Data Communication and Networks

Lecture 6

Networks:(Packet Switching: X.25. ATM, Frame Relay)

October 10, 2002

Joseph Conron

Computer Science Department

New York University

conron@cs.nyu.edu

X.251976Interface between host and packet

switched networkAlmost universal on packet switched

networks and packet switching in ISDNDefines three layers

Physical Link Packet

X.25 - PhysicalInterface between attached station and

link to nodeData terminal equipment DTE (user

equipment)Data circuit terminating equipment DCE

(node)Uses physical layer specification X.21

X.25 - LinkLink Access Protocol Balanced (LAPB)

Subset of HDLC see chapter 7

X.25 - PacketExternal virtual circuitsLogical connections (virtual circuits)

between subscribers

X.25 Use of Virtual Circuits

Virtual Circuit ServiceVirtual Call

Dynamically established

Permanent virtual circuit Fixed network assigned virtual circuit

Virtual Call

Packet Format

MultiplexingDTE can establish 4095 simultaneous

virtual circuits with other DTEs over a single DTC-DCE link

Packets contain 12 bit virtual circuit number

Virtual Circuit Numbering

Flow and Error ControlHDLC at the link layer(Chapter 7)Sliding window at the VC layer

Packet SequencesComplete packet sequencesAllows longer blocks of data across

network with smaller packet size without loss of block integrity

A packets M bit 1, D bit 0

B packets The rest

Zero or more A followed by B

Reset and RestartReset

Reinitialize virtual circuit Sequence numbers set to zero Packets in transit lost Up to higher level protocol to recover lost packets Triggered by loss of packet, sequence number

error, congestion, loss of network internal virtual circuit

Restart Equivalent to a clear request on all virtual circuits E.g. temporary loss of network access

Asynchronous Transfer Mode (ATM)

Protocol ArchitectureSimilarities between ATM and packet

switching Transfer of data in discrete chunks Multiple logical connections over single physical

interface

In ATM flow on each logical connection is in fixed sized packets called cells

Minimal error and flow control Reduced overhead

Data rates (physical layer) 25.6Mbps to 622.08Mbps

Protocol Architecture (diag)

ATM Logical Connections Virtual channel connections (VCC) Analogous to virtual circuit in X.25 Basic unit of switching Between two end users Full duplex Fixed size cells Data, user-network exchange (control) and

network-network exchange (network management and routing)

Virtual path connection (VPC) Bundle of VCC with same end points

ATM Connection Relationships

Call Establishment Using VPs

VP/VC CharacteristicsQuality of serviceSwitched and semi-permanent channel

connectionsCall sequence integrityTraffic parameter negotiation and usage

monitoring

VPC only Virtual channel identifier restriction within VPC

ATM CellsFixed size5 octet header48 octet information fieldSmall cells reduce queuing delay for high

priority cellsSmall cells can be switched more

efficientlyEasier to implement switching of small

cells in hardware

ATM Cell Format

Header FormatGeneric flow control

Only at user to network interface Controls flow only at this point

Virtual path identifierVirtual channel identifierPayload type

e.g. user info or network management

Cell loss priorityHeader error control

Generic Flow Control (GFC) Control traffic flow at user to network interface

(UNI) to alleviate short term overload Two sets of procedures

Uncontrolled transmission Controlled transmission

Every connection either subject to flow control or not

Subject to flow control May be one group (A) default May be two groups (A and B)

Flow control is from subscriber to network Controlled by network side

Single Group of Connections (1)Terminal equipment (TE) initializes two

variables TRANSMIT flag to 1 GO_CNTR (credit counter) to 0

If TRANSMIT=1 cells on uncontrolled connection may be sent any time

If TRANSMIT=0 no cells may be sent (on controlled or uncontrolled connections)

If HALT received, TRANSMIT set to 0 and remains until NO_HALT

Single Group of Connections (2)If TRANSMIT=1 and no cell to transmit on

any uncontrolled connection: If GO_CNTR>0, TE may send cell on controlled

connectionCell marked as being on controlled connectionGO_CNTR decremented

If GO_CNTR=0, TE may not send on controlled connection

TE sets GO_CNTR to GO_VALUE upon receiving SET signal Null signal has no effect

Header Error Control8 bit error control fieldCalculated on remaining 32 bits of headerAllows some error correction

HEC Operation at Receiver

Cell Based Physical LayerNo framing imposedContinuous stream of 53 octet cellsCell delineation based on header error

control field

Cell Delineation State Diagram

ATM Service CategoriesReal time

Constant bit rate (CBR) Real time variable bit rate (rt-VBR)

Non-real time Non-real time variable bit rate (nrt-VBR) Available bit rate (ABR) Unspecified bit rate (UBR)

Real Time ServicesAmount of delayVariation of delay (jitter)

ATM Adaptation LayerSupport for information transfer protocol

not based on ATMPCM (voice)

Assemble bits into cells Re-assemble into constant flow

IP Map IP packets onto ATM cells Fragment IP packets Use LAPF over ATM to retain all IP

infrastructure

Adaptation Layer ServicesHandle transmission errorsSegmentation and re-assemblyHandle lost and mis-inserted cellsFlow control and timing

Frame RelayDesigned to be more efficient than X.25Developed before ATMLarger installed base than ATMATM now of more interest on high speed

networks

Frame Relay Background - X.25Call control packets, in band signalingMultiplexing of virtual circuits at layer 3Layer 2 and 3 include flow and error

controlConsiderable overheadNot appropriate for modern digital

systems with high reliability

Frame Relay - DifferencesCall control carried in separate logical

connectionMultiplexing and switching at layer 2

Eliminates one layer of processingNo hop by hop error or flow controlEnd to end flow and error control (if used)

are done by higher layerSingle user data frame sent from source to

destination and ACK (from higher layer) sent back

Advantages and DisadvantagesLost link by link error and flow control

Increased reliability makes this less of a problem

Streamlined communications process Lower delay Higher throughput

ITU-T recommend frame relay above 2Mbps

User Data TransferOne frame type

User data No control frame

No inband signalingNo sequence numbers

No flow nor error control