Instrumentos de evaluación

Next Generation Transmission

AI

© Wray Castle Limited

AII © Wray Castle Limited

a

Address (Data Link Connection Identifier (DLCI))

2 d

TY2702/v3.1 © Wray Castle Limited A2.1

Frame Relay – Frame Switching and Addressing

A customer may use Frame Relay to interconnect several geographically disparate sites. Computer applications as well as VoIP (Voice over IP) services are typical of the services supported.

The FRAD (Frame Relay Assembler/Dissembler) takes the information in from the customer equipment and encapsulates the traffic into a Frame Relay frame. The frames are then addressed using a DLCI (Data Link Connection Identifier). The DLCI is bound to a fragment. The fragment is usually a number of TDM timeslots; in the example shown, two timeslots have been combined to form the fragment. These timeslots in turn are usually carried in a TDM structure such as an E1/T1.

The circuit path through the network may be either permanently set i.e. PVC, or dynamically established as an SVC. For SVCs a signalling protocol between the frame switches is required; this could be established using Q.933 and Q.922.

The circuit path in this example is a PVC – it has been configured centrally. Each switch has a simple look-up table based on the interface and DLCI values.

Thus if the FRAD associated with the laptop wishes to send information to the desktop PC, it will forward this into the network with a DLCI value of 1. This incoming value will be switched out of interface ‘c’ with a value of 8. The frames incoming at the second switch will have a incoming value of 8. The second switch will look up its table and forward the frames out of interface ‘c’ with a new value of 3. The final switch in the chain will see an incoming value of on its interface ‘c’, and as a result of its table settings it will forward the information towards the FRAD with a value of 1 on interface ‘d’.

Options for Layer 2 Virtual Circuits

Header Information FCS

1598 octets 2 octets1 octet FL

GA

FL GA

Address (6 bits) C/R EA (0)

Address (4 bits) DE EA

(1)

Frame Relay over TDM

The Frame Relay frame is usually carried in a TDM fragment, which in turn could be transported using a transmission technology such as SONET/SDH. This could be then transferred over fibre as a SONET/SDH frame or carried in an individual wavelength as part of a WDM network.

TY2702/v3.1

A2.2 © Wray Castle Limited

Frame Relay Frame Structure

An example of the frame relay frame is shown with the following fields:

 Flag

 FCS (Frame Check Sequence)

 Information Field

 Header

Flags are used to delimit the Frame Relay frame set to 01111110.

The FCS uses a 16-bit CRC (Cyclic Redundancy Check), which is computed across the frame to allow the receiver to check for transmission errors.

The Information Field is the payload of the frame. It can be up to a maximum of 1598 octets.

The header shown here is set to 2 octets, as indicated by the Extension bit (EA) set to 0 in the first octet and then 1 in the second, where the 1 indicates this is the end of the header, the header could be extended over 3 or 4 octets.

The header could be extended further to carry a longer address field. Here, the DLCI is a 10-bit value over two octets.

DE (Discard Eligible) is used in congestion control, as are the FECN (Forward Explicit Congestion Notification), which is used upstream to inform other devices that congestion has occurred, and the BECN (Backward Explicit Congestion Notification), which is used on any frames in the downstream direction to inform downstream switches of congestion.

C/R (1 bit) is used to designate whether the frame is a command or response. Its use is not defined by frame relay and as such is left to implementors to decide on its use. Therefore this field may not be used.

Access Rate

Discard on entry

Excess Information Rate (EIR) Attempt to deliver

Mark 'Discard Eligible'

Committed Information Rate (CIR) Deliver

CIR EIR

TY2702/v3.1 © Wray Castle Limited A2.3

QoS (Quality of Service)

The CIR (Committed Information Rate) is the data rate at the ingress point that has been contracted with the customer. It must be supported by the network under normal conditions.

The EIR (Excess Information Rate) is at a level above the CIR. The communications provider will attempt to deliver the frames that exceed the CIR but are below the EIR limit. However, it will mark these frames as ‘Discard Eligible’, and as they pass through the network

they will be discarded by a subsequent switch through which they intend to pass which is currently suffering from congestion. Any frames at the ingress above the EIR will be discarded on entry.

Options for Layer 2 Virtual Circuits

TDM

SONET/SDH Frame Relay

TY2702/v3.1

A2.4 © Wray Castle Limited

Frame Relay over TDM

The Frame Relay frame is usually carried in a TDM fragment, which in turn could be transported using a transmission technology such as SONET/SDH. This could then be transferred over fibre as a SONET/SDH frame or carried in an individual wavelength as part of a WDM network.

APPENDIX B

ATM

Next Generation Transmission

BI

© Wray Castle Limited

BII © Wray Castle Limited

VC Link 2 VC Link 1

Virtual Channel Connection (VCC) VCC

Switch 1 Virtual Path Switch 2

Virtual Path Switch 3

VPI-1 VPI-2 VPI-3 VPI-4 VPI-5

VCI-7 VCI-8

VP Link VP Link VP Link VP Link VP link

Virtual Path Connection 1 Virtual Path Connection 2 VC

TY2702/v3.1 © Wray Castle Limited B2.1

ATM Terminology – VPs (Virtual Paths) and VC (Virtual Channel) Switches

ATM is a connection-oriented switching technology within which the basic unit of transport is a fixed length packet known as a cell. Cells transport data flows over an ATM connection. An ATM connection is referred to as a VCC (Virtual Channel Connection). VCCs may be preconfigured, in which case they are known as PVCs (Permanent Virtual Connections), or they may be set up automatically using a suitable control protocol, in which case they are known as SVCs (Switched Virtual Connections).

VCs are identified using a two-layer channel identifier consisting of a VPI (Virtual Path Identifier) and a VCI (Virtual Channel Identifier). This allows for switching at two levels. Individual circuits may be identified and switched using both the VPI and VCI; devices capable of this are generally known as VC switches. Groups of circuits may also be switched using just the VPI; devices capable of this are known as Virtual Path switches. VC switches must be able to terminate a VP, and by implication are also VP switches.

In this example shown, an ATM VC has been established between VCC Endpoint 1 (A) and VCC Endpoint 2 (F) through several VP and VC switches. At A the VC is identified as VPI-1: VCI-7. This is transported over a physical connection (such as SDH) to Virtual Path Switch 1 (B). Virtual Path Switch 1 is not interested in the value of the VCI; it is simply configured to switch the traffic from VPI-1 to VPI-2.

Virtual Path Switch 2 (C) is similarly configured to switch VPI-2 to VPI-3. Thus VCI-7 is transferred over VPI-1, then VPI-2, and finally it arrives on the Virtual Channel switch (D) on VPI-3. The Virtual Channel switch (D) is configured to switch cells arriving on VPI-3: VCI-7 to VPI-4: VCI-8. The path is completed to Endpoint 2 (F) via Virtual Path Switch 3 (E).

Note that cells for this circuit arrive at Endpoint 2 (F) labelled as VPI-5: VCI-8. In the VP layer, cells have been transported over several VP links. A VP link exists between any pair of devices applying or swapping a VP label. There are two VP connections used in this example.

A VP connection is simply a concatenation of VP links. A VP connection terminates at points where the VCI must be acted on. In the VC layer, two VC links have been used; A-D and D-F. A VC link exists between a pair of devices applying or swapping the VCI. The complete connection between endpoints is known as a VCC. A VCC is simply a concatenation of VC Links. The ability to switch groups of circuits under a single VP label ensures that size of switching tables at a VP switch is minimized.

Options for Layer 2 Virtual Circuits

Customer Site Customer Site UNI

UNI

UNI NNI or PNNI

Private – ATM-F (UNI+OSPF) Public – ITU-T B-ISUP ITU-T:

Inter-VPI VCI face VPI VCI

TY2702/v3.1

B2.2 © Wray Castle Limited

ATM Interfaces and Adaptation Layers

ATM allows operators/vendors to manage bandwidth across any interface, while providing services to both circuit- and packet-switched information. These valuable assets allow any application software to own dedicated bandwidth in order for the application to communicate.

User data/traffic is no longer confined to 64 kbit/s timeslots on a physical layer transport system such as an E1 (PDH). Using ATM, greater or smaller bandwidths can be configurable, dependent on requirements.

Bandwidth may be changed by different means, dependent on the type of ATM virtual circuit created.

ATM switches fixed-length packets (cells) of information in the connection-oriented mode of operation, ensuring that all cells relating to a specific connection follow the same route through the network.

Despite its name, the ATM cell stream is synchronous, and is ideally suited for delivery over synchronous transmission systems such as SDH. It is the way in which cells relating to a particular connection are transported that can be asynchronous rather than the operation of the complete cell stream.

ATM has defined two interfaces, that of the UNI (User Network Interface), which connects customers to ATM networks, and the internal NNI (Network to Network Interface).

The signalling protocol used across the UNI is usually based on Q.931 i.e. ITU-T Q.2931. Both the ITU-T and ATM Forum use these standards for their UNI. UNI signalling would be required specifically to support SVCs (Switched Virtual Channels). Most major deployments do not use SVCs and as such do not deploy UNI signalling. A positive exception to this could be seen in 3G networks where Q.2931 signalling is used to provide circuits for real time voice services i.e. Circuit Switched.

The ATM Forum and ITU differ when it comes to the NNI. In the case of the ATM Forum this is PNNI (Private NNI) which is based on the UNI signalling with a dynamic routing protocol based on OSPF. The ATM Forum also extended some of OSPF's Link State Advertisements and renamed them as PTSEs (PNNI Topology State Elements). The PTSE LMI (Local Management Interface) uses VPI/VCI of 0/16.

The ITU-T version of the NNI is their PNNI (Public NNI) where the signalling is based on the existing SS7, where the SS7 messages have been extended to include ATM parameters. This approach is consistent with their view of ATM as part of B-ISDN.

ATM

Physical e.g. SONET/SDH, PDH, Packet

SAAL AAL

TY2702/v3.1 © Wray Castle Limited B2.3

The ATM Layered Reference Model

The ATM layered reference model consists of three layers: physical, ATM, and AAL (ATM Adaptation Layer).

The physical layer provides for the transport of cells between ATM cell switching interfaces. It performs functions such as cell delineation and the insertion of idle cells. Suitable physical layers include SONET/SDH.

The ATM layer provides for the switching of cells between interfaces. The choice of output virtual link for any given input link is based on routing table information held within the ATM switches.

The AAL provides for the support of multiple applications by adapting the higher-layer information for transfer across the ATM switching network. In effect, the AAL isolates the ATM layer from the complexity of functions associated with multiple applications. SAAL (Signalling AAL) is a specific implementation of AAL5 with two additional sublayers called SSCF (Service Specific Coordination Function) and SSCOP (Service Specific Connection Oriented Protocol).

The higher layer may consist of functions required to exchange user data. Higher layers may also include access signalling, i.e. Q.2931; core network (PNNI) signalling using B-ISUP. The higher layers will also support management protocols such as CMIP (Common Management Information Protocol) or SNMP (Simple Network Management Protocol).

Options for Layer 2 Virtual Circuits

In document La eficacia de los Programas de (página 142-151)