Overview• Principles of Packet Switching• Standards Structure of X.25• Packet-Switch DTE and DCE Concepts• Theory of Operation• Packet Sequencing• X.25 Interface Protocol Structures• Network Layer Functions• User Connectivity• X.75 Internetworking Protocol

Early Reasons for Packet Switching– Create standard interfaces between computing devices– More efficient use of expensive transmission bandwidth– Interconnection of a large number of computing devices

and– Ability to extend computer communication over noisy

transmission facilities

Principles of Packet Switching• Packet switching divides user data into blocks called the payload

information. Packet switching adds overhead to the user data blocks, resulting in a block of data called a packet

• The functions that use the packet overhead are– Link layer: performs error detection, multiplexing of multiple logical

connections, basic network management capability– Network layer: identify a uniquely addressed network station– Pack layer: allows multiple logical users to share a single physical network

access circuit by providing buffers which are used during times of overflow

Principles of Packet Switching (continue…)• Packet switching extends the concept of statistical multiplexing to an

entire network• Refer to Figure 9.1 (p. 325)• Packet switching employs queueing to control loss and resolve

contention at the expense of added, variable delay• There are two types of flow and congestion control

– Implicit: adaptively alter the rate of packets sent into the network by estimating loss and delay using a layer 4 protocol (ex. TCP)

– Explicit: the sender gets notified on congestion events, if the sender or receiver reacts to the explicit indication of congestion quickly enough, the loss can be avoided entirely

X.25 Packet Switching DefinedOrigins of X.25• The CCITT X.25 packet-switching standard was developed to provide

a reliable system of data transport for computer communications over the noisy, unreliable analog-grade transmission medium

• Many packet switches today can provide networking speeds up to and including DS1 and E1, with packet sizes up to 4096 bytes

Standards Structure of X.25• The CCITT set of “X dot” standards for the physical, link, and packet

layer protocols are known collectively as X.25• Refer to Figure 9.2 (p. 327)• The physical layer is defined by the X.21 and X.21bis standards. X.21

specifies an interface between DTE and DCE• The data link layer standard is based upon the High-level Data Link

Control (HDLC) ISO standard. The modified X.25 version of this called LAP-B

• The packet layer standard is called the X.25 Packet Layer Protocol (PLP). The packet layer defines Permanent Virtual Circuit (PVC) and virtual call, or Switched Virtual Circuit (SVC) message formats and protocols

Standards Structure of X.25 (Continue…)• User connectivity to the packet-switched network takes on mnay

forms. The suite of “X dot” standards is: X.121, X.21, X.25, X.28, X.29, X.3, and X.32.

• X.121 defines the network layer numbering plan for X.25• CCITT Recommendations X.3, X.28, and X.29 define the method for

asynchronous DTEs to interface with X.25 networks via a Packet Assembler/Dissembler (PAD) function

• A PAD delivers a stream of asynchronous characters to the DTE from a received X.25 packet

What does X.25 Packet Switching Provide?• X.25 packet switching provides logical user access and logical

channel multiplexing onto a single line by interleaving low-speed user channels into a single high-speed data channel which then interfaces to the network

• Refer to Figure 9.3 (p. 329)• Packet switches allocate user bandwidth in virtual circuits. This gives

the user the perception of a dedicated circuit of full bandwidth• Packet-switch networks are designed based upon the statistical chance

that not all users will want to transmit at the same time, nor for the same duration

Packet-Switch DTE and DCE Concepts• Packet switching’s first practical commercial application was designed

for host computers to communicate through multiple nodes over a packet-switched network.

• Refer to Figure 9.4 (p. 329)• The X.25 protocol defines the protocols and procedures that manage the

transfer of data between the DTE, typically a user terminal, and the DCE, typically the network node

• The end-user DTE accesses the X.25 network through a communications controller, FEP, or host

• A DCE packet assembler/dissembler (PAD) is often used within the network for DTE access

Theory of OperationTraffic Characteristics• The two primary requirements that best suit packet networks are

– burstiness: data transmission having a duration of less than five seconds

– delay insensitive traffic: packet switched networks that are not delay sensitive

• Refer to Figure 9.5 (p. 331)

Basic Operation• DTE devices transmit data to the network via synchronous mode X.25

protocol through the use of packets.• As packets flow through the network, each mode checks for errors

and then retransmits if necessary• All operations are transparent to the user• Refer to Figure 9.6 (p. 332)• An elaborate acknowledgment and retransmission scheme is provided

in packet switching, primarily because of the large amount of errors which could be experienced when using voice grade lines

Acknowledgments• under normal conditions, all packets are acknowledged by each

receiving node as they pass through the network• Refer to Figure 9.7 (p. 333)

Packet Sequencing• Packet switching utilizes queuing and store-and-forward protocols

with packet-sequencing information contained within each packet.• Sequencing errors can be caused by delay incurred by message

division (different packet delays) over simultaneous long and short hops

Example of X.25 Store-and-Forward Operation• The LAP-B protocol uses a store-and-forward approach to ensure

reliable delivery of packets across noisy, error-prone transmission links

• Two types of packets are exchanged between th enodes: Link Data (LD) and ACKnowledgment (ACK)

• Each message has a pair of numbers associated with it:– Receive Sequence number (RSN): indicates the next SSN that is expected by

the receiver– Send Sequence Number (SSN): number of packet that is being sent

• Refer to Figure 9.8 (p. 335)

Packet Error & Lost Packets• To alleviate these problems, each node only transmits

acknowledgments to the previous node when it passes a packet on to the next destination node.

• This scheme adds additional delay to the queuing process. If a certain level of packet loss can be reasonably tolerated, this scheme does not need to be implemented.

• Refer to Figure 9.9 (p. 336)

Packet duplication• It can be caused by a failure of a node to acknowledge a packet that

has continued through the network

X.25 Interface Protocol Structure• The X.25 interface protocols define two levels of control procedures:

– The subscriber link access procedure which defines the physical movement of data between two user locations

– The packet layer DTE/DCE interface which defines the transfer of information across the physical medium at the network layer

CCITT Recommendation X.25• X.25 acts as the interface between DTE and DCE for terminals and

computers operating in the packet switching mode• Refer to Figure 9.10 (p. 338)

X.25 and the OSI Reference Model• Refer to Figure 9.11 (p. 338)• Refer to Figure 9.12 (p. 339)

Data link Frame Structure• X.25 network access can be accomplished over

– a single-link procedure (SLP): uses standard ISO HDLC framing– a multilink procedure (MLP): uses LAP-B over multiple channels based on the

ISO MLP standard

• Refer to Figure 9.13 (p. 340)

The Packet Layer DTE/DCE Interface• Each packet to be transferred across the DTE/DCE interface is carried

or packaged within the data link layer HDLC (LAPB) frame information field

• Refer to Figure 9.14 (p. 341)

X.25 Layer 3 Packet Format• The X.25 layer is composed of a header and a user data, or

information, field• Refer to Figure 9.15 (p. 342)

Network Layer Functions• Each physical interface can establish one or multiple virtual circuits to

remote physical interfaces• Each of these circuits is then assigned logical channel numbers• Virtual circuits assure sequence integrity in the delivery of user data,

established either administratively as a PVC, or as an SVC through call control procedures.

• Over these PVCs flow both normal data and call control packets.• An X.25 “call” is set-up using a global X.25 address, and a SVC

channel number is negotiated and allocated between each switch

Network Layer Functions (continue…)• The X.25 global address is only present in call set-up and tear-down,

and all data in between is identified by its SVC channel number.• Thus, the X.25 address is global significant, and the SVC channel

number is locally significant• Datagrams are messages unto themselves that do not require control

procedures.• Datagrams may be sent in a best-effort mode, or have an explicit

receipt notification

Permanent Virtual Circuits and Virtual Calls• The network layer provides the user with up to 4095 logical channels

designated either as Permanent Virtual Circuits (PVCs) or Virtual Calls (VCs) over one LAPB physical channel

• PVCs are virtual circuits permanently established between a source and destination node

• The user always sees the virtual circuit as a dedicated circuit for his or her use only, whereas the network provides the same circuit as a shared resource to multiple users upon demand.

• Refer to Figure 9.16 (p. 343)• Switched Virtual Circuits are established and disabled on demand

Permanent Virtual Circuits and Virtual Calls (Continue…)

• Virtual Calls (VCs) or Switched Virtual Calls (SVCs) act as circuit-switched calls, with the characteristic of being connected and disconnected after the data has been sent between the source and destination node

• Therefore, one source can connect to many destinations at the different times.

• Refer to Figure 9.18 (p. 345)• SVCs can add even more delay than when using a PVC

Virtual Calls and Logical Channel Numbers• Logical Channel Numbers (LCNs) are assigned to each of the

incoming and outgoing virtual calls for each DCE and DTE, respectively, as well as to all PVCs

• Out of the 4095 logical channel numbers available per physical circuit, PVCs are assigned the lowest numbers, followed by one-way incoming virtual calls, then two-way incoming and outgoing calls, and the highest numbers are reserved for one-way outgoing virtual calls

• Refer to Figure 9.19 (p. 346)

X.25 Control Packet Formats• Control packets are used for virtual call setup and termination.• Refer to Figure 9.21 (p. 347)• Control packets perform many functions, including call request and

incoming call packets, call accepted and call connected clear request and clear indication, etc.

• Clear packets are used to clear the user-to-user session (DTE-to-DTE)• Interrupt packets are used when the user wants to bypass the normal flow

control protocol• Reset packets will reset the entire PVC or SVC during data transfer• Restart packets will reset all of the PVCs and clear all SVCs on a specific

physical interface port

X.25 Control Packet Formats (Continue…)• Diagnostic packets are used as the catch-all identifier of all error

conditions not covered by the other control packets• Reject packets reject a specific packet and retransmission occurs from

the last received packet

Normal Data Packet Formats• Normal data packets are transferred after call setup and before call

termination• PVCs only require normal packet formats, as the virtual circuit is

permanent and does not need to be set up or terminated• Refer to Figure 9.22 (p. 349)

Flow Control and Windowing• The send and receive sequence numbers in the X.25 packet layer are

also used to provide flow control between the packet layer source and sink.

• The send sequence number is a sequential number for the current packet

• Refer to Figure 9.23 (p. 349)• Numbers are incremented modulo the maximum window size.• The receive sequence number acts as an acknowledgement for all

packets up to one less than the receive sequence number

Flow Control and Windowing (Continue…)• The transmitter can send no more packets than the modulo minus 1

without acknowledgment• This is called a sliding window flow control protocol. This process

allows the receiver to control the maximum rate of transmission over a virtual circuit, and is therefore a form of traffic control

Datagram Function• Datagrams are single-packet messages which contain a destination

address and are independently routed through the packet-switched network

Datagram Function (Continue…)• All the information for a single transaction is contained within the

datagram packet.• These packets each select the best route through the network, do not

require acknowledgements, are unsequenced, and thus are a form of connectionless service.

• Datagrams are ideal for when the user interface data transfer is for a very short period of time and where response time must be fast, or is not even needed, and where user control over the transmission is not required.

• Datagrams were dropped from the X.25 standard

User Connectivity• The network user interface is accomplished through seven important

standards: X.121, X.21, X.28, X.29, X.3 and X.32

User Network Interface• CCITT Recommendation X.121 - Defines the international numbering

plan for packet-switched networks• CCITT Recommendation X.28 - Defines the operational control of

these functions between character mode terminal DTE and the DCE PAD

• CCITT Recommendation X.29 - Defines the same controls but for the host computer destination

User Network Interface (Continue…)• CCITT Recommendation X.3 - Defines a Packet

Assembler/Disassembler (PAD) concentrator function for start-mode or character-mode DTE devices

• CCITT Recommendation X.32 - Defines X.25 synchronous dial-up mode for DTE services

• There are two types of “user-to-network” interfaces using these protocols: – packetized virtual circuits: assure sequencing of user data and on ordered flow

of messages which require multiple packets– packetized datagram: are messages unto themselves and do not follow the

ordered packet flow of virtual circuits

User Applications• The primary application used with packet switching is the terminal-to-

host application access and vice versa.• Packet switching was designed for terminals and PC interactions, and

compressed voice can also be passed through HDLC over the packet-switched network.

Performance And Design Considerations• Performance in a packet-switched network is measured in packets per

second throughput• Packet delay through a typical packet switch node is 50 to 200 ms due

to packet processing

Performance And Design Considerations (Continue…)

• The switch performance should be constant, irrespective of packet size. Some packet switches drastically degrade performance and packet processing as the packet size increases above 128 or 256 kbits.

• Another consideration is the protocol conversion. The greater the amount of protocol conversion that the switch or PAD has to perform, the lower the throughput and performance.

X.75 Internetworking Protocol• X.75 can be used to connect the larger backbone packet switches to

one another• X.75 functions reside in the network layer above X.25 functions• All functions of the X.75 protocol are similar to the X.25 protocol• X.75 also supports the multilink procedures to support multiple links

between STE• STE is Signaling Terminal Exchange and acts as the internetwork

interface point and performs both packet-transfer procedures and packet signaling

• Refer to Figure 9.27 (p. 358)

Cisco

Virtual LANs (VLANs)

Objectives

• Describe Virtual LANs• Describe Frame Tagging• Describe Inter-Switch Link Routing• Describe Virtual Trunking Protocol

Virtual LANs (VLANs)

• Definition: A logical grouping of network users and resources

connected to administratively assigned ports on a switch.– Essentially…smaller broadcast domains– Organized by:

• Location• Function• Department• Application or protocol

Virtual LANs

Broadcast Control

• Broadcasts occur in every protocol• Bandwidth greedy & Broadcast applications• Flat network = one broadcast domain

• VLANs & Broadcasts• all devices in a VLAN are members of the same

broadcast domain and receive all broadcasts

Security

Flat network problems addressed

– By using VLANs and creating multiple broadcast groups, administrators now have control over each port and each user

Flexibility & Scalability

• Layer-2 switches only read frames– Can cause a switch to forward all broadcasts

• VLANs create broadcast domains• Reduces broadcast traffic• Add users to VLAN regardless of physical location• Additional VLANs can be created when network growth

consumes more bandwidth

• Switch fabric– Group of connected switches sharing same VLAN information

Traditional Collapsed Backbone

Switched & Routed Network

VLAN Memberships• Static VLANs

– Typical method of creating VLANs

– Most secure (switch port assigned to a VLAN always maintains that assignment until changed)

• Dynamic VLANs– Node assignment to a VLAN is automatic

• MAC addresses, protocols, network addresses, etc

• VLAN Management Policy Server (VMPS)• MAC address database for dynamic assignments

• MAC-address to VLAN mapping

Identifying VLANs

• Access links– A link that is part of only one VLAN

• Trunk links– Carries multiple VLANs

Frame Tagging

Frame Tagging Definition:Keeping track of users/frames as they travel the

switch fabric & VLANs

– User-defined VID (VLAN ID) assigned to each frame– Removed before exiting trunked & access links

VLAN ID (VID) Methods• Inter-Switch Link (ISL)

– externally tagging VLAN info onto Ethernet frame – Cisco proprietary– FastEthernet & Gibabit Ethernet only– Allows VLANs to be multiplexed over trunk line

• IEEE 802.1q– Must use if trunking between Cisco & non-Cisco

switch

Trunking & Routing

• Trunking (layer 2)• Single port can be part of multiple VLANs• Cisco switches use Dynamic Trunking Protocol

(DTP) for trunk management (using ISL or 802.1q)

• Routing (layer 3)– A layer-3 device (router or switch) is necessary for any

host or device to communicate between VLANs

VLAN Trunk Protocol (VTP)

• VTP Purpose:– Manage all configured VLANs across a switch

internetwork & maintain consistency– Allow an administrator to add, delete, &

rename VLANs– Permits trunking over mixed networks

• VTP server must be created to manage VLANs

VTP Modes

VTP Modes of Operation• Server

– Default for all Catalyst switches– Minimum one server for a VTP domain

• Client– Receives information + sends/receives updates– Cannot make any changes

• Transparent– Does not participate in a VTP domain but

forwards VTP advertisements– Can add/delete VLANs– Locally significant

Revision Number

Summary

• Described Virtual LANs• Described Frame Tagging• Described Inter-Switch Link Routing• Described Virtual Trunking Protocol