VCAT/LCAS

Tuan Nguyen-viet

Concatenation

Contiguous and Virtual Concatenation

Contiguous Concatenation (CCAT)

Virtual Concatenation (VCAT)

VCAT (2)

• Packet-oriented, statistically multiplexed technologies, such as IP or Ethernet, do not match well the bandwidth granularity provided by contiguous concatenation.

– VCAT is an inverse multiplexing technique that allows granular increments of bandwidth in single VC-n units.

– At the source node VCAT creates a continuous payload equivalent to X times the VC-n

– The set of X containers is known as a Virtual Container Group (VCG), and each individual VC is a member of the VCG.

– Lower-Order Virtual Concatenation (LO-VCAT) uses X times VC11, VC12, or VC2 containers (VC11/12/2-Xv, X = 1... 64).

– Higher-Order Virtual Concatenation (HO-VCAT) uses X times VC3 or VC4 containers (VC3/4-Xv, X = 1... 256), providing a payload capacity of X times 48 384 or 149 760 kbit/s.

VCAT Operation

• MFI: Multiframe Indicator, VCG: Virtual Container group, SEQ: Sequence Number• Virtual concatenation is required only at edge nodes• Sink node must compensate for the different delays

VCAT Graphically

CCAT vs. VCAT

H4 multi-frame (VCAT & LCAS scheme)

• H4 is part of the HO-Path OH• H4 is repeated every 125 µs• 16-byte multi-frames takes 16 ms• A complete multiframe of 4096 bytes takes 512 ms to repeat (125x4096=512 ms)

K4 multi-frame (VCAT & LCAS scheme)

• K4 is part of the LO-PO overhead and is repeated every 500 µs• 32 bits are sent in a complete multiframe which takes 16ms to repeat. (500 x 32=16

ms)• The bit-2 superframe is made up of 32 multiframes and takes 512 ms to repeat.

Ethernet service provided by VCAT/LCAS

• The link between node A and node Z transports Ethernet frames using a Virtual Concatenation Group (VCG) of three members.

• Three separate LCAS protocols constantly monitor each peer connection:– LCAS-a of node R talks with LCAS-a of node Z, LCAS-b(R) with LCAS-b(Z), ...

LCAS-n (R) with LCAS-n (Z)

Link Capacity Adjustment Scheme (LCAS)

• ITU-T as G.7042, designed to manage the bandwidth allocation of a VCAT path.• LCAS can add and remove members of a VCG that control a VCAT channel.• LCAS cannot adapt the size of the VCAT channel according to the traffic pattern. • Source to Sink messages:

– Multi-Frame Indicator (MFI) keeps the multi-frame sequence.– Sequence Indicator (SQ) indicates member’s sequence to reassemble correctly

the client signal that was split and sent through several paths.– Control (CTRL) protocol messages which can be fixed, add, norm, eos, idle, and

dnu.– Group Identification (GID) is a constant value for all members of a VCG.

• Sink to Source include:– Member Status (MST), which indicates to source each member status: fail or OK.– Re-Sequence Acknowledge (RS-Ack) is an ack of renumbering after a new eos

member.

LCAS protocol

Simplified LCAS Source and Sink State Machine

VCAT Channel managed with LCAS

• LCAS helps network operators to efficiently control NG SDH connections established at VCAT sites. The use of LCAS is not compulsory, but improves VCAT management

LCAS protocol: MADD and Path error

LCAS protocol: MREMOVE

• LCAS is a two-way handshake protocol resident in H4 and K4 and executed permanently between source and sink as many times as VCAT members.

Sink messages are fault tolerant

• Sink to Source messages (MST, RS-Ack) are redundant while Source to Sink are specific to each member.

• This means that Sink messages are repeated as many times as members in the group.

• It also means that the origin of sink messages is irrelevant, because all the members are sending the same information in a multi-frame.

LCAS applications

• VCAT bandwidth allocation, LCAS enables the resizing of the VCAT pipe in use when it receives an order from the NMS to increase or decrease the size.

• Network Resilience, In the case of a partial failure of one path, LCAS reconfigures the connection using the members still up and able to continue carrying traffic.

• Asymmetric Configurations, LCAS is a unidirectional protocol allowing the provision of asymmetric bandwidth between two MSSP nodes to configure asymmetric links

• Cross-Domain Operation, because LCAS resides only at edge nodes it is not necessary to coordinate more than one configuration centre.

Example VCAT flow diagram

• As shown in Figure above, VCAT enables individual user data streams to be mapped into discrete paths in virtual concatenation groups (VCGs) for transparent forwarding across the SONET/SDH transport infrastructure.

• The L2 framer can feed native user data (Ethernet frames, IP/PPP packets, Fibre Channel, ESCON, etc.) to the VCAT mapper, using a variety of standard encapsulation schemes (HDLC/POS, GFP, X.86, etc.) and rate adaptation coding.

• The Layer 2 framed, rate-adapted byte stream is then mapped into VCGs to match the client data flow with provisioned bandwidth levels and SLA commitments.

FIGURE-1

Figure-1 description

• The SONET/SDH transport network is completely unaware of virtually concatenated traffic streams because all VCAT mapping and de-mapping is handled by non-path terminating equipment.

• Thus, the network doesn't have to do anything new so existing infrastructures can seamlessly transport VCGs with no need for modification.

• The transport network only sees a framed STS-xx signal carrying standard synchronous payload envelopes (SPEs).

• Since the network considers each SPE/VC as a separate entity, they can be transported on completely different routes through the network.

• The VCAT function can either co-route or differentially route the client data packets.• At the end-point, de-skew processing is automatically performed to realign all paths in

a VCG prior to de-mapping.• After de-skewing and de-mapping, the Layer 2 unframed byte stream is fed to the L2

framing function and output as native user packet data.

Diagram showing the mapping of a GbE client into a SONET pipe without the use of VCAT

• Figure-2 illustrates the benefits of VCAT for right-sizing Ethernet over SONET/SDH pipes.

• The example in Figure-2 shows a GbE client data stream being mapped over a single STS-48c pipe, which is the closest SONET-defined tributary size that can accommodate a full GbE packet flow (STS-12c/STM-4 could be used for fractional rate).

• In this example, approximately 1.4-Gbit/s of the available 2.4-Gbit/s bandwidth is wasted; it is used only to send rate adaptation bytes (GFP or HDLC flags).

• Even more bandwidth is wasted if the user data rate on the GbE link averages less than 1 Gbit/s.

• Theoretically, two GbE ports could be packed into the same STS-48c pipe but it would require that they be stuck together from end-to-end by sharing the same source and destination points.

FIGURE-2

Mapping multiple GbE clients into SONET pipes using VCAT

• In contrast, the example in Figure-3 shows multiple GbE client data streams using VCAT mapping on to a SONET STS-48 channelized SPE circuit.

• VCAT allows each GBE port, or a fraction of it, to be mapped into a dedicated, right-sized tributary, thereby optimizing the user's purchased bandwidth.

• The remaining bandwidth can be used to map other GbE clients within the same STS-48, thereby optimizing the carrier's utilization rate and maximizing revenue.

• In addition, flexibility is enhanced because VCAT-mapped GbE clients can be transported to different end-points anywhere across the SONET network.

FIGURE-3

Diagram showing the LCAS signaling format

Diagram showing the capacity increases provided by LCAS