Module 4 netwok layer,routing ,vlan,x.25doc

MODULE 5 MCA-402 Computer Networks ADMN 2012-‘15

Dept. of Computer Science And Applications, SJCET, Palai Page 1

FRAME RELAY Is a packet switched WAN protocol that operates at the physical and data link layers of the OSI

reference model.

As fiber optic was introduced, the quality of circuits improved and there was no need for error

control.

Was developed in response to a high speed, high performance and greater efficient transmission.

It puts data in variable-size units called "frames" and provide minimal internal checking

support data transfer rates at

T-1 (1.544 Mb/s)

T-3 (45 Mb/s) speeds.

Enabling end stations to dynamically share the network medium and the available bandwidth.

Devices attached to a Frame Relay WAN fall into the following two general categories:

1. Data terminal equipment (DTE)

For a specific network and typically are located on the premises of a customer.

Example of DTE devices are terminals, personal computers, routers, and bridges.

2. Data circuit-terminating equipment (DCE)

Carrier-owned internetworking devices.

The purpose is to provide clocking and switching services in a network, which are

the devices that actually transmit data through the WAN.

Fig 5. 1 frame relay devices

Architecture

Frame Relay has 2 layers: physical and data link (LAPF, Link Access Procedure for Frame Mode

Bearer Services)



Fig 5.2 frame relay protocol architecture

Physical Layer

No specific protocol is defined for the physical layer in Frame Relay. Instead, it is left to the

implementer to use whatever is available.

Frame Relay supports any of the protocols recognized by ANSI

Data Link Layer

Link layer uses the services of the physical layer. It, in turn, provides the following services :

Flag recognition.

Frame check sequence (FCS) generation and checking.

Recognition of invalid frames.

Discard incorrect frames.

Routing.

Congestion control notification

Frame Relay Virtual Circuit

Virtual circuits provide a bidirectional communication path from one DTE device to another and

are uniquely identified by a number called data link connection identifier (DLCI).

When a virtual circuit is established by the network, a DLCI number is given to a DTE in order to

access the remote DTE.

Frame Relay virtual circuits fall into two categories:

switched virtual circuits (SVCs)

Permanent virtual circuits (PVCs).

1. Switched virtual circuits (SVCs)

Temporary connections, a new virtual circuit connection will be established each time a DTE

wants to make a connection with another DTE.

A communication session across a SVC consists of the following four operational states(Call setup

,Data transfer ,Idle and Call termination )

2. Permanent virtual circuits (PVCs)

Permanently established connections by the network provider that are used for frequent and

consistent data transfers between DTE devices across the Frame Relay network.

Always operate in one of the following two operational states(Idle and Data Transfer)



Protocol Architecture

Fig 5.3 frame relay protocol architecture

LAPF Core

LAPF frame

Fig 5.4 LAPF frame

The address area, which is 2 bytes in length, is comprised of 10 bits representing the actual circuit

identifier and 6 bits of fields related to congestion management.



DLCI field: 10-bit DLCI field represents the address of the frame and corresponds to a PVC.

Command/response (C/R): Designates whether the frame is a command or response.

Extended address (EA): used for expanding the number of possible addresses.

Forward explicit congestion notification (FECN):can be set by any switch to indicate that traffic

is congested. This bit informs the destination that congestion has occurred.

Backward explicit congestion notification (BECN):is set (in frames that travel in the other

direction) to indicate a congestion problem in the network.

Discard eligibility (DE): indicates the priority level of the frame. In emergency situations,

switches may have to discard frames to relieve bottlenecks and keep the network from collapsing

due to overload.

core functions of LAPF are used for frame Relay:

Frame delimiting and transparency

Frame mux and demux using addressing field

Ensure frame is neither too long nor short

Detection of transmission errors

Congestion control functions

LAPF-Control

The user terminals (DTEs) implement full LAPF protocol, which is also called LAPF-Control

Protocol.

The only difference b/w this protocol and LAPF-core is the inclusion of a control field.

Control protocol provides the functions of flow and error control that are missing from core

protocol

CONGESTION-CONTROL MECHANISMS

Frame Relay reduces network overhead by implementing simple congestion-notification

mechanisms. Frame Relay implements two congestion-notification mechanisms:

Forward-explicit congestion notification (FECN)

Backward-explicit congestion notification (BECN)

FECN and BECN each is controlled by a single bit contained in the Frame Relay frame header.

The Frame Relay frame header also contains a Discard Eligibility (DE) bit, which is used to

identify less important traffic that can be dropped during periods of congestion.



Fig 5.5 FECN

Fig 5.6 BECN

Four Cases of Congestion

Fig 5.7 four cases of congestion



ASYNCHRONOUS TRANSFER MODE (ATM)

ATM is a concept similar to frame relay which take advantages of modern digital facilities to

provide faster packet switching

is a connection-oriented, high-speed, low-delay switching and transmission technology

Allows multiple logical connections to be multiplexed over a single physical interface.

uses fixed sized packets called cells

Developed to enable simultaneous Voice, Video, and Data traffic on the same network with

minimal error and flow control

data rates of 25.6Mbps to 622.08Mbps

Design Goals

1. Use of high data rate transmission media (i.e fiber optic)

2. Interoperability with existing technologies

3. Implementation at reasonable cost

4. Support for existing telecommunications hierarchies

5. Reliable and predictable

6. Suitable for real-time and non-real-time services

Cell Networks

A cell network uses the cell as the basic unit of data exchange

ATM carries information on cells

The length of each cell is 53 Bytes

First 5 bytes are used as the cell header

Next 48 bytes are used as the payload carrying the data

Fixed Length Cell Advantage

Delay or latency is significantly reduced

ATM is therefore suited for voice and video transmission

Fixed length cells make it easier to switch data across multiple networks

ATM networks are built based on switches and not routers

Fixed length cell is similar to container based road transportation

Multiplexing with cells

The cells from the two lines are interleaved so that none suffers a long delay.

High speed of the links coupled with the small size of the cells means that cells from each line

arrive at their respective destinations in a continuous stream.

A cell network can handle real-time transmissions, such as a phone call, without the parties being

aware of the segmentation



Fig 5.8 multiplexing

Asynchronous Time-Division Multiplexing

ATM uses asynchronous time-division multiplexing to multiplex cells coming from different

channels.

It uses fixed-size slots, ie cells.

ATM multiplexers fill a slot with a cell from any input channel that has a cell; the slot is empty if

none of the channels has a cell to send

Fig 5.9 asynchronous multiplexing

Architecture

ATM Devices

ATM networks are built around two categories of devices

ATM Switch

ATM end-point

ATM switch can be connected to either another ATM switch or and ATM end-point.

ATM end point

contain and ATM end-point adapter

Examples of ATM end-points are Workstations,LAN switches, Routers etc

Two Types of Interfaces that interconnect ATM devices over point to point links:



User-Network Interface (UNI): connects an ATM end-system (client side) with an ATM

switch (network site).

Network-Network Interface (NNI): switches are connected through network-to-network

interfaces (NNIs).

Fig 5.10 ATM interfaces

Virtual Connection

Connection between two endpoints is accomplished through

1. Transmission Paths (TPs): is the physical connection between an endpoint and a switch or between

two switches. A transmission path is divided into several virtual paths.

2. Virtual Paths (VPs): provides a connection or a set of connections between two switches.

3. Virtual Circuits (VCs):SVC or PVC

A virtual connection is defined by a pair of numbers: VPI and VCI

ATM assigns each Virtual Connection a 24-bit identifier

1. Virtual Path Identifier (VPI), specifies the path the VC follows through the network.8 bits long.

2. Virtual Channel Identifier (VCI), specifies a single VC within the path.16 bits long.

Cell networks are based on Virtual Connection and all cells belonging to a single message follow

the same virtual circuit

Fig 5.11 ATM Virtual Circuit



Fig 5. 12 example of virtual path and Virtual Circuit

ATM Protocol Layers

Fig 5.13 ATM protocol layers

Physical Layer

It describes the physical transmission media.

We can use shielded and unshielded twisted pair, coaxial cable, and fiber-optic cable.

ATM Layer

The ATM layer is responsible for establishing connections and passing cells through the ATM

network.

It provides routing, traffic management, switching, and multiplexing.

ATM cell

Fig 5.14 ATM cell



Fig 5.15 a ATM Cell Header—UNI Format Fig 5.15 b ATM Cell Header—NNI Format

General Flaw Control (GFC): Provides local functions, such as flow control from end point

equipment to the ATM switch.

Payload Type (PT): Indicates whether the cell contains user data or control data.

Cell Loss Priority (CLP): Indicates whether the cell should be removed if it encounters errors as it

moves through the network.

Header Error Control (HEC): Contains Cyclic Redundancy Check (CRC) on the cell header.

Virtual Path Identifier (VPI): Identifies semi-permanent connections between ATM end points.

Virtual Channel Identifier (VCI): Have only local significance on the link between ATM nodes.

ATM Adaptation Layer (AAL)

It converts the submitted information into streams of 48-octet segments and transports these in the

payload field of multiple ATM cells.

Similarly, on receipt of the stream of cells it converts the 48-octet information field into required

form for delivery to the particular higher protocol layer.

AAL exists only in end systems, not in switches.

AAL Services

Handle transmission errors

Segmentation/reassembly (SAR)

Handle lost and misinserted cell conditions

Flow control and timing control

AAL is classified into four(The classification was made with respect to the ,following parameters:

Timing relationship between sender and receiver

Related



Not related

Bit rate

Constant bit rate

Variable bit rate

Connection mode

Connection-oriented

Connectionless

AAL is divided into two sub layers:

Convergence Sub layer: manages the flow of data to and from SAR sub layer.

Segmentation and reassembly sub layer:Packages data from CS into cells and

unpacks at other end

Fig 5.16 AAL classes

AAL 1 (Constant Bit Rate)

The CS layer divides the bit stream into 47-byte segments and passes them to the SAR sub layer

below

The SAR sub layer adds 1 byte of header and passes the 48-byte segment to the ATM layer.

The 1 byte header is divided into two 4-bit fields

Sequence number (SN)

Sequence number protection (SNP)



Fig 5.17 AAL1 Operation

AAL2

AAL2 was intended to support a variable-data-rate bit stream.

It is used for low-bit-rate traffic and short-frame traffic such as audio (compressed or

uncompressed), video, or fax

Allows the multiplexing of short frames into one cell.

It widely used in wireless applications

Fig 5.18 AAL2 operation

AAL 3/4

AAL3 was intended to support connection-oriented data services and AAL4 to support

connectionless services

Later they have been combined into a single format called AAL3/4

the convergence sub layer (CS) creates a protocol data unit (PDU) by adding a beginning header to

the frame ,a length field as a trailer and a variable-length pad



the segmentation and reassembly (SAR) sub layer fragments the PDU and append a header to it .

Then, the SAR sub layer appends a CRC-10 trailer to each PDU fragment for error control

The completed SAR PDU becomes the Payload field of an ATM cell

Fig 5.19 AAL3/4 operation

AAL 5

Is the primary AAL for data and supports both connection-oriented and connectionless data.

also known as the Simple and Efficient Adaptation Layer (SEAL)

The SAR sub layer simply accepts the CS-PDU and segments it into 48-octet SAR-PDUs without

adding any additional fields.

The CS sublayer appends a variable-length pad and an 8-byte trailer to a frame. The trailer

includes the length of the frame and a 32-bit cyclic redundancy check (CRC)

The SAR sub layer segments the CS-PDU into 48-byte blocks.

the ATM layer places each block into the Payload field of an ATM cell

Fig 5.20 AAL 5



TRANSPORT LAYER

Introduction

The transport layer is concerned with the provision of host-to-host user connections for the reliable

and cost effective transfer of user data

It Isolates upper layers from the network layer

The transport layer is responsible for process-to-process delivery of a packet.

At the transport layer, we need a transport layer address, called a port number, to choose among

multiple processes running on the destination host.

Transport Services

Provide logical communication between application processes running on different hosts.

There are two types of transport service. The connection-oriented transport service and connection-

less transport service.

transport protocols are used for providing transport services .transport protocols run in end systems

sender side: breaks messages into segments, passes to network layer

receiver side: reassembles segments into messages, passes to higher layer

more than one transport protocol available to apps

Internet: TCP and UDP

Elements of Transport Protocols

1. Addressing

2. Connection Establishment

3. Connection Release

4. Flow Control and Buffering

5. Multiplexing

6. Crash Recovery

1. Addressing

When an application process wishes to set up a connection to a remote application process, it must

specify which one to connect to

The method normally used is to define transport addresses is by using connection requests

The network layer address identifies a host. The transport layer address identifies a user process –

a service – running on a host

In the Internet, these endpoints are called ports or TSAP (Transport Services Access Points).

The endpoints in the network layer (i.e., network layer addresses) are called NSAPs (Network

Service Access Points).



Fig 5.21 TSAPs, NSAPs, and transport connections

Fig 5.22 IP addresses versus port numbers

2. Connection Establishment

Just send REQUEST, wait for ACCEPTED.

The problem occurs when the network can lose and duplicate packets.

Main problem is delayed duplicates



Fig 5.23 connection establishment strategy

Solutions for delayed duplicates

1. Using throw-away transport addresses

In this approach, each time a new transport address is needed,

When a connection is released, the address is discarded and never used again.

2. Give each connection a connection identifier

Each connection is associated with a connection identifier. Whenever a connection request comes,

transport entity update a table with connection.

After each connection is released, each transport entity could update a table listing obsolete

connections.

Whenever a connection request comes in, it could be checked against the table, to see if it

belonged to a previously-released connection.

3. Setting Packet lifetime

Packet lifetime can be restricted to a known maximum using one of the following techniques:

Restricted subnet design(Any method that prevents packets from looping)

Putting a hop counter in each packet (hop counter incremented every time the

packet is forwarded).

Time stamping each packet (Each packet caries the time it was created, with routers

agreeing to discard any packets older than a given time

Three-way handshake protocol

Used for connection establishment

Each packet is responded to in sequence

Duplicates must be rejected

Three protocol scenarios for three way hand shake

a) Normal operation

b) Old CONNECTION REQUEST appearing out of nowhere.



c) Duplicate CONNECTION REQUEST and duplicate ACK.

Fig 5.24 three way hand shake operation (a) Normal operation. (b) Old duplicate CONNECTION

REQUEST appearing out of nowhere. (c) Duplicate CONNECTION REQUEST

and duplicate ACK.

3. Connection RELEASE

There are two styles of terminating a connection:

asymmetric release

symmetric release

Asymmetric release

only 1 peer closes the connection.is abrupt and may cause data loss

CR: Connection Request

DR: Disconnect Request

Fig 5.25 Connection release Asymmetric release

The two-army problem



Fig 5. 26 two army problem

The blue army has 4 troops (2 on either side of valley) while the white army has 3 troops. If both

blue armies charge at the same time they can vanquish the white army. If only one of the blue

armies charges it will succumb (3 white troops against 2 blue troops). This means: the blue armies

have to synchronize their attack. But in order to synchronize they need to send a messenger

through the valley; of course the messenger can get caught by the white army (‘lost packet’).

Approach #1: The blue army #1 sends a messenger to tell blue army #2 to attack @ 1400.

Problem: The blue army #1 does not know if the messenger managed to convey message or if he

was caught. Thus blue army #1 will not attack.

Approach #2: The blue army #2 sends back a messenger to acknowledge to blue army #1 that it got the

message.

Problem: The blue army #2 does not know if acknowledge-messenger reached blue army #1. Thus

blue army #2 will not attack.

This play can be continued.

Symmetric release

Each direction is released independently of the other one.

Four protocol scenarios for releasing a connection:



Fig 5.27 four scenarios for symmetric release (a) Normal case of a three-way handshake.

(b) Final ACK lost (c) Response lost. (d)Response lost and subsequent DRs lost.

4. Flow control and Buffering

The sender process may send at much higher speed than the receiver process can handle the data

thus causing overflow (= packet loss).

Transport layer segments the data stream

Fig 5.28 flow control and buffering strategy



If most Segments are nearly the same size, it is natural to organize the buffers as a pool of

identically-sized buffers, with one Segment per buffer

If there is wide variation in Segment size, a pool of fixed-sized buffers presents problems.

Fig 5.29 types of buffering (a) Chained fixed-size buffers. (b) Chained variable-sized buffers. (c)

One large circular buffer per connection.

If the buffer size is chosen equal to the largest possible Segment, space will be wasted whenever a

short Segment arrives.

If the buffer size is chosen less than the maximum Segment size, multiple buffers will be needed

for long Segments, with the attendant complexity.

Another approach to the buffer size problem is to use variable-sized buffers.

The advantage here is better memory utilization, at the price of more complicated buffer

management.

A third possibility is to dedicate a single large circular buffer per connection

This system is simple and elegant and does not depend on segment sizes, but makes good use of

memory only when the connections are heavily loaded.

4. Multiplexing & De-multiplexing

In the transport layer the need for multiplexing can arise in a number of ways.

There are two types of multiplexing

i. Upward

ii. Downward



Fig 5.30 multiplexing and multiplexing

i. Upward multiplexing

Traffic from a “data stream” is distributed over several transport connections (TSAPs).

For Eg, if only one network address is available on a host, all transport connections on that

machine have to use it.

Fig 5.31 upward multiplexing

ii. Downward Multiplexing

Many “data streams” share the same transport connection using multiple NSAPs, possibly over

multiple network interfaces (load balancing).

Fig 5.32 downward multiplexing



6. Crash Recovery

A crash of one host (server) during the transmission leads to a connection loss which results in data

loss. Solution for this, the client retransmits only unacknowledged packets.

Does not work in all cases

Fig 5.33 normal crash recovery mechanism

A crash at layer N can only be handled at layer N+1 (a system crash is a crash at every layer).

Thus: It is left to the application layer to handle crashes of the remote host (client or server).

Generally applications detect that the remote host has died and then simply restart the connection

and retransmit everything.

Each client can be in one of two states

i. S1: 1 unacknowledged packet outstanding

ii. S0:No unacknowledged packet outstanding

The server can be programmed in one of two ways

i. First ACK, then write

ii. First write, then ACK

The client can be programmed in one of four ways

i. always retransmit the last segment,

ii. never retransmit the last segment,

iii. retransmit only in state S0,

iv. Retransmit only in state S1.



Three events are possible at the server

i. sending an ack (A),

ii. writing to the output process (W),

iii. crashing (C)

Fig 5.34 crash recovery states

A SIMPLE TRANSPORT PROTOCOL

Transport Service Primitives allows transport users (e.g., application programs) to access transport

service. Five primitives: CONNECT, LISTEN, DISCONNECT, SEND and RECEIVE.

Fig 5.35 connection primitives



The parameters for the service primitives and library procedures are as follows:

connum = LISTEN(local)

connum = CONNECT(local, remote)

status = SEND(connum, buffer, bytes)

status = RECEIVE(connum, buffer, bytes)

status = DISCONNECT(connum)

The LISTEN primitive announces the caller's willingness to accept connection requests directed at

the indicated TSAP.

The CONNECT primitive takes two parameters, a local TSAP (i.e., transport address), local, and a

remote TSAP, remote, and tries to establish a transport connection between the two.If it succeeds,

it returns in connum a nonnegative otherwise a negative number

The SEND primitive transmits the contents of the buffer as a message on the indicated transport

connection, in several units if needed. Possible errors, returned in status, are no connection, illegal

buffer address.

The RECEIVE primitive indicates the caller's desire to accept data. The size of the incoming

message is placed in bytes. If the remote process has released the connection or the buffer address

is illegal, status is set to an error code indicating the nature of the problem

The DISCONNECT primitive terminates a transport connection. The parameter connum tells

which one.

Possible errors are connum belongs to another process or connum is not a valid connection

identifier.

The transport layer makes use of the network service primitives to send and receive TPDUs.

The hardware and/or software within the transport layer that does the work is called the transport

entity.

Fig 5.36 network packets in transport layer



Transport entity: states of a connection

Fig 5.37 states of transport entity

Education

Module 4 netwok layer,routing ,vlan,x.25doc