intro,pnni,saal

7/29/2019 Intro,PNNI,SAAL

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Shivkumar KalyanaramanRensselaer Polytechnic Institute

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ISDN, B-ISDN, X.25, Frame-Relay,

ATM Networks:A Telephony View of Convergence

Architectures


[email protected]://www.ecse.rpi.edu/Homepages/shivkuma

Based in part on slides of Raj Jain (OSU), S. Keshav (Ensim)Based also on the reference books: by U. Black, J.C. Bellamy
http://www.ecse.rpi.edu/Homepages/shivkumahttp://www.ecse.rpi.edu/Homepages/shivkuma


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Switched Packet-Data Services

Integrated Services Vision and Concept Ingredients

History: X.25, ISDN, Frame Relay

ATM Networks: foundation for B-ISDN

ATM Key Concepts

ATM Signaling and PNNI RoutingATM Traffic Management

IP over ATM: setting the stage for MPLS

Overview


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3

A Telephony View of Convergence

Separate Voice network (PSTN) and Data Networks(Frame Relay, SMDS, etc.) PSTN sometimes used as a data network backbone, but

PSTN is circuit switched (voice-optimized) and PSTN-based WAN not efficient

Delay sensitive traffic such as voice not possible ondata networks since no guarantee of QoS

Initial attempts to converge data and voice network nottoo successful, i.e. ISDN

B-ISDN and ATM networks viewed as the convergenceend-point leading world-wide domination of telephonydriven standards


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Switched Packet-Data Services After the success of T1, the telephone carriers saw the

growth in packet switched networks Evolved their own flavors of packet switching, notably

X.25, ISDN, SMDS, Frame Relay, ATM etc Key concept: Switched services

Switched services: (aka dial-up service) Digital communications that is active only when the

customer initiates a connection. Subsumes both circuit switched and packet switched.

Customer to be billed only when the line is active. Led to activity-basedoraverage-load-basedpricingmodels that did not necessarily have a distance-basedcomponent Vspeak-rate and distance-sensitive T-carrier pricing


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Ingredients

Signaling and setup of a virtual circuit(I.e. nailing down aswitched path) is a common feature Signaling was heavyweight, and was coupled to

heavyweight QoS routing Contrast this to connectionless, best-effort Internet

Long 20-byte global addresses used only in signaling Short 4-byte local labels (aka DLCI etc) used in

packets (cells): label-switching

Large address space, low per-packet overhead ISDN/B-ISDN vision of an end-to-end integrated digital

network: Rich QoS capabilities developed: support for voice,

data, video traffic


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Ingredients (contd)

X.25 -> Frame relay/ATM: reduction of hop-by-hopprocessing complexities Led to the development ofhigh-speed switches and

networksA serious attempt to inter-network with a variety of

data-networking protocols (IP, Ethernet etc)

Integration (coupling) of too many features led to slowrollout, enormous overall complexity

Failure to attain the end-to-end market vision Current trend is to de-couple building blocks of the

architecture within the context of IP/MPLS, sacrificingstrict performance guarantees.


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Shivkumar KalyanaramanRensselaer Polytechnic Institute7

X.25


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X.25

First packet switching interface in the telephony world Issued in 1976 and revised in 1980, 1984, 1988, and

1992. Data Terminal Equipment (DTE) to Data Communication

Equipment (DCE) interface User to network interface (UNI) Slow speeds, used in point-of-sale apps (eg: credit-card

validation) and several apps abroad


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X.25 Virtual Circuits

Circuit: Pin a path, reserve resources, use TDM based transmission

Virtual Circuit = Virtual Call: pin a path, optionally reserve resources Connection-oriented: Setup an end-to-end association (data-

structure); path not pinned Connectionless: stateless. No path, no end-to-end association Two Types of Virtual Circuits:

Switched virtual circuit (SVC): Similar to phone call Permanent virtual circuit (PVC): Similar to leased lines

Up to 4095 VCs on one X.25 interface


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X.25 Protocol Layers

Note: the three modular layers were co-specified by thesame standards body

Layers: X.21 replaced by EIA-232 (RS-232C) LAP-B = Link access procedure - Balanced Packet layer = Connection-oriented transport over

virtual circuits


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X.25 Physical Layer

Electrical and mechanical specifications of the interface X.21 = 15-pin digital recommendation X.21bis = X.21 twice = X.21 second

Interim analog specification to allow existingequipment to be upgraded.

Now more common than X.21 => X.21 Rev 2

RS-232-C developed by Electronics IndustriesAssociation of America (EIA) is most common Uses 25-pin connector. Commonly used in PCs.


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Link Layer Roots: HDLC Family Original:

Synchronous Data Link Control (SDLC): IBM Derivatives:

High-Level Data Link Control (HDLC): ISO Link Access Procedure-Balanced (LAPB): X.25

Link Access Procedure for the D channel (LAPD):ISDN Link Access Procedure for modems (LAPM): V.42 Point-to-Point Protocol (PPP): Internet

Logical Link Control (LLC): IEEE Link Access Procedure for half-duplex links (LAPX): Teletex Advanced Data Communications Control Procedures (ADCCP):

ANSI V.120 and Frame relay also use HDLC


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HDLC (contd)

Primary station: Issue commands (master) Secondary Station:Issue responses (slave)

Hybrids:

Combined Station: Both primary and secondary: a.k.aAsynchronous Balanced Mode (ABM)Balanced Configuration: Two combined stations

Unbalanced Configuration: One or more secondary

Normal Response Mode (NRM): Response fromsecondary

Asynchronous Response Mode (ARM): Secondary mayrespond before command


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LAPB

Uses balanced mode subset of HDLC between DTE andDCE

Uses 01111110 as frame delimiter

Uses bit stuffing to avoid delimiters inside the frames

Uses HDLC frame format Point-to-point: Only two stations - DTE (A), DCE (B)

Addresses: A=00000011, B=00000001

Address = Destination Addresses in Commands


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HDLC frames

Information Frames: User data Piggybacked Acks: Next frame expected Poll/Final = Command/Response Supervisory Frames: Flow and error control

Go back N and Selective Reject Final No more data to send Unnumbered Frames: Control Mode setting commands and responses

Information transfer commands and responses Recovery commands and responses Miscellaneous commands and responses


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HDLC Operation

SABM: SetAsynchronousBalanced Mode

UA: UnnumberedACK

DISC: disconnect

RR: Receiver Ready

RNR: Receiver NotReady

I: information frame

Heavyweight Link-Setup and Per-Packet Acking !!


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HDLC Operation (Contd)


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X.25 Packet Level: Layer 3

Packet Level = End-to-end for X.25 networksBut really Layer 3 (network layer)

Packet level procedures:Establishment and clearing of virtual calls

Management of PVCs

Flow Control

Recovery from error conditions


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X.25 Packet Level (Layer 3) Signaling Operation

Redundant signaling and reliability functions at L2 and L3!


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X.25 Packet Format

GFI = Packet formatting information

PTI = 20 possible packet types (for de-multiplexing)

Logical Channel Group and Channel Numbers:

Virtual circuit identifier


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(Layer 3) Packet Format (contd)

Fragmentation/Reassembly support: M = More segments

Layer 3 reliability: P(R) and P(S) refer to packet sequence # Different from N(R) and N(S) - frame sequence #


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(Layer 3) Packet Format (Contd)

3-bit and 7-bit sequence number options possible Again, note: these are layer 3 sequence numbers


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ISDN: Integrated Services Digital Network


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ISDN: End-to-End Digital Services Vision


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ISDN Configurations


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BRI and PRI Services

* Basic Rate ISDN and Primary Rate ISDN.* BRI can transmit data up to 128 kbps.

* PRI (transmitted over a T1 line) can transmit data up to 1.536 Mbps.An LDN (Local Directory Number): customer's 7-digit ISDN phonenumber.

A SPID (Service Profile Identifier): unique ID of an ISDN line or serviceprovider (10+ digits long and includes the LDN).


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Basic Rate ISDN (BRI): contd

Basic Rate ISDN service divides a standard telephoneline into three digital channels capable of simultaneousvoice and data transmission. The three channels are comprised of two Bearer (B)

channels at 64 kpbs each and a data (D) channel at

16 kbps, also known as 2B+D. The B channels are used to carry voice, video, and

datato the customer's site (hence the term integratedservices).

The D channel is used to carry signalingandsupplementary services.

Multiple B channels can be used at the same time.The D channel can also be used to carry packetizeddata.


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BRI and Reference Model


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BRI Reference Model Details

U-interface:U-interface is a 2-wire digital telephone line that runs

from the telephone company's central office to an NT1 device.

NT1 (Network Termination Type 1):NT1 is a Basic Rate ISDN-onlydevice that converts a service provider's U-interface to a customer'sS/T-interface. Stand-alone or integrated into a terminal adapter.

S/T-interface:S/T-interface is a common way of referring to eitheran S- or T-interface. This can be used to connect directly to an ISDN2B+D NT1 or an NT2 device with a terminal adapter. This type ofinterface is often found on Terminal Equipment Type 1.

TE1:TE1 (Terminal Equipment Type 1) is ISDN-ready equipmentthat can directly connect to the ISDN line (often using an S/ T-interface). Eg: ISDN phones, ISDN routers, ISDN computers, etc.


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BRI Ref Model Details: Contd

TA (terminal adapter):TA is a device that allows non-ISDN-ready equipment to connect to an ISDN line. Thisdevice can have an integrated NT1.

R-interface: R-interface is a non-ISDN interface such asan EIA-232 or a V.35 interface. This type of interface isoften found on TE2.

TE2 (Terminal Equipment Type 2):TE2 is equipmentthat cannot directly connect to an ISDN line. A commonexample of this device is a PC, or a non-ISDN-readyrouter. A TA must be used to connect to the ISDN line.


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Primary Rate ISDN (PRI)

Primary Rate Interface (PRI) ISDN is a user-to-networkinterface (UNI) consisting of: Twenty-three 64 kbps bearer (B) channels, and One 64 kbps signaling (D) channel (aka 23B+D) Cumulatively carried over a 1.544 Mbps DS-1 circuit. The B channels carry data, voice or video traffic. The

D channel is used to set up calls on the B channels.


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ISDN Reference Model


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LAPD Framing in ISDN


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Q.931: ISDN Signaling


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Frame Relay


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Dis-economics of Leased Lines

Multiple logical links => Multiple connections Four nodes => 12 ports (full mesh!!)

12 local exchange carrier (LEC) access lines, 6 inter-exchange carrier (IXC) connections One more node => 8 more ports, 8 more LEC lines, 4

more IXC circuits (same issues as full mesh in LANs) Charged both by bandwidth and by the mile!


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X.25/Frame Relay Niche 6 IXC circuits (star vs full mesh: FR network is like a hub or switch in

a star-topology) One more node: 1 more port, 1 more access line, 4 more IXC circuits Share local leased lines to LECs (aka Virtual Private Networks

(VPNs) or closed-user groups (CUGs))

Tradeoffs: PacketizedL2 (FR) or L3 (X.25) service instead of digital L1 service

(T-carrier) Service guarantees weaker(delay, jitter, loss; PIR/CIR vs peak rate)


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X.25 vs Frame Relay

X.25 Message Exchanges Frame Relay Message Exchanges

FR obviously more efficient from a protocol standpoint than X.25,

in addition to the compelling economics vs leased lines


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X.25 vs Frame Relay

X.25: interface between host and packet-switchingnetwork 3 layers: phy, link, packet Heavyweight: error control at every linkas well as

layer 3: twelve messages for one packet transfer!! X.25 offers no QoS capability

Frame relay breaks up link-layer into two parts: LAPF-core and LAPF-control

Network nodes only implement LAPF-core Frame Switching is a service that implements both

Frame relay uses a separate VC for control channelin vsin-band control approach used in X.25


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Frame Relay Overview Frame Relay: digital packet network providing benefits

dedicated T-1 link, but without the expense of multiplededicated circuits. Frame Relay leverages the underlying telephone network Frame Relay distance-insensitive and average-rate

pricingis an ideal, cost-effective solution for networkswith bursty traffic Especially those that require connections to multiple locations

and where a certain degree of delay is acceptable. FR also allows a voice circuit to share the same virtual

connection as a data circuit, again, saving money. Frame Relay assumes higher-speed, low error-rate

underlying PHY. Switches do not perform hop-by-hop error correction (other than

discarding corrupted frames) or flow control(other than settingFECN/BECN bits)


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Frame Relay: Key Features

X.25 simplified No flow and error control Out-of-band signaling Two layers Protocol multiplexing in the second layer Congestion control added Higher speed possible. X.25 suitable to 200 kbps vs Frame relay suitable to 2.048 Mbps. Frame Relay = Unreliable multiplexing service X.25 Switching = Relaying + Ack + Flow control + Error

recovery +loss recovery


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Frame Relay Reference Model & Lingo

PVC: Permanent Virtual Circuit DLCI: Data Link Connection Identifier CIR: Committed Information Rate CSU: Channel Service Unit UNI: User-to-Network Interface NNI: Network-to-Network Interface DTE: Data Terminal Equipment DE: Discard Eligible FRAD: Frame Relay Access Device

DSU: Data Service Unit


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Frame Relay Lingo (contd) Frame Relay Access Device FRAD:generic name for a device

that multiplexes/formats traffic for entering a Frame Relay network. Access Line:A communications line interconnecting a Frame

Relay-compatible device to a Frame Relay switch. Bursty/burstiness:Sporadic use of bandwidth that does not use the

total bandwidth of a circuit 100% of the time. CIR (Committed Information Rate):The committed rate (usually

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