.
From: Voice over IP Fundamentals, Cisco Press
The SS7 Network
• SSP: Service switching point. Originates the messages requesting bandwidth through the network.
• STP: Signal transfer points. Packet switches for signaling messages.
• SCP: Service control points. Servers that host routing instructions and enhanced services.
From: Voice over IP Fundamentals, Cisco Press
Keshav, Chapter 15
SS7 Protocol Details
• Note that SS7 is it’s own protocol with layers.– Much like the TCP/IP stack
• Layers 1-3 are MTP and are switched through STPs (similar to routers)– Layer 1 is T1 connections – Layer 3 routes on point codes (like IP address)
SS7 Protocol Details (cont)
• 3 different stacks on top of MTP– TCAP/SCCP
• Services requiring database lookup:– Calling cards, interactive dialing
– TUP: Telephone User Part• Basic telephone services
– ISUP: ISDN User Part• Enhanced services like:
– User to User signaling, VPNs, Caller ID, Call Forwarding