
2/22

Network Layer Protocols (cotd.)

Connection Oriented Protocols:

Employ virtual circuits

Routing is decided at the VC setup time

Provides End-to-End Flow and Error Control

Delivers all packets in proper order

Less overhead in packets

Loss of node/VC causes loss of all packets in VC

Unable to adapt to network traffic conditions


3/22

Network Layer Protocols (contd.)

Connectionless Protocols

No end to end connectivity

No flow control or error control by network layer. (It is

left to the Transport layer.)

Each packet is independent and each finds its route.Packet header must carry full information.

Packets can arrive at the destination out of order.

No circuit set up time involved.

More robust.

Can adapt to network changing network traffic

conditions.

Loss of a node causes only the loss of packets there.


4/22

Network Layer Protocols (contd.)

X.25 Protocol

Connection Oriented Protocol

ITU-T Recommendation Introduced in 1976.

Defined ad the interface between DTE and DCE. i.e. theinterface between Packet Mode Terminal and Packet

Switching Exchange.

DTE Data Terminal Equipment

DCE Data Circuit-terminating Equipment

Employs Virtual Circuits (either Switched Virtual Circuit (SVC)

or Permanent Virtual Circuit (PVC)).


5/22

X.25 (contd.)

X.25 Layers

Physical layer X.21 or X.21 bis (EIA RS232 equivalent)

Link Layer LAP B (HDLC in ABM)

Network Layer X.25 LAP B

I frames to send data

S frames for flow and error control

U frames to setup and manage connection

Eg. SABM followed by UA followed by several I frames and

then a DISC and UA to terminate.


6/22

X.25 (contd.)

X.25 Packet Layer

Called the Packet Layer Protocol (PLP)

Responsible for (a) Establishing connection, (b) Transferringdata, and (c) Terminating connection.

Responsible for creating the virtual circuit and negotiatingservice options between DTEs.

Note:

Frame layer (i.e. link layer) is responsible for making

connection between DTE and DCE.Packet layer (X.25) is responsible for making connectionbetween Two DTEs. (i.e. end-to-end connection.)


7/22

X.25 (Contd.)

X.25 Flow and Error Control

Done at both frame and network layers

Frame layer does it across a link

Network layer does it end-to-endNote:

X.25 Virtual Circuit is created at the network layer and not at

the link layer.

(In contrast, the Frame Relay and ATM are created at the linklayer.)

X.25 Packet Format


8/22

X.25 (Contd.)

X.25 Packet Format

General Format Identifier

Identifies packet parameters i.e., data or controlinformation, what kind of window flow control, whether

delivery confirmation is required. Logical Channel Identifier

Identifies the virtual circuit across the local DTE/DCEinterface.

Packet Type Identifier

Identifies the packet type if it is a control packet.

User Data

This field carries the user data.


9/22

X.25 (Contd.)

X.25 Disadvantages

Was designed for PSTN which was slow at the time.

Max. speed available is 48 kbps. This is insufficient for modern

day data transactions. Addressing scheme (X.21) is based on the telephone

numbering system.

Has superfluous error control that makes the throughput

small. Some X.25 provisions (such as the operation of Character

Mode Terminals) have become obsolete.


10/22

X.25 Related Other Protocols

X.121

Provides the global addressing scheme used in X.25 packet

addressing.

Address Format:DNIC (4 digits) NTN (10 digits)

DNIC Data Network Identification Code defines the specific

network. First 3 digits define the country and the 4th defines

the network inside that country.NTN National Terminal Number defines the DTE inside the

particular network.


11/22

Frame Relay Networks

Frame Relay Features and Advantages

Operates at the link layer

Virtual circuit mode operation

No flow control or error controlFrame error check done only to see if the frame should be passed on

or dropped.

Less overhead compared to X.25 in frame format

Relies on improved transmission media, such as fiberIt particularly suits for protocols like TCP/IP since there is no

duplication at the network layer


12/22

Frame Relay Networks (Contd.)

Frame relay doesnt need fixed bit rate (like X.25) and thus is

more suitable for bursty data.

Frame relay frame size can be up to 9000 bytes and thus can

accommodate all LAN frames.

Frame Relay Disadvantages

Frame relay can operate at bit rates up to 44.376 Mbps. But

this is not enough for some services.

Frame relay allows variable frame lengths. This can createunfair delays for some users.

Because of the variable delay (due to variable frame length)

frame relay is not suitable for real time traffic.


13/22

Frame Relay Operation

FR provides both switched and permanent VCs

FR provides connection of LANs or Main Frame Computers to

each other in a WAN.

The facility that provides the connection on the LAN or Main

Frame side is the DTE, and could be a Router, a Bridge, or a

Man Frame Computer.

The facility that provides switching of Frames in the WAN is

the DCE, and is a switching centre.

Since Frame relay operates with VCs, the packets carry VC

identifiers and not addresses. This identifier is called the Data

Link Connection Identifier (DLCI).


14/22

Frame Relay Layers

Physical Layer

No specific protocol is specified for the physical layer of the

Frame Relay. Frame relay supports any of the protocols

recognized by ANSI.

Data Link Layer

At the data link layer FR employs a simplified version of HDLC

called LAP F.

Flag Address Information FCS Flag

Note that the Control field is missing

The Address field is divided into several subfields:


15/22

Frame Relay Data Link Layer (contd.)

Address Field Format:

DLCI C/R EA DLCI FECN BECN DE EA

6 bits 1 bit 1 bit 4 bits 1 bit 1 bit 1 bit 1 bit

DLCI (6+4=10 bits) gives the VC identifier.Command/Response bit is for the use of the upper layer.

EA is Extended Address bit. If it is 0 then the address field has

another byte to follow. If it is 1 the current byte is the last.

FECN is Forward Explicit Congestion Notification.BECN is Backward Explicit Congestion Notification.

DE is Discard Eligibility bit. If 1 do not discard.


16/22

Congestion Control in Frame Relay

Congestion occurs when inflow data at a node exceeds its

handling capacity. Congestion can bring down a network quite

rapidly.

X.25 has flow control at both link and network layers. At link

level it controls the flow of data across a link. At network layer

it provides end-to-end flow control.

Frame Relay has no flow control.

Frame Relay allows the user to send bursty data. This makes

Frame Relay networks vulnerable for traffic congestion.

Therefore some congestion control mechanism at frame level

is required. Two bits, BECN and FECN, do this.


17/22

Congestion Control in Frame Relay (Contd.)

BECN warns the sender about the congestion in the network. Thiswarning can come (from the receiver) as a separate (special) frame,or in another receiving frame if it is a full-duplex link.

FECN warns the receiver that congestion has occurred in thenetwork. Frame relay it self cannot do anything about it but it cancommunicate with the sender and seek assistance from higher layerprotocols.

When senders are not responding to congestion notifications theFrame Relay switches will discard frames, depending on the statusof the Discard Eligibility bit.

Senders are informed by the upper layers (such as Transport layerwhich will recognize the congestion) about the discarded frames,and will be requested to resend those discarded frames.


18/22

Frame Relay Congestion Control

Leaky Bucket Algorithm

Behavior of Frame Relay Switch is similar to a leaky bucket

that has a small hole at the bottom.

The water leaks from the bucket at a constant rate

irrespective of the inflow to the bucket (as long as the bucket

is not empty).

If the inflow to bucket is too rapid the bucket may overflow.

Each Frame Relay Switch can be set to send data at a certain

rate. If data is received at a rate faster than the sent out rate

the switch is congested and the data frames will have to be

discarded.


19/22

Frame Relay Congestion Control

Ex. A one inlet one outlet frame relay switch with buffer has aninput data stream that comes in bursts at a rate of 40 Mbps.The burst duration is 100 ms. The gaps between data burstsare 1 min. long. If the switch has to avoid congestion, what

should be the minimum buffer size?Minimum buffer size = 40 Mbps x 100 ms = 4 Mb

Ex. If the output line has a capacity of 1.544 Mbps cancongestion be avoided with this buffer?

Time taken to empty the buffer = 4/1.544 sec.

Since this is less than 1 min. the congestion can be avoided.


20/22

Frame Relay Traffic Control

Congestion strategies require Frame Relay to take Traffic

Control measures to determine when to set BECN, FECN, DE,

etc. and to discard frames.

For this four different traffic attributes are taken into account.

Access rate

Committed Burst Size

Committed Information Rate

Excess burst rate These are set during the VC establishment phase after

negotiations between the user and the network.


21/22

Frame Relay Traffic Control (contd.)

Access Rate

For each connection the access rate (bps) is defined.

The value depends on the bandwidth of the channel connecting theuser to the network.

User cannot exceed this rate.Eg.If the access is through a G703 link, then the access rate cannotexceed 2 Mbps.

Committed Burst Size

This is the maximum number of bits the network is committed to

transfer in a predefined period, without discarding any frames (orsetting DE bit).

Eg. 400 kb in 4 sec. (could be 300 kb in 1st sec. +100 kb in last sec.)


22/22

Frame Relay Traffic Control (contd.)

Committed Information Rate

This is the average rate of information the user can send to

the network. As long as this is not exceeded the frames will

not be discarded.

The value of Committed Information Rate (CIR) is related to

Committed Burst Size (CBS).

CIR = CBS/Burst Time

Excess Burst Size

This is the maximum number of bits in excess of CBS that a

user is allowed to send in a predefined period. Network will

honor this if there is no congestion.

Documents

Network Layer Protocols.ppt