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