© Copyright 1997, The University of New Mexico 5-1 WAN Technologies Dialups X.25 Frame Relay Asynchronous Transfer Mode.

5-1© Copyright 1997, The University of New Mexico

WAN Technologies

• Dialups• X.25• Frame Relay• Asynchronous Transfer Mode


Analog Remote Access

PSTN

PCs w/modems

Notebooksw/modems

CorporateNetwork

UNIX

NetWare

MAC

PC

DEC VAX

Modems

TerminalServer


Definitions

• Central Office (C.O.)– Telephone company facility where subscribers' lines are

joined to switching equipment for connecting other subscribers to each other, locally and long distance.

• CSU/DSU– A device which terminates a digital channel on a

customer’s premises– Line coding/conditioning & loopback testing from C.O. – Similar to a modem except it doesn’t perform dial-up

• Channel Bank (Multiplexor)– A multiplexor that merges several low-speed voice or data

lines into one high-speed (typically T1) line and vice versa.


Definitions cont.

• Analog loop (local loop)– “Last mile” voice-grade line from the central office to the

customer’s premises

• Pulse Code Modulation (PCM)– The most common and most important method a telephone

system in North America can use to sample a voice signal (analog) and convert that sample into an equivalent digital code (An A/D or D/A converter , CODEC)

– An electronic circuit that converts audio (analog) into digital code (and vice versa) using techniques such as pulse code modulation

• P.O.T.S.– Plain Old Telephone System


What is T-1?

• Digital transmission facility operating at 1.544 Mbps (DS1)

• Comprised of 24 64-Kbps DS0s– 24 x 64 Kbps = 1.544 Mbps– Channelized or non-channelized

• Transmission is over 2 sets of twisted pair wires (UTP)

• Full duplex: 2-way simultaneous transmission• Time-division Multiplexed

– Allows the bit stream to be channelized into 24 x 64 Kbps channels


T-1 Channel Formats

64 64 64 646464 64 64 64 64 64 64 646464 64 64 64 646464 64 64 64F

• DS0 1, DS0 2, DS0 3 . . . . . DS0 24• Total of 24 64 Kbps DS0s = 1.536 Mbps• 8 Kbps framing overhead: 1.536 + .8 = 1.544 Mbps

Non-Channelized

1.536 Mbps payload

1.536F

- Point-to-point leased line

Channelized


Why T-1?

• Reduced wide area network costs– Less wiring– Less network terminating equipment– Less recurring monthly phone costs

• Improved reliability and availability• Improved serviceability - easier to isolate

faults– T-1 line problems can be isolated and solved by the telco

• Improved quality of information: faster and less errors


Analog Remote Access

Ethernet

Central Office

Central Office

Modems

Modem

PC

T1Channel

Bank

T1Channel

Bank

D

A

D

A

D

D

A

- Digital Signal

- Analog Signal


T-1 Remote Access

- Digital Signal

- Analog Signal

Customer Premises

Central Office

Modems

Modem

PC

T1Channel

Bank

T1Channel

Bank

Ethernet

D

A

D

D

D

A

AT-1


T1 Channel Bank Remote Access

ETHERNETT1

T1 CSU/DSU

RemoteAccessServer

Modem

..

Modem

Modem

Modem

Modem

Modem

Modem

T1Channel

Bank

PSTN


Communication Servers

T1

ETHERNET

T1 CSU/DSU

T1Multiplexor

RemoteFunctions

..

Modem

..

Modem

Modem

Modem

Modem

Modem

Modem

DS0

..

DS0

DS0

DS0

DS0

DS0

DS0

Codecs


DEC VAX

PSTN

PCs w/modems

Notebooksw/modems

CommunicationServer

Enterprise Network

UNIX

NetWare

MAC

PC

T-1

24 Channels

Channelized T1 Access


T1 Hardware Features

• Integrated T-1 CSU/DSU for direct connection to a T-1 line

• Built-in T-1 multiplexor to split out individual DS0s

• 8, 16, or 24 built in V.34 modems for 28.8kbps remote access

• Single power supply that draws only 75W• Drop-and-insert (DSX-1) interface

– Allows access to individual T-1 DS0s– Bi-directional– Allows sharing of a T-1 between a PBX and remote

access server– Allows DS0s configured as data channels to be

connected to external data equipment such as routers– Enables multiple units to be cascaded to provide fault

tolerance


T1 Hardware Features

• Audible Ring generation– Generates ring tone back to calling party

• Diagnostics and management– Full internal testing of all band channels and modems on

power-up– Dedicated out-of-band channel for monitoring and control of T1

engine

• Supported T1 signaling and framing options– Loop start, ground start, E&M Type II (Wink, Immediate)– D4 and ESF frame formats– AMI and B8ZS line coding– Full facilities data link (FDL) supports: ANSI & AT&T

• Map any DS0 to any modem


Modem Specifications

• V.34, V.FC, V.32bis, V.32, V.22bis, V.22A/B, V.23 and V.21

• V.42 LAPM and MNP 2-4 error correction• V.42bis and MNP 5 data compression• Fax modem send/receive rates up to 14.4

Kbps– V.17, V.29, V.27ter, and V.21 channel 2

• MNP 10 data throughput enhancement• Line quality monitoring and retrain• Flow control and speed buffering


Enterprise Remote Access Applications

DEC VAX

PSTN

PCs or Macs w/modems

Notebooksw/modems

XylogicsRA 6100

EnterpriseNetwork

UNIX

NetWare

MAC

PC

T-1

24 Channels


Enterprise Internet Access

router

Internet

PSTN

PCs or Macs w/modems

Notebooksw/modems

CommunicationServer

T-124 Channels

CentralSecurityServer


X.25

• Defines a telephone network for data communications

• Provides users with WAN connectivity across public data networks (TELENET, TYMNET)

• Developed by common carriers to increase subscription to PDNs


X.25 Standard

• ITU-T• Truly a global standard• Specification defines a point-to-point

interaction between DTEs and DCEs for remote access terminals and computers

• Maps to Layers 1 through 3 of the OSI reference model


X.25 Key Features

• Speeds -- 9.6 to 64 Kbps• IUS -- Variable up to 4096 bytes• Addressing -- Variable up to 14 decimal

digit IDNs• Connectionless -- No• PVCs -- Yes• SVCs -- Yes• VC Flow Control -- Yes• L2 Error Correction -- Yes


X.25 and OSI Reference Model

• L3 --> PLP (Packet Link Protocol)• L2 --> LAPB (Link Access Procedure

Balanced)• L1 --> X.21 bis (Physical Interface,

Substitute EIA-232)


X.25 Data Link Layer (LAPB)

• Concerned with framing and delivery of error free data

• LAPB is a subset of HDLC • Three frame types:

– Information (I) Frame -- Carry upper layer information– Supervisory (S) Frame -- provide control information– Unnumbered (U) Frame -- unsequenced control

information


X.25 Data Link Layer (LAPB)

• Frame Format: [Flag|Address|Control|Data|FCS|Flag]

• LAPB commands must be acknowledged within specific time or frame is retransmitted

• Suitable for error prone links• High processing overhead, low transfer

rates


X.25 Network Layer (PLP)

• Concerned with establishing and maintaining DTE/DCE connections

• Three VC operational procedures:– Call setup– Data transfer– Call clearing

• Supports both SVCs and PVCs• Segments and reassembles user

messages


X.25 Key Features

• In-band call control packets (set/reset VCs)

• Intermediate nodes must maintain VC state tables

• L3 VC multiplexing• Hop-to-hop L2 & L3 flow and error

control• Considerable overhead• Designed for links with high error rates


X.25

• Defines a telephone network for data communications: DTEs connect to DCEs which connect to Packet Switching Exchanges (PSEs) or Switches

DTE

DTE

DCE

DTE

DCE

DCE

PSE

PSE

PSE PSN


X.25

• Standard calls for three layers of functionality: lowest 3 layers of OSI model

Packet Level

Link Access, Frame Level

Physical Level

Packet Interface

Frame Interface

Physical Interface

DCE

DTE


X.25

• Layer 3: describes packet formats and exchange procedures (ISO 8208)

• Layer 2: uses Link Access Procedure, Balanced (LAPB), defines packet framing (ISO 7776)

• Layer 1: defines the electrical and mechanical procedures


X.25

• End-to-end communication via a bi-directional link called a virtual circuit (no physical medium is dedicated)

• Permanent Virtual Circuits (PVCs): permanent circuit, used for most data transfers

• Switched Virtual Circuits (SVCs): temporary or switched circuit, used for sporadic data transfers

• Layer 3 is concerned with end-to-end communication involving PVCs and SVCs


X.25

• DCEs use the virtual circuit number to route packets through the X.25 network

• X.25 provides multiplexing• A DTE can establish up to 4095

simultaneous full-duplex virtual circuits with other DTEs over a single physical DTE-DCE link

• Each packet contains a 12-bit virtual circuit number: 4-bit logical group number and an 8-bit logical channel number)


X.25

• X.25 Layer 2 LAPB frame

Flag FlagAddressControl FCSDATA

1 1 1 Variable 2 1

length in bytes


X.25

• Flag: delimits the LAPB frame, bit stuffing is used to avoid flag occurring within body of frame

• Address: indicates whether frame carries a command or a response


X.25

• X.25 Layer 3 packet• General Format Identifier (GFI): 4-bit

group number, identifies the general format of the packet header

• Logical Channel Identifier (LCI): 12-bit virtual circuit number

• Packet Type Identifier (PTI): identifies one of 17 packet types

GFI LCI PTI User Data


X.25

• The network connects two logical channels, each with an independent LCI, on two DTE/DCE interfaces to establish a virtual circuit

• Addressing fields in call setup packets provide source and destination DTE addresses

• Addresses are used to establish the virtual circuits

• ITU-T X.121 recommendation specifies address formats; addresses are also referred as International Data Numbers (IDNs); up to 14 decimal digits long


X.25

• X.121 address format• Data Network Identification Code (DNIC):

4 digits, first 3 digits specify the country, last digit specifies PSN

• National Terminal Number (NTN): used to specify a DTE on the PSN

Destination DTEaddress length

Source DTE address lengthCountry PSN

National terminalnumber

DNIC 4 digitsNTN up to 10 digits4 bits 4 bits


ISDN


ISDN

• Frame Relay: most important technical innovation to come out of narrowband ISDN; 64Kbps basic unit of switching, circuit-switching orientation

• Asynchronous Transfer Mode (ATM): most important technical innovation to come out of broadband ISDN; 100s of Mbps, packet-switching orientation


Frame Relay

• Major push done by Cisco, StrataCom, Northern Telecom, and Digital Equipment Corporation

• Provides a packet-switching network (routers, bridges, hosts, switching nodes)

• DTE and DCE terminology from X.25 is valid

• PSN can be privately owned or a public network


X.25

• Addressing fields are only necessary when an SVC is used, and then only during call setup. When call is established, the PSN uses the LCI field of the data packet header to specify the virtual circuit to remote DTE

• Layer 3 uses three virtual circuit operational procedures: call setup, data transfer, and call clearing– SVCs use all three procedures– PVCs are always in data transfer mode


X.25

• Layer 3 segments and reassembles user messages if they are too long for the max. packet size. Each packet is given a sequence number for error and flow control

• Layer 2 is implemented by LAPB; three frame formats– Information (I) frame– Supervisory (S) frame– Unnumbered (U) frame


Frame Relay

• Industry standard, switched data link protocol that handles multiple VCs using HDLC encapsulation between connected devices. Frame Relay is more efficient that X.25, the protocol for which it is generally considered a replacement.


X.25

• Information (I) frame: carry upper-layer information and some control information (full-duplex)

• Supervisory (S) frame: provide control information. Request or suspend transmission, report on status, acknowledge the receipt of I frames, do not have information field

• Unnumbered (U) frame: frames are not sequenced, used for control purposes

• Control: further qualifications of command and response frames, indicates frame format (I, S, or U), frame function, send.receive sequence number


X.25

• Data: carries upper-layer information. Max. size determined by PSN and subscriber

• Frame Check Sequence (FCS): ensures integrity of transmitted data


X.25

• X.25 layer 1 uses X.21 bis physical layer protocol• Equivalent to EIA/TIA-232-C (formerly RS-232-C)• Derived from ITU-T recommendations V.24 and V.28• X.21 supports up 19.2 Kbps, synchronous, full-duplex

transmission over four-wire media, max. distance between DTE and DCE is 15 meters

• Integrated Services Digital Network (ISDN) refers to a set of digital services that are becoming available to end users

• Involves digitization of the telephone network (voice, text, data, graphics, music, video, and other sources) and provision of services

• Want a worldwide network much like the present telephone network


ISDN

• Effort to standardize subscriber services, user/network interfaces, and network to internetwork capabilities

• Applications: high-speed image and file transfers, voice and audio applications, video conferencing, and services to the telecommuting industry

• Many carriers are beginning to offer ISDN under a tariff

• Same type of protocol as X.25• Differs from X.25 in its functionality and format


Frame Relay

• FR is a more streamlined protocol, gives higher performance and greater efficiency

• FR statistically multiplexes many logical data conversations over a single physical connection; provides a more flexible and efficient use of channel bandwidth

• FR exploits advances made in WAN technology: digital transmission and optical fiber

• X.25 was developed for an analog and copper-based telephone network


Frame Relay

• A form of packet switching based on the use of variable-length, link-layer frames. There is no network layer, and many of the basic functions have been streamlined or eliminated to provide for greater throughput.


Frame Relay

• Originally conceived for use over ISDN interfaces

• Exploits recent advances in WAN transmission technology, including fiber media to produce higher reliability and lower bit error rates.

• Flow control is suited better for upper layer protocols, only congestion notification


Frame Relay Standards

• T1S1• ITU-T• LMI Extensions (Local Management

Interface)• Maps to Layer 1 through 2 of the OSI

reference model


Frame Relay Key Features

• Speeds -- 56 Kbps to 2 Mbps (45 Mbps)• IUS -- Variable up to 4096 bytes• Addressing -- Fixed length 10-bit DLCI• Connectionless -- No• PVCs -- Yes• SVCs -- Proposed (future)• VC Flow Control -- No• L2 Error Correction -- No


LMI Extensions

• VC status messages– Common, provides communication and synchronization

between DCE and DTE

• Multicasting– Optional, allows sender to transmit a single frame but

have it received by multiple recipients


LMI Extensions

• Global addressing– Optional, makes Frame Relay network resemble LANs

were address resolution protocols perform over Frame Relay networks just like they do over LANs

– Simple flow control– Optional, provides for an XON/XOFF flow control

mechanism that applies to the entire Frame Relay interface


Frame Relay and OSI Reference Model

• L2 --> LAPF (Link Access Procedure For Frame-Mode)

• L1 --> V.35 (Physical Interface, Substitute EIA-449)


Frame Relay Data Link Layer (LAPF)

• Concerned with preservation of the order of the frame transfer with small probability of loss

• Enhanced version of LAPD• Only core functions of LAPF used• Frame Format: [Flag|Address|Data|FCS|

Flag]• If an error detected frame is discarded


Frame Relay Congestion Control

• Discard strategy -- DE bit• Congestion avoidance -- BECN bit• Congestion avoidance -- FECN bit• Congestion recovery -- Sequence

number in higher-layers


Frame Relay Traffic Rate Management

• CIR -- Committed information rate (bps)


Modern Digital Communications Networks

• High-quality• Reliable transmission links• High data rates


Frame Relay

• Logical outband call control packets (set/reset) VCs

• No Intermediate nodes VC state tables• L2 VC multiplexing• No hop-to-hop flow and error control• Higher layers End-to-end flow and error

control


Asynchronous Transfer Mode (ATM)

• Another name - “cell relay”.• Similar in concept to “frame relay”.• Provides faster packet switching than

X.25.• Supports data rates several orders of

magnitude higher than frame relay.


Similarities with X.25 and Frame Relay

• Data is transferred in discrete chunks.• Multiple logical connections are

multiplexed over a single physical interface.


The Cell advantage

• Information flow on each logical connection is organized into fixed-size packets called cells.

• ATM protocol has minimal error and flow control capabilities which leads to– reduced overhead of processing ATM cells.– reduced number of overhead bits required with each

cell.– high data rates due to less overhead.

• Fixed size cell reduces processing need at each ATM node leading to high data rates.


Protocol Architecture

• Standards issued by ITU-T are based on B-ISDN.

• Figure shows the ATM reference model.


Physical Layer

• Provides specification of a transmission medium.

• Provides specification of a signal encoding scheme.

• Specified physical layer data rates:– 155.52 Mbps– 622.08 Mbps– others are possible based on SONET physical layer.


ATM Layer

• Provides common services to all higher layers.

• Provides packet transfer capabilities.• Defines transmission of data in fixed

sized cells.• Defines use of logical connections.


ATM Adaptation Layer (AAL)

• Choice of a particular AAL depends on the type of service which is being provided.

• Supports information transfer protocols not based on ATM.

• Maps higher layer information into ATM cells and passes them on to the ATM layer.

• Collects information from ATM cells and passes them on to higher layers.


Different Planes of the Protocol Reference Model

• User Plane– provides for user information transfer.– provides flow control, error control.

• Control Plane– performs call control and connection control functions.

• Management Plane– plane management

• performs management functions related to a system as a whole.• provides coordination between all the planes.

– layer management• performs management functions relating to resources and

parameters residing in its protocol entities.


Virtual channel connection- the ATM logical connection

• Virtual channel connection referred to as VCC.• Analogous to a virtual circuit in X.25 or a data link

connection in frame relay.• VCC is the basic unit of switching in ATM.• VCC connects two end users over the network.• VCC allows exchange of information in the form of

fixed cells at variable rates.• VCCs allow for user-network information exchange

also called “control signaling”.• VCCs allow for network-network information

exchange for the purposes of network management and routing.


Virtual Path Connection

• Also referred to as VPC.• A VPC is a bundle of VCCs having the

same endpoints.• All the cells flowing over all the VCCs in

a single VPC are switched together.• Grouping connections that share a

common path over the network reduces control costs.


VPC advantages

• Simplified network architecture– easy separation of network transport functions into those related to a

single logical connection (virtual channel) and those related to a group of logical connections (virtual paths).

– Increased network performance and reliability– reduces the aggregate number of entities that the network has to deal

with.

• Reduced processing and short connection setup time.– bulk of control work is done at virtual path setup.– capacity reservation on VPCs for anticipated future calls allows for VCC

establishment with minimal control functions at the end points of VPC.– transit nodes do not have to do call processing for new calls.


Call Establishment Process

• A Virtual path connection to the destination node must exist in order to set up a virtual connection.

• The Virtual path in which the new virtual channel will be contained must have – sufficient available capacity to support the virtual channel. – sufficient capacity to support requested quality of service.

• Virtual channel set up is done by storing the required state information - VC number/VP number mapping.

• Control mechanisms for Virtual Paths include– calculating routes.– allocating capacity.– storing connection state information.


VP/VC Terminology

• Virtual Channel• Virtual Channel Link• Virtual Channel Identifier(VCI)• Virtual Channel Connection(VCC)• Virtual Path• Virtual Path Link• Virtual Path Identifier (VPI)• Virtual Path Connection (VPC)


Virtual Channel defined

A generic term used to describe a unidirectional transport of ATM cells associated by a common unique identifier value.


Virtual Channel Link defined

A means of unidirectional transport of ATM cells between a point where a VCI value is assigned and the point where that value is terminated or translated.


Virtual Channel Identifier(VCI)

• Identifies a particular VC link for a given VPC


Virtual Channel Connection (VCC) defined

• A concatenation of VC links that extends between two points where the adaptation layer is accessed.

• VCCs are provided for the purpose of user-user, user-network, or network-network information transfer.

• Cell sequence integrity is preserved for cells belonging to the same VCC.


Virtual Path defined

• A generic term used to describe unidirectional transport of ATM cells belonging to virtual channels that are associated by a common unique identifier value.


Virtual Path Link defined

• A group of VC links, identified by a common value of VPI, between a point where a VPI value is assigned and the point where the value is translated or terminated.


Virtual Path Identifier (VPI)

• Identifies a particular VP link.


Virtual Path Connection defined

• A concatenation of VP links that extends between the point where the VCI values are assigned and the point where those values are translated or removed, thus extending the length of a bundle of VC links that share the same VPI.

• VPCs are provided for the purpose of user-user, user-network, r network-network information transfer.


Uses of VCCs

• VCCs can be established between:– end users– network entities– end user and network entity

• Cells must be delivered in the same sequence, over the VCC, in which they were received.


VCC between end users

• Can be used to carry– end to end user data– control signaling between end users

• VPC between end users provides them with overall capacity and how the VPC is broken into VCCs depends on the end users.– for example there could be two data VCCs and a control

VCC in the VPC between the end users.

• Set of VCCs must not exceed the VPC capacity.


VCC between an end user and a network entity

• Used to carry control signaling between a network and the end user.

• A VPC between an end user and a network may carry several control VCCs.


VCC between two network entities

• Used for network management and routing functions.

• A VPC is used to define a common route for exchange of network management information.


VP/VC characteristics

• Quality of service• Switched and semi-permanent virtual

channel connections• Cell sequence integrity• Traffic parameter negotiation and usage

monitoring• Virtual channel identifier restriction

within a VPC


Quality of service

• Each VCC is provided with a quality of service specified by parameters such as– cell loss ratio– cell delay variation


Switched and Semi-permanent VCCs

• both are switched connections• both require call-control signaling• both provide dedicated channels


Cell sequence integrity

• sequence of transmitted cells within a VCC has to be preserved


Traffic Negotiation & Usage Monitoring

• Can negotiate traffic parameters between a user and a network for each VCC.

• Input of cells to the VCC is monitored by the network to ensure that negotiated parameters are not violated.

• Types of negotiated parameters are:– average rate– peak rate– burstiness– peak duration

• The network can disallow new VCCs and/or terminate old ones in case negotiated parameters can not be maintained.


Virtual channel identifier restriction within a VPC

• One or more VCC may be reserved for use by the network.

• The VCI for this VCC is not available to the user of the VPC.

• Such VCCs may be used for network management.


ATM cells

• Fixed size cells• Cell size is 53 octets

– 5 octet header field– 48 octet information field


ATM cell advantages

• Reduced queuing delay for high priority cells which arrive slightly late than a low priority cells, as they need to wait for only 53 bytes to go by.

• Fixed size cells can be switched more efficiently.

• It is easier to implement the switching mechanism in hardware for fixed size cells.

• So, switching can be implemented in hardware, which is very fast rather than implementing in software, which is slow.


ATM cell’s 5 octet header

• It contains the following fields– Generic Flow Control ( GFC ).– Virtual Path Identifier ( VPI ).– Virtual Channel Identifier ( VCI ).– Payload type.– Cell loss priority ( CLP ).– Header error control ( HEC ).


Generic flow control field

• 4 bits are allocated for this field.• Performs end-to-end user’s functions.• Used for control of cell flow at only user-

network interface.• This field is not retained for network-

network interface.• Details of its applications are for further

study, not yet defined.


Virtual path identifier

• 8 to 12 bits.– 8 in the case of user-network interface.– 12 in the case of network-network interface.

• It is the routing field for the network• 4 extra bits in network-network

interface allows support for an expanded no. of VPC’s internal to the network, which can be used for network management.


Virtual channel identifier

• Also known as VCI.• 12 bits in length.• It is used for routing from end user to

end user.• It functions as service access point.


Payload type

• 3 bits• Indicates the type of information in the

information field.• First MSB indicates whether it is a user

information or network management information.– 0 for user– 1 for network management

• Middle bit indicates whether congestion has been experienced or not during flow.


Payload type (contd.)

• Last bit is to convey information from user-user, also known as AAU (ATM user to ATM user) indication bit.

© Copyright 1997, The University of New Mexico 5-1 WAN Technologies Dialups X.25 Frame Relay Asynchronous Transfer Mode.

Documents

university of new mexico

t1 channel bank pstn

digital channel

t1 line

channel bank multiplexor

leased line channelized

channel formats

voice signal analog